Pump overrun is killing my heating!

[milleniumaire]

Happy New Year.

Okay, maybe the subject is a little dramatic, but hopefully I've got your attention for a few minutes 😀

After a recent upgrade of my CH system, which included a LLH and a new circulation pump, my trusty old Vaillant ecoTEC 637 Plus system boiler seems to be spending a lot of time in pump overrun mode, especially in the mornings, when it first comes on.

It now spends the first 50 minutes heating up the water (DHW priority) before it starts to even think about heating the radiators. I've noticed a lot of that 50 minutes is spent in pump overrun mode, which I think means the hot water is cycling round the boiler circuit and is not getting to the unvented hot water cylinder to heat the water. During pump overrun the new circulation pump is off.

Am I correct in my assessment of the situation? Is there anything that can be done to resolve this issue so that the hot water is heated faster allowing the radiators to be heated, just like it used to do?

 

[ShaunCorbs]

No both pumps should be on maybe depending on how the hot water is piped up is it after the llh ? / any pics ?

 

[milleniumaire]

No both pumps should be on maybe depending on how the hot water is piped up is it after the llh ? / any pics ?

Hi Shaun, thanks for replying so quickly. A little more info about my system.

The system has been expanded over the last 6 years and the recent changes included an additional UFH zone, making it 3 zones (radiators, kitchen UFH, bathroom UFH) and DHW.
To achieve this a VR 71 wiring centre, VRC 700 and 2 X VR 91 remote thermostats were added. The LLH and new pump off the LLH flow was added to improve the efficiency of the different heating zones.

Boiler is top top right, Vaillant unvented cylinder to the left and the new Grundfos pump is to the left of the new LLH, fitted to the system flow. After the pump, each zone has its own 2 port valve.

At the time the heating engineer did the work, he had to order a new component, called an X40, which was a mini circuit board that plugged into the main boiler circuit board. Vaillant told him to connect the new pump upto this, rather than into the VR 71 wiring centre. Here's a photo of the boiler circuit board with the X40 on the right, perpendicular to the main board:

The engineer still seemed a little unsure exactly how to connect up the new pump as there are 4 connectors on the X40 and after reading the information that came with it, he connected it upto the yellow "rel 1".

The pump definitely does not come on when the boiler is running in pump overrun mode and I suspect therefore that the pump in the boiler is causing water to pump around the short boiler circuit between the boiler and the LLH? I assume it does this to try to cool down the water? The old system only had the boiler pump and so I assume when in pump overrun, water was being pumped around the system with the relevant zone valve open (DHW in this case), which possibly means that even in pump overrun mode, the boiler was heating the water in the cylinder? I'm guessing at this and I'm sure you guys will know better, but this would seem to explain why the current system takes so long to heat up the hot water and the previous version of the system did not.

 

[EvilDrPorkChop]

Certainly sounds like a wiring issue, in HW Priority I’d expect your cylinder to reheat in 20/25 mins. Have you set the correct kw output of the coil for HW generation on the board?

Personally I’d of wired the additional pump to either the pump relay on the main board, or the S/L of the motorised valves.

 

[milleniumaire]

Certainly sounds like a wiring issue, in HW Priority I’d expect your cylinder to reheat in 20/25 mins. Have you set the correct kw output of the coil for HW generation on the board?

Personally I’d of wired the additional pump to either the pump relay on the main board, or the S/L of the motorised valves.

The boiler and cylinder were installed 10 years ago, so I assume the correct kw output would have been set and used over this time. I'm not sure if the engineer would have changed any of the boiler settings during the recent upgrade (mid November), but it sounds like this particular setting wouldn't need to be changed?

Obviously, in terms of installing the VRC 700 (to replace a VRC 430), there were a number of config settings that had to be programmed to identify zones, heat curves and other stuff, but I guess none of this would affect the setting you mention.

Originally, the engineer had assumed the new pump would be connected upto the new VR 71 wiring centre, but after talking to Vaillant he had to order the X40 so that the pump could be controlled by the boiler, rather than by the wiring centre. At the time I thought this made sense i.e. any time the boiler pump is on, the new circulating pump should also be on, but this doesn't appear to be the case during pump overrun, when only the boiler pump is running.

I think the "pump relay" you mention is actually the X40. I believe this now comes as standard with new Vaillant boilers, but didn't with mine, which is now 10 years old. To connect the pump to the S/L of the motorised valves, it would need to be wired upto 4 different zones in the VR 71 and I believe this was also my engineers first thought, until Vaillant told him to connect it upto the X40.

 

[EvilDrPorkChop]

The boiler and cylinder were installed 10 years ago, so I assume the correct kw output would have been set and used over this time. I'm not sure if the engineer would have changed any of the boiler settings during the recent upgrade (mid November), but it sounds like this particular setting wouldn't need to be changed?

Obviously, in terms of installing the VRC 700 (to replace a VRC 430), there were a number of config settings that had to be programmed to identify zones, heat curves and other stuff, but I guess none of this would affect the setting you mention.

Originally, the engineer had assumed the new pump would be connected upto the new VR 71 wiring centre, but after talking to Vaillant he had to order the X40 so that the pump could be controlled by the boiler, rather than by the wiring centre. At the time I thought this made sense i.e. any time the boiler pump is on, the new circulating pump should also be on, but this doesn't appear to be the case during pump overrun, when only the boiler pump is running.

I think the "pump relay" you mention is actually the X40. I believe this now comes as standard with new Vaillant boilers, but didn't with mine, which is now 10 years old. To connect the pump to the S/L of the motorised valves, it would need to be wired upto 4 different zones in the VR 71 and I believe this was also my engineers first thought, until Vaillant told him to connect it upto the X40.

Did you previously have HW priority before? The settings for this are I think D.75, D77, D.78. D.77 should be set to the KW rating of your Vaillant Coil.

Regarding the X40, yes you are probably correct. The newer Vaillants over a certain Kw have the relay built in now.

Has the probe been for the cylinder been properly wired in?

 

[ShaunCorbs]

The second pump after the llh should run with any demand eg hot water, heating or ufh

 

[SJB060685]

Are you sure the second pump is actually running when theres a demand?

 

[milleniumaire]

Did you previously have HW priority before? The settings for this are I think D.75, D77, D.78. D.77 should be set to the KW rating of your Vaillant Coil.

Regarding the X40, yes you are probably correct. The newer Vaillants over a certain Kw have the relay built in now.

Has the probe been for the cylinder been properly wired in?

Yes, the system has always been setup for DHW priority.

D.75 = 45 (factory setting)
D.77 = 37 (factory default and maximum output I believe)
D.78 = 80 (factory default)

So, it looks like these settings are all defaults.

Yes, the cylinder temperature probe has been connected to S9 on the VR 71 wiring centre as shown in the attached schema diagram. This diagram was supplied by Vaillant as the nearest to my setup as none of the diagrams in the VRC 700 System Diagrams booklet were any good! However, this diagram shows each zone has having its own pump (and mixing valve), which isn't the case in my system.

 

[milleniumaire]

Are you sure the second pump is actually running when theres a demand?

Yes, most definitely.

The Grundfos pump has LED's that show green when the pump is running. The VRC 700 is very good at showing what the system is doing and which zone valves are open. I spent a bit of time ensuring all the zone valves and the new pump were operating correctly for each zone when the upgrade was completed.

 

[milleniumaire]

To be clear about pump overrun, from memory, the boiler display shows a circle with a black triangle indicating the internal pump is running. To the left of this is a tap (hot water supply active) and the i button shows S.7 - Pump over run. During this time the LED on the Grundfos pump is NOT on. I also see this from time to time during the day, but instead of the tap, it is showing the radiator symbol in the display. Again, the Grundfos pump is not on, so while in this mode, no hot water is being pumped around this system, even when the thermostats are calling for heat!

