Priority domestic hot water PDHW

[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.