Pump overrun is killing my heating!

[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.