 

[ShaunCorbs]

There can’t be a demand then or cylinder isn’t upto temp as my thought is a soon as it’s finished heating the cylinder it should switch to heating if heating is calling if it’s not then it should turn off or pump overrun depending on what vaillants do

 

[milleniumaire]

There can’t be a demand then or cylinder isn’t upto temp as my thought is a soon as it’s finished heating the cylinder it should switch to heating if heating is calling if it’s not then it should turn off or pump overrun depending on what vaillants do

I believe the cylinder is taking too long to get upto temp, so the system is in DHW mode for 50 minutes and a lot of that time it is showing pump overrun. Every time it goes to pump overrun (S.7) the new Grundfos pump turns off, so all that hot water that is running around the boiler side isn't going anywhere. Eventually, I assume it cools down and the pump is then turned on again and the water starts to be pumped around the cylinder. Then the boiler circuit heats up and its back to S.7. This continuous overrun mode seems to cause it to take much too long to heat up the cylinder, but eventually, the DHW circuit closes (I guess when it finally does get to temperature) and the other CH circuits open and finally the radiators etc. start to warm up.

At least, that's my novice understanding of what might be happening, but I could be completely wrong.

In the morning I will check the state of the system and the individual zones (specifically DHW) to be sure there is a demand whenever the boiler is in pump overrun mode. Given that it's supposed to be heating up the hot water cylinder, I would hope there is a demand.

 

[ShaunCorbs]

Is the pump on full speed ?

 

[milleniumaire]

Is the pump on full speed ?

No, it is on the slowest speed (setting 1 of 3). The engineer left it on full speed when it was installed and when I called him and asked if there was any reason for that he told me there wasn't, so I turned it down.

It seems to me that the slowest speed setting allows the heat in the water time to dissipate while circulating around the system, so there's more chance of it being at a lower temperature on its return to the boiler/LLH. I'm not actually convinced the boiler is condensing as much as it used to, but I believe that is one of the side affects of using LLH's. But that's potentially another issue for another day 😀

 

[SJB060685]

Could this not be a control fault ie PCB. The function of pump overrun is to dissipate the residual heat after a demand is satisfied to protect the boiler. It shouldn't be active at the same time as a demand, although obviously the internal pump will be.

 

[milleniumaire]

I've just noticed the boiler is currently showing the radiator and pump symbol, but the new Grundfoss pump is not on. "i" shows S.0, which surprised me as according to the VRC 700, the radiator zone is requesting heat i.e. it is on and the valve is open. The system status is showing "heat.mode". So, right now, there is demand by one of the zones, but the external pump is switched off and the internal boiler pump is running. This is what I assumed was pump overrun mode, but the boiler is showing S.0, not S.7!

This could be a different scenario to what is happening in the mornings, so I still need to monitor it first thing.

 

[milleniumaire]

Could this not be a control fault ie PCB. The function of pump overrun is to dissipate the residual heat after a demand is satisfied to protect the boiler. It shouldn't be active at the same time as a demand, although obviously the internal pump will be.

That's the thing I don't understand about pump overrun. Isn't the best way to loose heat to pump the hot water around the system, without heating more hot water i.e. have only the pump running? You're saying there shouldn't be demand at the same time as pump overrun meaning all the 2 port valves would be closed, so any pumping would result in the water just cycling round the boiler (via the diverter valve)? Isn't this just wasted energy and at the same time a zone is waiting to be heated?

 

[ShaunCorbs]

Sounds like it’s not set up / wired right next thing to do is speak to vaillant and make sure everything is plugged into the right kit

 

[milleniumaire]

Sounds like it’s not set up / wired right next thing to do is speak to vaillant and make sure everything is plugged into the right kit

Yes, I have contacted Vaillant already about the setup, given the installer had a few issues, I got Vaillant to confirm all the wiring connections, which was when they provided me with the schema diagram.

I will ask them again, specifically about the pump connection, and explain the behaviour I'm seeing in the mornings.

 

[SJB060685]

As shaun says it does sound like it's not wired/set up correctly.
Ideally yes you would use the residual heat with pump overrun but in most cases it just circulates through a bypass to slowly drop the temperature of the heat exchanger, of course whats happening in your case is you're circulating quite a small volume so its no surprise its taking time for that temperature to drop.

 

[milleniumaire]

As shaun says it does sound like it's not wired/set up correctly.
Ideally yes you would use the residual heat with pump overrun but in most cases it just circulates through a bypass to slowly drop the temperature of the heat exchanger, of course whats happening in your case is you're circulating quite a small volume so its no surprise its taking time for that temperature to drop.

So, do you think with my old system (pre LLH and external pump) that in pump overrun mode that hot water would have continued to circulate around the radiators/cylinder? I guess this depends on the 2 port valves being open, but if they weren't how would the water have circulated?

 

[EvilDrPorkChop]

Certainly sounds like a wiring issue.

Basically what I said first time round 😉😜

After having a fair bit of experience with Vaillants, my concern would probably be whether your new controller and wiring centre is compatible with your older PCB. They say they are, but i'm not always convinced.

You need to set D.77 on 16kw (From memory this is what the coils are in the Vaillant uniStors) Otherwise you're going to get a lot of anti cyciling which may not be helping issues.

After heating each circuit the boiler will overrun the pump with the zone valve open, so after a HW reheat it'll over run and after a CH demand it'll over run. I can't exactly remember if it over runs also once the zone valve closes.

If your secondary pump out of the LLH isn't operating though on any demand, then that would explain your HW issue in a morning. The boiler will try and heat the cylinder (Failing) and then at 45 mins decide it can't do it, over run the pump then switch to CH mode.

 

[SJB060685]

If there was no circuit you wouldn't of had flow. Did you have a bypass?

 

[EvilDrPorkChop]

So, do you think with my old system (pre LLH and external pump) that in pump overrun mode that hot water would have continued to circulate around the radiators/cylinder? I guess this depends on the 2 port valves being open, but if they weren't how would the water have circulated?

There is a bypass in the boiler, but yeah the Vaillant controls usually hold the zone valve open for a period of time. Well they do on the newer system from what I recall.

 

[EvilDrPorkChop]

If there was no circuit you wouldn't of had flow. Did you have a bypass?

It would go round the LLH. That's effectively your bypass, plus an integrated one.

 

[milleniumaire]

If there was no circuit you wouldn't of had flow. Did you have a bypass?

The only bypass is the one in the boiler.

 

[SJB060685]

It would go round the LLH. That's effectively your bypass, plus an integrated one.

He said prior to the llh lol

 

[SJB060685]

There is a bypass in the boiler, but yeah the Vaillant controls usually hold the zone valve open for a period of time. Well they do on the newer system from what I recall.

I wish all boilers were this advanced lol

 

[milleniumaire]

It would go round the LLH. That's effectively your bypass, plus an integrated one.

Yes, in the new setup it can cycle to the LLH and back. If the external pump was on, it could cycle further (and cool down quicker, while at the same time continuing to provide heat - to the cylinder in the morning) and I guess this is the crux of the issue that this doesn't happen in pump overrun mode.

 

[EvilDrPorkChop]

Out of interest have you checked to see if there's power to the pump? I've had a couple of them newer UPM3s fail quite early on tbh.

 

[milleniumaire]

Out of interest have you checked to see if there's power to the pump? I've had a couple of them newer UPM3s fail quite early on tbh.

Yes, there's definitely power as the LEDs are on and I can feel the pump vibrating, which increases as I increase the speed.

