Priority domestic hot water PDHW

[Southsealee]

Hi everyone,

I have a customer who needs a new system boiler, currently has a 22kw which I've roughly calculated as enough heat output for the heating (its a 6 bed victorian terrace house), but this doesn't take the 200l unvented cylinder demand into account. Currently the system is setup on S plan, with no priority domestic hot water. When I fit the new boiler, I would like to try and set it up with PDHW for obvious reasons, but I'm not sure if this is possible with the existing two x 2 port valves (both normally closed).

I know PDHW is a hot topic at the mo, but I've not delved into it as yet (mostly combis in my area). I've found a wiring diagram for W plan using a three port (2 position) valve, but can't find any wiring diagram for a two x 2 port valve arrangement. I've found a little info on X plan, but again, not much info out there and it looks like it's something to do with Intergas boilers. I will be fitting a Vaillant on this particular job.

If any of you guys can point me in the right direction that would be great.

Thanks, Lee.

 

[ShaunCorbs]

Have you looked at the Viessmann’s 4 pipe system?

if you must go with vaillant you will require there controls best speak with your local rep they should have a drawing and will help you to set it up

 

[SJB060685]

What a coincidence.

I've just joined a Facebook heating and hot water group. One of the recent comments on a post mentioned PDHW, although I'm still not sure what this as a system is. I have found no information online and only a little on X plan system.
Can someone explain to me how a PDHW system works and secondly what an x plan system is?
From what little I have found on x plan is it ramps the boiler up to full load drastically decreasing recovery times? The CH side is on a normally open 2 port valve

 

[ShaunCorbs]

What a coincidence.

I've just joined a Facebook heating and hot water group. One of the recent comments on a post mentioned PDHW, although I'm still not sure what this as a system is. I have found no information online and only a little on X plan system.
Can someone explain to me how a PDHW system works and secondly what an x plan system is?
From what little I have found on x plan is it ramps the boiler up to full load drastically decreasing recovery times? The CH side is on a normally open 2 port valve

priority hot water even if heatings calling it will do hot water first (full output of the boiler goes to hot water) so the recovery times are great has its plus and minus points tho

 

[SJB060685]

And am I right in thinking the x plan is this which ramps boiler up to match cylinder coil rating? Does the primary flow temperature ramp up as well to match the 80-85°c most cylinders state their reheat times at?

 

[ShaunCorbs]

And am I right in thinking the x plan is this which ramps boiler up to match cylinder coil rating? Does the primary flow temperature ramp up as well to match the 80-85°c most cylinders state their reheat times at?

yes ramps upto max load and then waits for the dry pocket sensor to say when to turn off / back onto heating

believe it’s intergas versions of a Viessmann 4 pipe

 

[SJB060685]

Thanks buddy. I know this is gas stuff at the moment but hopefully oil will get there eventually

 

[ShaunCorbs]

Thanks buddy. I know this is gas stuff at the moment but hopefully oil will get there eventually

it’s like anything need more demand for progress else manufacturers won’t look into it

 

[SJB060685]

Exactly. Domestic oil boilers have been fixed rate for a long time. With pressure jet technology coming along we are now starting to see modulating domestic boilers, although still a long way to go. When it does take off then the oil industry will be able to utilise opentherm, weather compensation, load compensation etc technology.
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Shaun above is a screenshot of the X plan from Intergas.
As you can see on the 24 volt side of PCB is feed to DHW timer and a NTC cylinder sensor.
Am i right in thinking these sensors can be set to a set temperature and must be set just below cylinder stat setting?
If that sensor was set to say 70°c and a cylinder stat to 60°c then that sensor will still require a load even though HW zone valve has closed and CH has opened again. This will obviously keep burner running at full load.

 

[ShaunCorbs]

No stat the sensor replaces the stat/cylinder stat

just used as an overheat stat eg 70

 

[SJB060685]

Ok so the cylinder stat in picture is just the high limit stat with manual reset?
Is the NTC sensor adjustable or factory set?

 

[ShaunCorbs]

I would say so no reason for it to be there with a pocket sensor other than overheat

would say software set

 

[SJB060685]

Yes I've actually found a similar wiring diagram from intergas, only the limit stat and NTC sensor are wired in series after the DHW timer. That makes sense more. From the wiring diagram above its a little misleading. When the sensor requires heat it brings on boiler and feed sent through limit stat, opening HW valve and closing CH. When sensor reads correct temperature it shuts down but if timer comes on 5 minutes later you have a feed back to PCB bypassing the sensor. This in turn would keep the boiler on max output for the duration of timer demand?

