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.
[automerge]1589927400[/automerge]
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.