 

[EvilDrPorkChop]

Yes, there's definitely power as the LEDs are on and I can feel the pump vibrating, which increases as I increase the speed.

I meant when it isn't running and the boiler is saying it should be.

 

[milleniumaire]

I meant when it isn't running and the boiler is saying it should be.

Ah, sorry, I see what you mean.

The only situations I'm aware of where the external pump doesn't start, when the internal pump does start is S.0 and S.7. It is possible there are other status codes that I haven't come across, but these are the two I regularly see.

However, when there's demand and neither of the above status codes are showing on the boiler, then both pumps are on at the same time.

 

[EvilDrPorkChop]

Ah, sorry, I see what you mean.

The only situations I'm aware of where the external pump doesn't start, when the internal pump does start is S.0 and S.7. It is possible there are other status codes that I haven't come across, but these are the two I regularly see.

However, when there's demand and neither of the above status codes are showing on the boiler, then both pumps are on at the same time.

Ah right OK, as I said in an earlier post then i'd configure your HW configuration correctly on the PCB first and see if that makes a difference. You're probably getting a lot of anti cycling.

 

[EvilDrPorkChop]

Yes, the system has always been setup for DHW priority.

D.75 = 45 (factory setting)
D.77 = 37 (factory default and maximum output I believe)
D.78 = 80 (factory default)

So, it looks like these settings are all defaults.

Yes, the cylinder temperature probe has been connected to S9 on the VR 71 wiring centre as shown in the attached schema diagram. This diagram was supplied by Vaillant as the nearest to my setup as none of the diagrams in the VRC 700 System Diagrams booklet were any good! However, this diagram shows each zone has having its own pump (and mixing valve), which isn't the case in my system.

Can I just ask, who did that drawing for you? It's great, I struggle to get an answer for basic things out of my rep (Although he isn't bad to be fair).

 

[milleniumaire]

Can I just ask, who did that drawing for you? It's great, I struggle to get an answer for basic things out of my rep (Although he isn't bad to be fair).

I don't think Vaillant created that drawing especially for me. If you look at the date, it is 30/07/2018.

The VRC 700 came with a printed System diagram book, unfortunately none of the diagrams seemed to come close to matching my setup and I pointed this out to Vaillant when asking them to check the installation for me (via email) as I wasn't 100% convinced that everything had been connected up correctly by the heating engineer. He's a great guy and does all my servicing and upgrading, but even he admits its difficult to keep up with technology! As an IT consultant, I'm used to getting into the detail and like to understand how things work, so had no problem liaising with Vaillant support and have made a couple of changes as a result.

As I explained earlier, although the diagram reflects the zones I have (DHW via cylinder, 2 x UFH and radiators) there are some differences, mainly in the area I'm having issues; the external pump. The diagram shows each zone having its own pump, so I suspect these would be wired up in the same way as the zone valves so that when the valve opens the pump switches on. It appears to be a little more complicated to use a single pump for all zones and even my engineer was surprised that the pump had to be connected to the boiler, rather than to the wiring centre!

One day, when I have time, I may create my own schema diagram using Microsoft Visio 😀

 

[milleniumaire]

Just found the installation guide for the X40 expansion board, which plugs into the boiler PCB and provides for the external pump connection.

Here are the relevant pages:

I'm trying to understand how the above relates to the actual device and the way the pump has been wired upto it:

On the boiler d.27 = 5 and d.28 = 2

What's the difference between "Circulation pump", "External Pump" and "Storage charging pump"?

Based on the photo, showing the pump is connected to the yellow "rel 1" connectors, which 1, 2, 3, 4, 5 or 6 does this relate to?

Because it is wired to rel 1 (open) and d.28 is 2, I assume it relates to 2 in the table. Are 1 and 2 mutually exclusive. 1 shows a ZP & tap and 2 shows a radiator, so I'm thinking they relate to DHW and heating! I'm not sure what "ZP" means.

Note: Just realised the last paragraph is nonesense as d.28 relates to "rel 2", so I'm now wondering if either the connection is wrong or the setting of d.27 is wrong!

 

[ShaunCorbs]

Those connections need to be done better as there’s cores nearly touching each other

 

[SJB060685]

As shaun says those connections are poor and the chance of a short circuit are real. Looking at the amendment should external pump go to relay 2?

 

[EvilDrPorkChop]

Just found the installation guide for the X40 expansion board, which plugs into the boiler PCB and provides for the external pump connection.

Here are the relevant pages:

View attachment 46989

View attachment 46991
I'm trying to understand how the above relates to the actual device and the way the pump has been wired upto it:

View attachment 46990

On the boiler d.27 = 5 and d.28 = 2

What's the difference between "Circulation pump", "External Pump" and "Storage charging pump"?

Based on the photo, showing the pump is connected to the yellow "rel 1" connectors, which 1, 2, 3, 4, 5 or 6 does this relate to?

Because it is wired to rel 1 (open) and d.28 is 2, I assume it relates to 2 in the table. Are 1 and 2 mutually exclusive. 1 shows a ZP & tap and 2 shows a radiator, so I'm thinking they relate to DHW and heating! I'm not sure what "ZP" means.

Note: Just realised the last paragraph is nonesense as d.28 relates to "rel 2", so I'm now wondering if either the connection is wrong or the setting of d.27 is wrong!

Should be down as external pump I think.

 

[milleniumaire]

Should be down as external pump I think.

So, assuming external pump is correct, does the chart suggest it can be wired in either of two different ways:

a. Connect to rel 2 and set d.28 to 2
b. Connect to rel 1 and set d.27 to 2

I'm concerned about the distinction between "circulation pump" (1) and "external pump" (2). Does this imply the pump will only work during hot water demand (1) or heating demand (2), but not both?

 

[SJB060685]

Your observation is like mine. Possibly connect to relay 2 and set to D.28 and see what happens?

 

[John.g]

Following this very interesting thread.
Just a few basic observations.
Pump overrun should not occur unless CH and/or HW satisfied, if boiler output still too high on max turndown then burner off but boiler circ pump and secondary pump should continue to run but overrun should not be called for. When pump overrun is called for normally, would expect secondary pump to be off. Suggest to check if pump overrun comes on with burner off with call from CH or HW.
Slow rad heat up/output not mentioned but with a LLH the boiler circ pump and external pump should be matched with boiler circ pump flowrate slightly higher?. so possibly wrong settings with boiler circ pump or more likely secondary pump or faulty pump. Also assume LLH connections correct.

 

[John.g]

Also S.0 refers to "no heating demand" which I would think is indicative of the boiler sensing no heat demand even with zone valve(s) open, I would have a good look at that Grundfos Pump as it may be the main cause of your problem, hopefully.

 

[ShaunCorbs]

Also don’t forget about if there’s flow through the pump the leds will light up as the unit is acting like a generator eg there’s no power going to the pump plug

 

[milleniumaire]

Good morning. Just catching up on the new posts.

Also don’t forget about if there’s flow through the pump the leds will light up as the unit is acting like a generator eg there’s no power going to the pump plug

Shaun, are you saying the LED's on the pump would still work even if the pump didn't have power? When I press the speed button on the front of the pump, I can hear/feel the pump toggling through the different speeds. Also, if the pump wasn't working, surely the radiators/UFH and hot water would never warm up as there's no way the internal boiler pump could push water past the LLH and around the system!

Also S.0 refers to "no heating demand" which I would think is indicative of the boiler sensing no heat demand even with zone valve(s) open, I would have a good look at that Grundfos Pump as it may be the main cause of your problem, hopefully.