 

[John.g]

I have seen that some (if not all) quick recovery coils not only specify flow temperatures of ~ 85C but also very high flow rates to achieve this, I think I have seen flow rates of 25 to 35 LPM specified even for 200 litre cylinders which is often more than twice the normal flow rates but while most boilers now can be configured to raise the temperature SP on HW duty I have never seen where they increase the circ pump speed to increase the flow rate, maybe the PDHW does.

 

[Southsealee]

Have you looked at the Viessmann’s 4 pipe system?

if you must go with vaillant you will require there controls best speak with your local rep they should have a drawing and will help you to set it up

Thanks Shaun, will give Vaillant a buzz. I know their Exclusive Green IQ system boiler is 4 pipe. I didn’t appreciate that PDHW delivers higher flow temp to cylinder compared to heating, so makes sense that it will only work using a 4 pipe system boiler. Just need to convince the customer to fork out a bit more for PDHW rather than keeping the existing S plan arrangement and standard system boiler.

Thanks for the replies all.
Lee.

 

[ShaunCorbs]

Let us know how you get on

 

[fixitflav]

Thanks Shaun, will give Vaillant a buzz. I know their Exclusive Green IQ system boiler is 4 pipe. I didn’t appreciate that PDHW delivers higher flow temp to cylinder compared to heating, so makes sense that it will only work using a 4 pipe system boiler. Just need to convince the customer to fork out a bit more for PDHW rather than keeping the existing S plan arrangement and standard system boiler.

Thanks for the replies all.
Lee.

Hello Lee
I’ve come up with a wiring diagram giving HW preference. Assume spring return 2-port valves, boiler and pump powered from auxiliary contacts in the valves (the usual way). Sketch attached. I think it works but any comments welcome.

If using a programmer, it might not be convenient to bring the wire from the cylinder stat to the room stat through the programmer, depends on the house layout. But it’s not a big problem, if you don’t want heating, just turn the room stat down.

This is for a conventional boiler, heat-only or system. I can’t see how the make of boiler makes a difference. If the boiler has different flow temperatures for HW and CH I assume it has an input to tell it which is calling. If it takes 240 volt, that could be from one or both of the stats. If weather compensation is used there would have to be different temperatures as the flow temperature might be turned down below the cylinder stat setting.

 

[Scott_D]

On the ideal logic and vogue you can have weather comp on the heating and full temp if hot water is calling for heat.

 

[fixitflav]

On the ideal logic and vogue you can have weather comp on the heating and full temp if hot water is calling for heat.

I assume the boiler needs an input to tell it which is calling.

 

[Scott_D]

here is the wiring diagram for a logic

 

[fixitflav]

here is the wiring diagram for a logic
View attachment 43843

Could be wrong, but I don't think either of them gives PDHW. The Y-plan doesn't, and if you want
PDHW you would go for W-plan with either/or diverter valve. Cheaper and simpler.
With S-plan as shown there, if CH and HW are called, both valves open.
Interesting that there's now talk about PDHW, as that was the system way back, as in my first house in 1973, but then Y-plan and S-plan came out, presumably thought to be an improvement as no need to wait for HW to be satisfied before CH available.

 

[SJB060685]

As above, neither of them look like they give PDHW

 

[EvilDrPorkChop]

Have quite a bit of experience of PDHW on systems;

Firstly you can make any traditional system PDHW without any changing of valves with a NC DP contactor quite easily. Basically on a HW demand the DP contactor opens the circuit and kills the power to the Heating zone valve, closing that and reheating the HW. When the HW stat is statisfied the DP resets to NC and the heating fires back up - simple. Dependant on how many zones you've got will determine which DP contactor you require. This is obviously a solution for a traditional system with one set flow temperatures.

The advantage to this is that you can have CH without HW on the clock, where as most PDHW systems require you to have HW & CH on at the same time.

I have this on my own system and it works well - I have it on a switch so you can knock it on and off as required.

Secondly; Vaillants systems will use HW priority when using their controls & wiring centres. You don't just need the GreenIQ 4 pipe boiler to do it. Any other their heat & system boilers will do it when using the Wiring centre and Vaillants control. You can either use a diverter or 2 two port valves, and I like to use the Probe on the tank too (VR10 I think). You can set DHW temp then, along with DHW flow temp (For reheat) and maximum time spent reheating DHW.

I don't have as much experience with Viessman but from my limited experience their system is very similar to Vaillants.

Thirdly; external controls will offer the same solution. Honeywell Evohome are just in the process of adding it to the next firmware update which does HW priority software based. I'm sure there will be a couple of other controllers that will do the same.