Given my comments above about the pump and the fact that I've done tests on each thermostat to force demand for each zone and can see the pump LED's turning on/off as demand is requested/removed, I'm feeling very confident the external pump is working. Whether it is connected and configured correctly is another matter!

Following this very interesting thread.
Just a few basic observations.
Pump overrun should not occur unless CH and/or HW satisfied, if boiler output still too high on max turndown then burner off but boiler circ pump and secondary pump should continue to run but overrun should not be called for. When pump overrun is called for normally, would expect secondary pump to be off. Suggest to check if pump overrun comes on with burner off with call from CH or HW.
Slow rad heat up/output not mentioned but with a LLH the boiler circ pump and external pump should be matched with boiler circ pump flowrate slightly higher?. so possibly wrong settings with boiler circ pump or more likely secondary pump or faulty pump. Also assume LLH connections correct.

So, I set the heating and hot water to start at 06:30am this morning, which is half an hour later than I usually set it, but I wanted to monitor what happened. At 07:00am the system was still heating the cylinder. The external pump was on, the system status (via the VRC 700) was showing DHW and the boiler status was S.24 (Burner ignited), so all looked to be okay, apart from that fact that the cylinder was still not to temperature (65 degrees C) after 30 minutes. There was absolutely no heat in the radiators or UFH.

10 minutes later it was showing S.20 (Warmstart demand). At 1 hour since turning on I noticed it had switched to heating demand and the radiators finally started to warm up.

I confess, I didn't see any sign of pump overrun this morning. Maybe I just didn't look at the right time (in the first 30 mins) or maybe I'm mistaken about this. However, should it really take an hour to heat up the water in a 260 litre Vaillant unistor cylinder?

Looking at the current config of the external pump. It is currently wired to rel 1 (open) and according to the chart, this suggests that d.27 determines what will happen. As this is currently set to 5, the chart says "External gas valve" for this value. I wonder if this is just incorrectly set and the engineer set d.28 to 2 when he meant to set d.27 to 2 as according to the chart, d.28 has no effect unless the pump is connected to rel 2 (open). As far as I'm aware, my system doesn't have an "External gas valve", so I'm not sure how this impacts on the pump starting/stopping.

From my observations yesterday (not this morning) it appeared that whenever the gas burner was on, the external pump was on. So, when in pump overrun mode, the gas burner wasn't on and therefore the external pump wasn't on. Again, this could be a complete red herring, but I'm trying to make sense of the current configuration and what is happening, given the fact that setting 5 for d.27 mentions "gas valve".

As the pump cable connections are inside the boiler, I don't want to take the front off to change them, so it may be better for me to simply change d.27 to value 2, rather than 5 and see what impact (if any) that has.

I appreciate the connections need to be re-done and I'll ask the engineer to look at them as he's due back soon to service the boiler.

So, although I didn't see any evidence of pump overruns this morning, I've definitely seen it happening on other days, so I'm not ruling this out completely just yet.

If it is expected to take an hour to heat the cylinder to 65 deg C then it would appear everything is working as normal and I'll just have to turn on the hot water earlier in the morning to give this time to happen, followed by time to heat up the house. However, this never used to be a problem prior to the recent upgrade, when the heating/hot water started up at 06:00 every morning and the house was toasty by the time I got out of bed at 07:00am.

 

[Murdoch]

What happens if you set the water to run more than an hour before the heating comes on? i.e. allow the water to run and then turn off before running the heating?

 

[milleniumaire]

Following this very interesting thread.
Just a few basic observations.
Pump overrun should not occur unless CH and/or HW satisfied, if boiler output still too high on max turndown then burner off but boiler circ pump and secondary pump should continue to run but overrun should not be called for. When pump overrun is called for normally, would expect secondary pump to be off. Suggest to check if pump overrun comes on with burner off with call from CH or HW.
Slow rad heat up/output not mentioned but with a LLH the boiler circ pump and external pump should be matched with boiler circ pump flowrate slightly higher?. so possibly wrong settings with boiler circ pump or more likely secondary pump or faulty pump. Also assume LLH connections correct.

Sorry, I meant to comment on your post.

Regarding matching the flowrates for the boiler and external pumps, the external Grundfos pump has a touch button that allows me to increase/decrease the flowrate and it is currently on minimum. I'm not sure exactly how the flow rate of the internal pump is controlled, possibly through a boiler config parameter, but I seem to recall it being on a low setting.

When you say assume LLH connections correct, I assume you are referring to flow and return connections to the system and boiler? With my basic knowledge they appear to be correct and I know my engineer has fitted this type of LLH before, so I would hope he got it right. If they weren't connected up correctly, I'm sure I would be seeing other, more obvious issues, such as heating zones not warming up etc.

I think I understand what you are saying about pump overrun. Basically, you wouldn't expect the external pump to be turned on when pump overrun is on. This is what I am seeing. Usually, when the boiler shows 2.7 (pump overrun) the burner symbol (hour glass) is NOT displayed.

 

[milleniumaire]

What happens if you set the water to run more than an hour before the heating comes on? i.e. allow the water to run and then turn off before running the heating?

Do you mean set the timer to stagger the start times for hot water and heating, so:

06:00 Water on
07:00 Water off
07:10 Heating on

I would expect the hot water demand to end before 07:00 i.e. boiler and pump turn off, due to the cylinder reaching the required temperature. This morning, the heating demand started around 55 mins after the hot water demand started, suggesting it took 55 minutes to heat up the 260 litre cylinder.

If I've understood you correctly, I'm not sure what this would prove.

I have wondered it it is possible to change the settings so that hot water does NOT have priority i.e. heat up both the cylinder and radiators/UFH at the same time?

 

[Murdoch]

run the water at 5am .......

Water heating is generally "slower" if the system is also doing radiators if I recall things correctly

I must admit I'm a bit puzzled by the depth you are going to - can't you just get the engineer back?

 

[John.g]

Yes, re pump overrun but it shouldn't IMO be ever on pump overrun with any demand for CH or HW, I read that the minimum output of this boiler is 12 kw? so if the coil is not absorbing this 12 kw then the boiler will cycle, maybe this is when the pump overrun is showing up. If the coil is absorbing 12 kw then that should heat a 260 litre cylinder from say 25C to 65c in exactly 1 hour, obviously if a 20kw coil, in 36 minutes. If the coil inlet temp is < say 68/70C then the cylinder will never reach setpoint temperature. You can monitor the flow/return temps on the temperature gauges which will tell you something and if you had/have access to a energy monitor you can measure the secondary pump power in watts from which the flow rate can be derived and then you will know exactly what's happening re boiler/coil power.

And presume this is a heat only boiler and not some form of combi!!.

 

[milleniumaire]

run the water at 5am ******.

Water heating is generally "slower" if the system is also doing radiators if I recall things correctly

I must admit I'm a bit puzzled by the depth you are going to - can't you just get the engineer back?

I could get the engineer back, but to what end? He has setup the system as it is, to the best of his knowledge, and it doesn't appear to be working "as well" as it used to be before the upgrade. I know he was on the phone to Vaillant at least three times while setting it up, which doesn't exactly inspire me with confidence! It is possible that introducing the LLH, external pump, VR 71 wiring centre and new VRC 700 controller has changed the way it works such that it doesn't work "as well" as it used to and I'll just have to accept that.

By "as well" I mean the hot water heated up and the house warmed up within an hour of everything starting up in the morning.

Yes, I could start the water earlier at 5am and this would probably resolve the issue, but isn't it best to first determine if there is an issue with the way things are working rather than just working around the issue?

 

[John.g]

Before installing the LLH the system flow temperature would always be the same as the boiler flow (setpoint) temperature but with the LLH, depending on (any) mixing then the system flow temperature may be lower but easy to see this by your temperature gauges.

 

[milleniumaire]

Before installing the LLH the system flow temperature would always be the same as the boiler flow (setpoint) temperature but with the LLH, depending on (any) mixing then the system flow temperature may be lower but easy to see this by your temperature gauges.

Yes, I can see this through the analogue temp gauges I have on the pipes. The photos I've posted don't show that I have also fitted a couple of gauges to the boiler flow and return as well, so I now have 4 gauges showing the temps between the boiler and LLH and the LLH and system circuit. You are correct, the flow and return around the system, from/to the LLH are always slightly lower than those on the boiler side, but I wouldn't have thought they were low enough to make a huge difference.

Currently, there is heating demand from all three heating areas (radiators and 2 x UFH zones), as has been the case all morning and the thermostats are showing:

LLH flow from boiler: 58 deg C
LLH return to boiler: 48 deg C
LLH flow to system: 56 deg C
LLH return from system: 46 deg C

I confess, I'm a little surprised the return from the system isn't lower, given it is going through 14 radiators, kitchen/sitting/dining UFH and bathroom UFH!

I have seen the boiler side temps get up to 75 deg C.

 

[John.g]

Is the boiler setpoint 75C ?, and was it the same prior to changes?. if the system flow temp never reaches 65C then it doesn't matter how long the boiler is on as the cylinder will never reach its SP, you could temporarily reduce it to say 55C.
Also most rapid heat recovery coils are designed with flow temperatues of up to 85C and with very high circulation rates to get their rated output.

 

[SJB060685]

With LLH if the system flow is equal to boiler flow then there should be know mixing and an equal temperature maintained, likewise if the system flow was less than boiler flow. If the system flow was greater than boiler flow then there will be a mixed lower temperature going to system circuit/circuits.
The connections in the picture are correct. All a LLH header is an empty tube effectively giving a means of separating two systems with a low pressure loss and low velocity zone. They are also know as a low velocity header.

 

[milleniumaire]

Is the boiler setpoint 75C ?, and was it the same prior to changes?. if the system flow temp never reaches 65C then it doesn't matter how long the boiler is on as the cylinder will never reach its SP, you could temporarily reduce it to say 55C.
Also most rapid heat recovery coils are designed with flow temperatues of up to 85C and with very high circulation rates to get their rated output.

Sorry John, I'm not sure what you mean by setpoint.

On the boiler, the dials are set such that hot water is at its maximum setting of 65 deg C. The radiators are set to 70 deg C. Is this the setpoint?

What is confusing is that the VRC 700 allows the desired hot water temperature to be set more than 65 deg C, but the boiler dial never shows more than this. The VRC 700 hot water setting and boiler hot water setting don't seem to be in sync. Not sure what this is about, possibly an example of where the 10 year old boiler isn't as compatible with the new VRC 700 controller as it could be.

When scrolling through the menus on the VRC 700, there are lots of temperatures mentioned, but the only one's I've changed are the "desired" temperatures, so 21 deg C for the radiator circuit (controlled by VRC 700, which is in the hallway - the coldest part of the house) and 20 deg C for the two UFH circuits.

 

[John.g]

It seems that this controller, for whatever reason is telling your boiler to run at 58C, based on this and your return temps and assuming room temperatures of 20C then these rads will only be emitting 56% of their rated output which would account for the slow heat up. This controller may be some smart form of outside temperature compensation, must have a read up of it.

 

[milleniumaire]

It seems that this controller, for whatever reason is telling your boiler to run at 58C, based on this and your return temps and assuming room temperatures of 20C then these rads will only be emitting 56% of their rated output which would account for the slow heat up. This controller may be some smart form of outside temperature compensation, must have a read up of it.

Yes, the system does use weather compensation and each zone has its own heat curve. While this is supposed to make the system more efficient, it also makes it more difficult to understand what or why things are happening e.g. why is there no heat demand for a zone when the desired temp is set to 21 and the current temp for the zone is showing 19 and the rooms are cold!

This kind of scenario usually ends up with me increasing the heat curve for the zone, making the system less efficient! Of course, the outside temperature also has a large impact and usually, at this time of year, when it is much colder outside, the heating system achieves the desired indoor temperatures more often than not.

 

[John.g]

Just had a quick scan through it and it does compensate for the outside air temperature with a outside temperature sensor, you can see on page 12 that you can change the heating curves, for example if currently set on curve 2, it will call for a boiler SP (reqrired boiler temperature) of 65C at OT of 0C, 55C at 5C, and 45C at 10C. If you select heating curve 2.5 then the numbers are 73C at 0C, 63C at 5C, and 52C at 10C and so on. The HW required temperature seems to be set by default to 60C but don't see any corresponding boiler SP, if working properly, I would expect to see the boiler SP temperature rising with the actual cylinder temperature. You obviously need a cylinder sensor connected.
It also mentions somewhere about switching off the heating pump with no heating demand, maybe this is S.0?

Just saw your post.

 

[milleniumaire]

Just had a quick scan through it and it does compensate for the outside air temperature with a outside temperature sensor, you can see on page 12 that you can change the heating curves, for example if currently set on curve 2, it will call for a boiler SP (reqrired boiler temperature) of 65C at OT of 0C, 55C at 5C, and 45C at 10C. If you select heating curve 2.5 then the numbers are 73C at 0C, 63C at 5C, and 52C at 10C and so on. The HW required temperature seems to be set by default to 60C but don't see any corresponding boiler SP, if working properly, I would expect to see the boiler SP temperature rising with the actual cylinder temperature. You obviously need a cylinder sensor connected.

I just beat you to posting!

The cylinder does have a temperature sensor, which is wired upto the VR 71 wiring centre, so I assume this is being used to get the cylinder to the desired temperature of 65 deg C.

I've been regularly checking the boiler today, since 7:00am and haven't once seen it go into pump overrun. Typical!

 

[John.g]

Re zone temperature control, Presume if a number of zones in service then it should base the boiler SP on the highest heat curve and if only one, on that zones heating curve.

I know you have HW priority and the reason you can't have HW & CH on together is probably the conflicting boiler SP requirements? although one might think that the controller could be configured to select the boiler SP based on the HW requirement or something like this to allow both systems on together.
When HW next selected can you note the actual cylinder temperature, the boiler SP temperature and the actual (thermometer measured) flow&return temps, this will give you a very good feel for what's going on there.

Can you also state the minimum output of your boiler.

 

[EvilDrPorkChop]

What size is the cylinder, 65oc seems a high set point tbh and may explain your reheat issues in the morning.

 

[milleniumaire]

What size is the cylinder, 65oc seems a high set point tbh and may explain your reheat issues in the morning.

It's a 260 litre Vaillant unistor unvented cylinder.
60 Deg C is obviously the lowest setting to avoid legionella.
Since the upgrade I'm fairly sure it has been necessary to turn up the shower temperature. I had assumed the engineer may have turned down the hot water setting, so I turned it up to the maximum of 65 Deg C using the boiler control. The shower temperature setting still needs to be set higher e.g. 7 out of 10, instead of 5.

 

[EvilDrPorkChop]

It's a 260 litre Vaillant unistor unvented cylinder.
60 Deg C is obviously the lowest setting to avoid legionella.
Since the upgrade I'm fairly sure it has been necessary to turn up the shower temperature. I had assumed the engineer may have turned down the hot water setting, so I turned it up to the maximum of 65 Deg C using the boiler control. The shower temperature setting still needs to be set higher e.g. 7 out of 10, instead of 5.

The Vaillant system has an Anti Legionella Setting that you can turn on, personally 60oc is too hot for me mines set at 55oc. I'd be careful as many showers have a Max inlet temp of 65oc also, so if your probe is measuring 65oc at the bottom it'll be more like 70oc at the top of the cylinder.