HW priority IMO isn't right for every system, it some situations it can be work really well. In other's I don't think there's much advantage.

 

[John.g]

Is there any simple way that you can also increase the DHW flow temp on a weather compensated boiler which may be only running at 40C when the hot water needs a boost, say maybe by having a additional resistor (controlled by another relay) wired into the outside air temperature sensor to fool it to increase the boiler SP temp.

 

[SJB060685]

My understanding John is its preset in the PCB, so when you have a HW demand it ramps up boiler output and flow temp for fast recovery times. When the cylinder sensor is satisfied it will close HW zone valve and re open CH, which then reverts back to temperature curve set for weather compensation

 

[John.g]

OK thanks, so (does) that implies that all boilers with weather compensation also have PDHW.

 

[SJB060685]

As you know I'm an oil man, I'm not the one to ask about gas. What I know is what I've recently studied and PDHW is still fairly new. Whether its done with two 2 ports and the CH a normally open with cylinder thermistor, or with the now 4 pipe option from some gas manufacturers which has a heating flow and return and a flow and return for HW, each with their own pump i believe, or a way to separate the flow direction? Its not something I'm aware of in the oil industry as we are still technically a long way behind gas.

 

[EvilDrPorkChop]

OK thanks, so (does) that implies that all boilers with weather compensation also have PDHW.

In theory they should do, but some of the control systems don't implement this and always work. I know there's been issues with Opentherm and determining a CH and HW demand with some manufactures.

This was why when the new ERP energy laws with new controls only came into force on Combi systems - traditional systems where there is stored HW there was a concern where if the Flow temperature was limited due to a weather compensation system or Open Therm then there was a slim possibility cylinders may not be heated to the required temperatures to kill off legionella.

Is there any simple way that you can also increase the DHW flow temp on a weather compensated boiler which may be only running at 40C when the hot water needs a boost, say maybe by having a additional resistor (controlled by another relay) wired into the outside air temperature sensor to fool it to increase the boiler SP temp.

In short no. Some boilers have a terminal where you can wire a live in on a HW demand where the boiler goes into full load. Others are software based when used with their own controls.
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As you know I'm an oil man, I'm not the one to ask about gas. What I know is what I've recently studied and PDHW is still fairly new. Whether its done with two 2 ports and the CH a normally open with cylinder thermistor, or with the now 4 pipe option from some gas manufacturers which has a heating flow and return and a flow and return for HW, each with their own pump i believe, or a way to separate the flow direction? Its not something I'm aware of in the oil industry as we are still technically a long way behind gas.

The issue with the 4 pipe systems is that they aren't always practical to do, especially retro fitting in existing properties. On a standard system you're always only going to have a flow and return back to the airing cupboard/cylinder usually then CH pipes from there to rads.

 

[John.g]

You & others above mention where the boiler goes to full load, if the boiler is directed to full load and if its output is higher than the coil output, then what controls the boiler before it reaches its hi limit safety stat limit, does it cycle on/off until the cylinder temp is achieved.?.

 

[EvilDrPorkChop]

You & others above mention where the boiler goes to full load, if the boiler is directed to full load and if its output is higher than the coil output, then what controls the boiler before it reaches its hi limit safety stat limit, does it cycle on/off until the cylinder temp is achieved.?.

Most boiler controls systems with PDHW allow you to set the coil rating of the cylinder - well Vaillant’s do. If you’re using an external control the boiler will modulate accordingly as it would usually.

 

[John.g]

That's interesting as any coil output is not constant while the cylinder is heating up, there will be, IMO, a big difference between a cold and hot cylinder, no so much if the HW setpoint hysteresis is say 5 to 10C so should work reasonably well under these (re heat) conditions but one would think generally that the coil rating would need to be understated to enable the HW SP to be reached before the boiler reached its hi limit setting, but obviously it works.

 

[fixitflav]

You & others above mention where the boiler goes to full load, if the boiler is directed to full load and if its output is higher than the coil output, then what controls the boiler before it reaches its hi limit safety stat limit, does it cycle on/off until the cylinder temp is achieved.?.

Depending on the boiler, it either stops/starts on the control-stat, or modulates the gas flow down. And then stops if the minimum modulated flow gives more heat than the cylinder takes.

 

[John.g]

So full loading is basically just increasing the boiler temperature SP to its max?? and in the Vaillant's case it limits the max boiler output to the inputted coil rating?.