This is probably your problem too, getting a 260l cylinder to 65oc, and with the boiler possibly anti cycling because your HW output isn't set correctly then your going to be taking a good 30/45 minutes to get to the set point. Also if you're drawing off in this period it isn't going to help.

 

[John.g]

Spec Sheet:
the unistor260 contains 250 litres of water, it has 22.3kw heating performance at a circulation flow rate of 22.3 LPM (no heating temp given) with a 35 min heating time, primary heating surface 0.75M2.

So presume the controller is looking at the rate of temperature rise and adjusting the boiler flow temp, I think it was mentioned that 75C was noted sometimes?.
It may be of some help to increase the heating output from 16kw to 22kw which may help to get the temperature up within a hour. Possibly some fouling of the heating surfaces as well which won't help.

 

[EvilDrPorkChop]

Spec Sheet:
the unistor260 contains 250 litres of water, it has 22.3kw heating performance at a circulation flow rate of 22.3 LPM (no heating temp given) with a 35 min heating time, primary heating surface 0.75M2.

So presume the controller is looking at the rate of temperature rise and adjusting the boiler flow temp, I think it was mentioned that 75C was noted sometimes?.
It may be of some help to increase the heating output from 16kw to 22kw which may help to get the temperature up within a hour. Possibly some fouling of the heating surfaces as well which won't help.

It was my understanding that the coil was 16.6w output? (According to Vaillants Website).

On HW priority the cylinder will reheat with a flow temp of 80oc.

 

[SJB060685]

Spec Sheet:
the unistor260 contains 250 litres of water, it has 22.3kw heating performance at a circulation flow rate of 22.3 LPM (no heating temp given) with a 35 min heating time, primary heating surface 0.75M2.

So presume the controller is looking at the rate of temperature rise and adjusting the boiler flow temp, I think it was mentioned that 75C was noted sometimes?.
It may be of some help to increase the heating output from 16kw to 22kw which may help to get the temperature up within a hour. Possibly some fouling of the heating surfaces as well which won't help.

But an output of 22kw with a DT of 20°c which my understanding is is what the boiler is looking for is 15.8 lpm. Boiler pumps are burner linked in modern gas boilers?

 

[John.g]

Don't know what the EN standard is but assuming heating 250 litres from 20C to 65C with 22.3kw, 23.3LPM in 35 minutes gives, by calculation, a deltaT of 8C, rapid heat recovery systems don't care what the deltaT is they use the highest possible boiler temps with the greatest flow rates to achieve their object.

Can't attach files for some reason.

 

[John.g]

EN12897.....temperature difference between water (65°C) and ambient (20°C) of 45K

The spec above is for the "VIH GB 260/2/S"

Total capacity litres 120 155 180 210 260 310
Actual capacity litres 119,2 149,1 176,6 211,2 250,3 298,5
Hot water capacity litres 104,0 136,7 166,4 203,9 247,5 270,5
Maximum supply pressure to pressure reducing valve MPa (bar) 1,2 (12)
Rated pressure of cylinder MPa (bar) 0,7 (7)
Maximum operating pressure of heating coil MPa (bar) 0,35 (3,5)
Operating pressure MPa (bar) 0,35 (3,5)
Pressure reducing valve MPa (bar) 0,35 (3,5)
Expansion relief valve MPa (bar) 0,6 (6)
Temperature and pressure relief valve °C,
MPa (bar)
90,
0,7 (7)
Charge pressure of hot water expansion vessel MPa (bar) 0,4 (4)
Maximum temperature of heating circuit °C 85
Maximum temperature of potable hot water °C 85
Standing heat loss kW/24 h 1,26 1,53 1,66 1,89 2,07 2,26
Heat up time according to EN 12897 mins 18 23 24 28 35 42
Recovery time (70% capacity) mins 16 18 17 20 25 30
Primary heat exchanger performance kW 18,6 18,8 22,0 22,6 22,3 20,4

 

[SJB060685]

Don't know what the EN standard is but assuming heating 250 litres from 20C to 65C with 22.3kw, 23.3LPM in 35 minutes gives, by calculation, a deltaT of 8C, rapid heat recovery systems don't care what the deltaT is they use the highest possible boiler temps with the greatest flow rates to achieve their object.

Can't attach files for some reason.

How are you arriving at those figures?
I'm right with you right up until the DT

 

[EvilDrPorkChop]

Interesting, the older UniStore is 22.3kw coil and the newer model is 16.6kw. They must have changed who manufactured the cylinders.

 

[John.g]

How are you arriving at those figures?
I'm right with you right up until the DT

I think you spotted my deliberate mistake SJB.

KW = LPM*deltaT*60/860 so deltaT = Kw*860/LPM/60 = 22.3*860/23.3/60 = 13.7C.

 

[SJB060685]

That was the figure I had !

 

[John.g]

@milleniumaire, have you noted the flow/return temps with HW heating on?, @EvilDrPorkChop says 80C flow temp, also I think the cylinder temp is set in the controller and not on the boiler front, maybe the boiler front acts as a max limiter. There is also reference to pump overrun in the controller manual which would seem to state that pump overrun will take place for a limited period once the cylinder reaches temperature (with the zone valve opened) and the "charging" pump running, there seems to be a 10 min overrun time judging by the 10 min delay between HW off @ 0700 and CH on @ 0710, whatever about the "charging" pump run status.

 

[milleniumaire]

@milleniumaire, have you noted the flow/return temps with HW heating on?, @EvilDrPorkChop says 80C flow temp, also I think the cylinder temp is set in the controller and not on the boiler front, maybe the boiler front acts as a max limiter. There is also reference to pump overrun in the controller manual which would seem to state that pump overrun will take place for a limited period once the cylinder reaches temperature (with the zone valve opened) and the "charging" pump running, there seems to be a 10 min overrun time judging by the 10 min delay between HW off @ 0700 and CH on @ 0710, whatever about the "charging" pump run status.

Hi guys, I did some checking of temps this morning as requested. The system (heating and hot water) started at 06:30am and I checked it at 07:10am. It was still in DHW mode, trying to heat up the cylinder.

The analogue pipe thermostats showed:
Boiler flow to LLH: 68
Boiler return from LLH: 63
LLH flow to system: 68
LLH return from system: 60

On the VRC 700, I could see the following temperatures shown:
DHW current flow temp: 79
System flow temp: 76
DHW Target flow temp: 90
Domestic host water: 65
Current temp: 63
Cyl. temp top: 73
Cyl. temp bottom: ---

In the DHW menu, I also noticed the following settings:
Max. cyl. charg. time: 60 min.
DHW req. anti cy time: 60 min.
Ch. pump overrun time: 5 min
Parallel cyl. charging: off

It would appear the reason I'm seeing the heating start to come on an hour after it has started is due to the "Max. cyl. charg. time" setting of 60 mins? So, even after an hour the cylinder isn't reaching 65 deg C, but then switches to heating mode due to this setting. Not sure of the impact of the other DHW settings.

Last night, I happened to catch the boiler showing S.7 (pump overrun). The external pump was off, but it showed the internal pump was on and there was a DHW demand. My son had recently had a shower. While I watched it, the S.7 changed to S.0 (no demand). I can't say how long it had been at S.7, but I did notice the radiators were luke warm, so suspect the DHW demand had been running for some time.

Right now, 3 hours after starting, the system is in Heat mode, DHW target flow temp is showing 0 and DHW current temp is showing 65, so clearly it has eventually reached the required hot water temperature.

As suggested, I think I will turn the boiler temperature down to 60 deg C and see what impact that has in the morning. Just need to figure out if it should be turned down using the boiler dial or the VRC 700. I'll probably do both, just to be sure!

Yesterday, I emailed a bunch of questions off to Vaillant, including details of how the Grundfos pump was connected/configured and how long they would expect it to take for the cylinder to heat up, with examples of the current settings in the hope they might see something that needs to be changed.

 

[ShaunCorbs]

It’s not getting rid of the heat that return should be under 50 if the cylinder is calling

 

[milleniumaire]

It’s not getting rid of the heat that return should be under 50 if the cylinder is calling

So, the only way to loose the heat is within the cylinder coil? Does that suggest the water in the cylinder is being kept at a fairly high temperature, even when not being heated i.e. during the night, but then why does it take so long to heat up in the morning if it's already reasonably warm/hot? If I could drag myself out of bed before 06:30am I could check on the "Start" cylinder temperature, before the boiler turns on. Maybe I'll give that a try tomorrow morning.

The cylinder is only about a meter from the boiler, so not much exposed pipework between them.

 

[John.g]

The deltaT through the secondary circuit is 8C (68-60), unfortunately we don't know the pump power, if we did then we could calculate very accurately what the flowrate is and thence the cylinder input, if the boiler Hx pressure loss was available then could get a fairly accurate picture as well. However I can't see the pump head being greater than 2.5M so the flowrate is at least IMO 25 LPM, with a 8C deltaT gives a cylinder input of at least 14kw, the cylinder top is showing 73C, if this sensor is also used for temperature control then why didn't it shut the zone valve at 65C?, maybe worth checking the zone valve operation, the few that I saw failing did so with the valve "1/2" open and with the end switch still closed which would mean that the cylinder would remain heating until the 1 hour period had elapsed, this assumes that the end switch is used for boiler control with this particular controller.
You also said "I think I will turn the boiler temperature down to 60 deg C and see what impact that has in the morning", maybe you mean the cylinder temperature?, if you turn that down to say 50C 0n the controller then IMO the cylinder should reach that temperature very well within the hour limit and the boiler should then switch to CH.

 

[ShaunCorbs]

So, the only way to loose the heat is within the cylinder coil? Does that suggest the water in the cylinder is being kept at a fairly high temperature, even when not being heated i.e. during the night, but then why does it take so long to heat up in the morning if it's already reasonably warm/hot? If I could drag myself out of bed before 06:30am I could check on the "Start" cylinder temperature, before the boiler turns on. Maybe I'll give that a try tomorrow morning.

The cylinder is only about a meter from the boiler, so not much exposed pipework between them.

I would check the cylinder temp before you go to bed and if it’s at 60 would turn hot water off and see how it runs the next morning

It might be upto temp and the boiler / controller is waiting for the temp to drop and after an hour it moves on to heating

 

[milleniumaire]

Just had a response from Vaillant technical support regarding one of my emails and their response is that I need to wire the external pump upto the VR 71 wiring centre! The exact opposite of what they told my heating engineer. I'm taking this with a pinch of salt as at the time I emailed them I wasn't aware that the value of d.27 was 5 and they have assumed it is 2 (apparently this is the default). Also, they are telling me that a value of 2 means the external pump would NOT work during DHW mode and so if using the VR 40 to connect up the pump, I would need to connect a SECOND external pump to relay 2 and set d.28 to 3. The fact that I'm seeing the external pump work in both DHW AND heating mode doesn't match what they are telling me, possibly due to the d.27 value of 5 (I've now asked them about this).

So, currently, they are telling me I either have to wire the pump upto the VR 71 or get a second pump, which I believe is overkill for my system. I'm still waiting for answers to a number of questions I asked yesterday.

Based on feedback so far I think I need to investigate the DHW 2 port value to make sure it is functioning correctly. If this isn't fully opening then even if the pump is running I guess the flow wouldn't go anywhere, but then I would assume the cylinder water would never get hot, which it clearly is (eventually).

Reducing the cylinder temperature setting may help the situation, so I've now reduced it to 60 deg C and I will monitor the impact in the morning and also try to determine what the start temperature of the cylinder is before it starts being heated.

I'm still on the fence about the pump wiring/configuration, but wouldn't be surprised if there was potentially an issue, even though the pump always seems to come on when the gas burner is on i.e. when there is demand from one of the zones/DHW.

As far as pump overrun goes, I'm not really seeing that as much as I expected to, so although that is the subject line for my post, it may be a red herring! I still can't help thinking it would be better if the external pump was on during pump overrun, providing at least one of the zone valves was open. Maybe this doesn't happen as pump overrun tends to happen when the heating demand has been met and it is cooling down the water temperature in the boiler to protect the heat exchanger, so to continue to pump hot water around the system might increase room/cylinder temperatures beyond what was desired? Just a thought.

 

[John.g]

I think post #23 suggests that the zone valve is kept open during the pump overrun and because of the LLH should entail the secondary pump running also?.

Have you ever watched the system when there is a call for hot water normally at any time when the cylinder temperature falls below its SP minus its hysteresis to see if it just reheats the cylinder and then stops or changes back to CH?.

 

[milleniumaire]

I think post #23 suggests that the zone valve is kept open during the pump overrun and because of the LLH should entail the secondary pump running also?.

Have you ever watched the system when there is a call for hot water normally at any time when the cylinder temperature falls below its SP minus its hysteresis to see if it just reheats the cylinder and then stops or changes back to CH?.

I've just tried to replicate what you have asked.

I did all this on the VRC 700.

System Status was showing Standby, so no demand from any of the 3 heating zones or from the DWH.

Increased desired temp for radiator zone to 22, current temp was showing 21. System Status quickly changed to Heating mode and the zone valve was shown as open.

Increased DHW temp to from 60 to 65, cylinder temp was showing as 59. System Status quickly changed to DHW mode and cyl. charging pump changed from off to on. Checked the external pump and LED showing. Checked Honeywell zone valve and no resistance in level suggesting it was definitely open.

Decreased DHW temp from 65 to 59, cylinder temp still showing 59. system Status changed back to Heating mode and valve shown as open. Pump wasn't on and boiler showed S.0 (no demand), although radiator zone valve had no resistance, so definitely on. Checked the VRC 700 and it was showing desired temp was 22 and current temp was 21.5, so possibly the heating curve was causing it not to demand heat (this happens a lot).

So, from what I can see, everything I expected to happen did happen exactly as it should have happened. DHW took priority over heating and then switched back to heating.

I'm not exactly sure what you meant by "SP minus its hysteresis". Is "hysteresis" a typo? I assume there's some flexibility before the cylinder calls for heat and this is what you mean by "hysteresis"?

 

[John.g]

That's right, the cut in temperature is the SP (or cut out temperature ) minus the hysteresis, which might normally be 5C, but is settable in some controllers. A Gas boiler SP setting works some what differently because you don't want the boiler burner to cut out once it reaches its setpoint as it can generally modulate down to maintain this SP temperature, when the heating load is less than the boiler minimum output, the boiler temperature will start climbing and the burner will cut out at the SP+5C, it will then re fire when the temperature falls to SP-5C.

Did the boiler go to overrun after your test, above?.

 

[milleniumaire]

Did the boiler go to overrun after your test, above?.

No, I didn't notice it going into pump overrun, but I didn't leave it in those states for very long. I was actually surprised how responsive it was to me changing the various temperatures.

 

[milleniumaire]

Just to give an update after setting the hot water temp to 60 deg C yesterday.

I dragged myself out of bed this morning before 6:30am when the heating/DHW is due to start and checked the current cylinder temperature on the VRC 700. It showed 44 deg C, so since I last checked at 11:00pm last night, which is when the heating/DHW turns off, it has lost 16 deg C overnight.