 

[SJB060685]

As Dr said above John, certain boilers have set options to set the boiler output to coil output, or a % of coil output as well as setting flow temperature. As you know most modern cylinders coil rating is based on a flow temperature of 80-85°c with a minimum flow rate and as you also know as the delta T changes so does the coil output.
The sensors used at the cylinder would be a thermistor whos resistance would change depending on the temperature, the resistance signal received at PCB would shut the burner down, or revert to CH if that was called for at the lower temperatures. At no point should the flow temperature exceed the preset value and come close to high limit stat setting, obviously system boilers high limit stat can be set to over 100°c.
As Dr also said if thermistors aren't used and a traditional cylinder stat is then the burner will modulate down when the delta T decreases.
Thats my understanding anyway and having said all that a diverter valve or mid position used as a diverter valve will give PDHW for older systems/boilers without this modern fancy stuff

 

[fixitflav]

Firstly you can make any traditional system PDHW without any changing of valves with a NC DP contactor quite easily.

Is the NC DP contactor in addition to the cylinder and room stats? I think the OP would need a wiring diagram.
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So full loading is basically just increasing the boiler temperature SP to its max?? and in the Vaillant's case it limits the max boiler output to the inputted coil rating?.

If yours is a reply to my #33, yes. The control-stat setpoint adjustable by the user. The boiler must have controls to stop or reduce gas flow when system demand is below boiler max output, whether in HW or CH mode.

 

[John.g]

The way I see PDHW operating is on HW demand (1) the HW zone valve opens and the CH zone valve closes and (2) that the boiler temperature SP is automatically increased to ~ 83/85C and the boiler modulates to maintain this 83/85C until the cylinder HW temperature is reached. (The Vaillant seems to introduce a refinement on this?) So what set point are you referring to here "The control-stat setpoint adjustable by the user."

 

[fixitflav]

a diverter valve or mid position used as a diverter valve will give PDHW

It might be possible to wire up a mid-position valve so it works either/or, but as the way it's intended to be used it allows flow to both CH and HW simultaneously. That's the whole point.

will give PDHW for older systems/boilers without this modern fancy stuff

If it's old enough it will be PDHW (eg Honeywell W-plan) because that was before Y and S-plans came in.

The OP has gone a bit quiet, but the way I read him he just wants to replace a boiler in a system with existing 2-port valves. Not sure why he wants to go PDHW, that and both-on have pros and cons. Unless weather compensation is used, when PDHW (or at least separate HW and CH cycles) is unavoidable. My sketch in #18 is a suggested way to do it.

 

[SJB060685]

It might be possible to wire up a mid-position valve so it works either/or, but as the way it's intended to be used it allows flow to both CH and HW simultaneously. That's the whole point.

I know a mid position is designed for flow through both ports, i was just pointing out that one could be wired in such a way to act as W plan and give priority to HW.

 

[fixitflav]

The way I see PDHW operating is on HW demand (1) the HW zone valve opens and the CH zone valve closes

That's right, but the circuit to achieve it has to be designed.

So what set point are you referring to here "The control-stat setpoint adjustable by the user."

I was assuming boiler has an adjustable control-stat, either old-fashioned type with a knob, or temperature set on a screen. When on weather compensation there's an additional control to stop or modulate the gas at some lower temperature, depending on outside temperature. Bypassed when HW called, to allow temperature to rise to
control-stat setpoint, via an input to the boiler to tell it it's HW mode. Obviously the control-stat would be set high enough to give realistic HW recovery time.
But no doubt there are variations from different manufacturers.

 

[EvilDrPorkChop]

The OP has gone a bit quiet, but the way I read him he just wants to replace a boiler in a system with existing 2-port valves. Not sure why he wants to go PDHW, that and both-on have pros and cons. Unless weather compensation is used, when PDHW (or at least separate HW and CH cycles) is unavoidable. My sketch in #18 is a suggested way to do it.

I think the main reasons being is that the boiler isn't large enough to reheat the HW and CH at the same time. For example - I have a customer with a 30kw Logic Plus system. The coil is rated around 15kw, the output of the rads must be in the region of 30kw nearly. So when the HW AND CH was calling the HW wouldn't reheat and the CH wouldn't get warm (in fact if the cylinder was remotely warm it would take the heat out the cylinder). So it would just try and reheat everything and basically not get anywhere. PDHW allows the cylinder to reheat efficiently then input the full 30kw into the heating system when completed.

Is the NC DP contactor in addition to the cylinder and room stats? I think the OP would need a wiring diagram.
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Yes the NC DP contactor is in addition. As i've said quite simple to add in, if OP would like a wiring diagram I will try and sketch something up.