By 07:05am I noticed the radiators were starting to warm up, suggesting the cylinder had been heated from 44 to 60 deg C in around 30 minutes. It seems strange that to increase the temp by another 5 deg C, to 65, it would take more than another 30 minutes (based on the hot water demand running for an hour each morning prior to this morning, before heating started).

 

[John.g]

I think you have two temp measurements, one on the cylinder top and one that shows the target temp? which I assume is where the reading for control is taken, yesterday you had the top reading 73C and the target 65C, what are the corresponding readings now.
Also if you heated 250 litres of water from 44 to 60C in 30 minutes then the cylinder input was 9.3 kw, less than half of its rating, did you notice the secondary flow/return temps, if they were similar to yesterday at 8c then it means that the circulation rate was ~ 17 LPM which sounds reasonable for that ups3 pump, if the flow temp was 75/80 deg then it points to a fouled "coil", I think you have "tank in tank" cylinder heating.
Do you live in a hard water area?.

If your boiler can only modulate to 12kw then it should have cycled also.

 

[milleniumaire]

Unfortunately, I didn't go to the boiler only the controller, so don't have any other details and now the system has been in heating mode since around 07:00am, so nothing will reflect the DHW period between 06:00 and 07:00.
I'll do the check again in the morning and try to get to the boiler before the heat mode starts.
We don't live in a hard water area. Limescale is not an issue for us.

 

[John.g]

You also seem to have a huge cylinder standing heat loss based on the overnight numbers, the cylinder lost ~ 2.5C/hr = 17.4kwh/24 hrs, spec states 2.07 kwh/24 hrs = 0.3C/hr, mine looses ~ 0.45C/hr, so something amiss there, maybe leaking/dripping HW tap except someone had a secret shower during the night.

 

[EvilDrPorkChop]

You also seem to have a huge cylinder standing heat loss based on the overnight numbers, the cylinder lost ~ 2.5C/hr = 17.4kwh/24 hrs, spec states 2.07 kwh/24 hrs = 0.3C/hr, mine looses ~ 0.45C/hr, so something amiss there, maybe leaking/dripping HW tap except someone had a secret shower during the night.

It isn’t that great, mine loses 7oc over night on a 170L that’s located in a well insulted garage and pipes lagged.
The reintroduction of cold water just by running a hot tap will lower the temp, and depending where the probe is located then this will pick up that change pretty quick.

 

[milleniumaire]

You also seem to have a huge cylinder standing heat loss based on the overnight numbers, the cylinder lost ~ 2.5C/hr = 17.4kwh/24 hrs, spec states 2.07 kwh/24 hrs = 0.3C/hr, mine looses ~ 0.45C/hr, so something amiss there, maybe leaking/dripping HW tap except someone had a secret shower during the night.

It isn’t that great, mine loses 7oc over night on a 170L that’s located in a well insulted garage and pipes lagged.
The reintroduction of cold water just by running a hot tap will lower the temp, and depending where the probe is located then this will pick up that change pretty quick.

It appears my 20 year old son (currently back from uni) did have a shower at 11:30pm last night, so that explains some of the heat loss from the cylinder. He's now aware that I'm monitoring the hot water so won't do it again tonight!

During our conversation he also mentioned he's had to turn up his shower dial to almost maximum setting to get hot water, which ties in with what we are also doing with our shower.

This got me thinking as surely a 60 deg C water temperature in the cylinder shouldn't have that big an impact on the heat delivered to the shower, and don't forget it has been set to 65 deg C until yesterday! The required increase in shower settings seems to have coincided with the CH upgrade, but I don't really understand what could have changed. As it's an unvented cylinder, I did wonder if the water pressure may have had an effect, but I don't see why that would have changed! There is a pressure reducing valve near the cylinder.

So, as well as the cylinder heating issue, the hot water appears to be not as hot as it used to be by the time it gets to the showers!

 

[EvilDrPorkChop]

It appears my 20 year old son (currently back from uni) did have a shower at 11:30pm last night, so that explains some of the heat loss from the cylinder. He's now aware that I'm monitoring the hot water so won't do it again tonight!

During our conversation he also mentioned he's had to turn up his shower dial to almost maximum setting to get hot water, which ties in with what we are also doing with our shower.

This got me thinking as surely a 60 deg C water temperature in the cylinder shouldn't have that big an impact on the heat delivered to the shower, and don't forget it has been set to 65 deg C until yesterday! The required increase in shower settings seems to have coincided with the CH upgrade, but I don't really understand what could have changed. As it's an unvented cylinder, I did wonder if the water pressure may have had an effect, but I don't see why that would have changed! There is a pressure reducing valve near the cylinder.

So, as well as the cylinder heating issue, the hot water appears to be not as hot as it used to be by the time it gets to the showers!

You’ve probably got a shower back feeding or mixer tap. Had the same issue before.

 

[John.g]

It could be be that you havn't a balanced hot and cold shower supply, someone here will explain what the problem may be. You should have no problem getting a hot shower with hot water at 60C and a showering temp of 40C, if mains water is presently say 7C then the shower should mix 6.2 litres of hot water with 3.8 litres of cold water to give 10 litres (or LPM) of 40 deg water which a thermostatic shower should mix perfectly. (or the same,manually).

 

[milleniumaire]

You’ve probably got a shower back feeding or mixer tap. Had the same issue before.

Both showers are Mira Excel rear fed, thermostatic valves and have been working brilliantly for many years.

It could be be that you havn't a balanced hot and cold shower supply, someone here will explain what the problem may be. You should have no problem getting a hot shower with hot water at 60C and a showering temp of 40C, if mains water is presently say 7C then the shower should mix 6.2 litres of hot water with 3.8 litres of cold water to give 10 litres (or LPM) of 40 deg water which a thermostatic shower should mix perfectly. (or the same,manually).

The cold water supply into the house is reduced to 3.5 bar, the same as the hot water feed and this hasn't changed. I'll Google balancing, but haven't had to do that before (except for the radiators 😀).

Don't get me wrong, it's possible to get nice hot showers in both showers, but the temperature setting knob has had to be turned up quite a bit.

 

[ShaunCorbs]

Easy way to test turn mains cold water off at the cylinder open a hot tap does it stop

yes no back feed
No there’s a back feed somewhere

 

[milleniumaire]

Easy way to test turn mains cold water off at the cylinder open a hot tap does it stop

yes no back feed
No there’s a back feed somewhere

A few weeks ago before Xmas I was fitting a new Mira Platinum digital shower in our refurbished bathroom and had to turn off the mains cold supply. I then ran the hot and cold taps until they stopped. It took longer for the hot taps to stop, but they did eventually. That seems to match the test and suggests there isn't a cold water back feed?

 

[John.g]

Not at the mains, shut the cold water only at the HW Cylinder, then open a HW tap is the suggestion, I think.

 

[EvilDrPorkChop]

There's three things it could be;

1 - Something back feeding - shower or mixer tap.
2 - Your incoming PRV on the cold main has failed and letting too much pressure through.
3 - Turning up your cylinder to 65oc has knackered the cartridges in on the two older Mira Excels (Had this before on other makes) Max input on showers is 65oc and it's probable your HW was nearer 70oc. The easiest way to check is whether your Digital shower is still outputting the same temp on the same setting.

I'd be going with option 3 tbh if everything else seems OK.

 

[John.g]

Thermostatic control should give very tight control, I have one on my HW cylinder set to 60C, and it controls to within a few degrees of this from a HW cylinder temp of 85C (summer solar) to 65C from oil fired boiler.