 

[John.g]

Above contactor installation is indeed a very neat and simple way to shut off the CH when HW required.

 

[Southsealee]

Wow - thanks for all the feedback and advise, really impressed with the support and knowledge on this forum.

I've just read through all this and will read again, digest and check out the wiring diagrams. The customer has now placed an order, but only has a budget for a new boiler and sweet F.A upgrades - so wired up to the existing 2 channel prog and stat. So frustrating when you have a potentially interesting job and the customer doesn't buy into it. I'll probably alter the wiring anyway, just to try PDHW out for myself and get a better understanding. Good to know that if the customer upgrades to Vaillant controls at a later date, then it can work as true PDHW with a higher flow on cylinder demand.

Thanks again for all the tips and knowledge you have shared. Lee.

 

[SJB060685]

Yes the NC DP contactor is in addition. As i've said quite simple to add in, if OP would like a wiring diagram I will try and sketch something up.

Can you provide me a rough wiring diagram using an NC DP contactor please sir?

 

[fixitflav]

Can you provide me a rough wiring diagram using an NC DP contactor please sir?

I've sketched a circuit using a NC SP contactor (or relay as I'd usually call it) so not sure why EvilDr uses a NC DP.

But my circuit in #18 does it with neither. Cylinder stats are (or can be) double-throw, C, NO and NO. And I could add a manual switch to override the PDHW and bring the CH on.

 

[EvilDrPorkChop]

Can you provide me a rough wiring diagram using an NC DP contactor please sir?

Yeah. I’m working today so I’ll sketch something up later on.

I've sketched a circuit using a NC SP contactor (or relay as I'd usually call it) so not sure why EvilDr uses a NC DP.

But my circuit in #18 does it with neither. Cylinder stats are (or can be) double-throw, C, NO and NO. And I could add a manual switch to override the PDHW and bring the CH on.

There was a reason, can’t really remember. Probably just that DP is more readily available.
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I've sketched a circuit using a NC SP contactor (or relay as I'd usually call it) so not sure why EvilDr uses a NC DP.

But my circuit in #18 does it with neither. Cylinder stats are (or can be) double-throw, C, NO and NO. And I could add a manual switch to override the PDHW and bring the CH on.

Also you can’t do it just using a cylinder stat. As there is a reason because of it - the main one being you have to have the HW ‘on’ on the clock for it to work (So clock in gravity mode). Where as my method you can still use both independently of one and other.

 

[SJB060685]

Thanks Dr, much appreciated

 

[John.g]

Havn't got my crayons but looking at the S plan, if one used a NC SP relay, take a feed off terminal 8 on the wiring centre to switch the relay, connect the relay contacts between "CH" and terminal 4 on the wiring centre, if the HW zone and the cylinder stat are calling for HW then the relay opens and switches off all the CH zone valves, if either the HW zone is switched off or the cylinder stat is satisfied, the contacts reclose and restores power to the CH zones, a manual o/ride manual switch can also be added. ( I think this will work?)

 

[fixitflav]

Also you can’t do it just using a cylinder stat. As there is a reason because of it - the main one being you have to have the HW ‘on’ on the clock for it to work (So clock in gravity mode). Where as my method you can still use both independently of one and other.

I agree my scheme needs HW ‘on’ on the clock, but that's how system W has always worked from way back. My guess is that's what's usually meant by PDHW. The only addition could be the ability to have CH when HW is "off" on the clock. I doubt that the OP wants that (perhaps he'll come back sometime!), but it could be done with an extra switch on my circuit. But IMO there's very little point in having HW on clock times vs always on. The small amount of additional heat loss goes into the house anyway.

Look forward to seeing your sketch.
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Havn't got my crayons but looking at the S plan, if one used a NC SP relay, take a feed off terminal 8 on the wiring centre to switch the relay, connect the relay contacts between "CH" and terminal 4 on the wiring centre, if the HW zone and the cylinder stat are calling for HW then the relay opens and switches off all the CH zone valves, if either the HW zone is switched off or the cylinder stat is satisfied, the contacts reclose and restores power to the CH zones, a manual o/ride manual switch can also be added. ( I think this will work?)

Yes, I think that works, it's same as my sketch with NC SP relay (not the sketch I posted). Difference between that and my posted sketch is mine needs HW "on" on the clock, and the cylinder stat satisfied for CH to be available. But as I said earlier, that's how system W works, HW on continuously, CH via the clock.
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Thanks again for all the tips and knowledge you have shared. Lee.

Lee - apologies, I hadn't noticed this post when I commented on your disappearance in my #50!