Wiring up combi for s-plan

[Madrab]

Without having to trawl through every post @omph

> what combi boiler is it? This will determine if the system flow temps can be controlled separately which would be needed for any PDHW system
> If you are talking PDHW then you will have a cylinder, is it open vent or unvented?

There are boilers that utilise Open Therm (OT) and that allows for a 'standard' PDHW system setup - if there is such a thing - and others that use their own propriety controls, a bespoke system layout, an addition of hardware/wiring to their standard configuration or an adapter interface or similar.

The most straightforward system, I believe, as has been suggested is the 'X' plan with a NO and a NC 2 port setup or 'D' plan that utlises a 3 port diverter valve, which of these is normally decided by the current system configuration. It can also be easier to have a boiler that natively supports OT 'out of the box' and a well designed OT enabled programmable stat as the user's interface.

Quickest and Easiest explanation I found for PDHW systems is - https://heatingacademynorthampton.co.uk/category/pdhw/ it does get into the nitty gritty of the various ways manufacturers have sought to implement PDHW across a plethora of different systems but it still gets my goat why these manufacturers can't agree to do things the same way and have a standard - which was the whole point of the OT development but they now feel that they all have to be just different enough to make things a lot more complicated when it comes to a independent professional looking to maintain them.

 

[dilalio]

I've just watched his video again. He mentions that it is pin 5 which makes it all work.

What does pin 5 normally do on these boilers when it isn't being used for D-Plan? In the manual it never seems to get connected and doesn't seem to do anything! Although, maybe that's because the manual only shows the standard S-Plan and Y-Plan electrical diagrams with just a switched live coming into X2.

It seems like this boiler can operate in two different ways. The standard S-Plan way where the zone valves basically control the boiler firing. But also in reverse, where the boiler controls the zone valves.

Dan just sent this to me...

Pin 5 has always been for controlling external valves. But there's lots of ways you could do it.

Even controlling multiple temperature zones off an opentherm stat plus an on/off stat.

 

[MNW67]

Dan just sent this to me...

Pin 5 has always been for controlling external valves. But there's lots of ways you could do it.

Even controlling multiple temperature zones off an opentherm stat plus an on/off stat.

Great, thanks. Straight from the horse's mouth!

 

[MNW67]

> what combi boiler is it? This will determine if the system flow temps can be controlled separately which would be needed for any PDHW system

There are boilers that utilise Open Therm (OT) and that allows for a 'standard' PDHW system setup - if there is such a thing

Might it be possible then that any combi which has OT could do PDHW using the right controls?

 

[Madrab]

Might it be possible then that any combi which has OT could do PDHW using the right controls?

Most definitely - any boiler be it a combi, regular or system could do PDHW if it's OT enabled that's managed by an OT enabled control.

The answer was directed to the Post's title question but then I re-read @omph 's original post where he states it's not really hardware specific. It is quite relative though IMO as not all boilers are OT enabled or have other means to separately control flow temps, That would dictate what would be used, how the system could be designed and implemented with some manufacturers hardware being more complicated and overly expensive than others to implement PDHW.

 

[MNW67]

It basically uses a cylinder sensor to create a DHW demand which then causes the boiler to fire at max rate and move the 3 port diverter to move over to supply the cylinder with very hot water.
A cylinder with a large coil and therefore fast recovery will use this heat very quickly to replenish the DHW and therefore CH will only be starved for a short period and not noticeable through heat loss.

What happens if you plug the cylinder thermostat into X4 but don't have D/X-Plan set up? It looks like that would trigger the higher DHW flow temperature (parameter n). And if there was also a CH demand at the same time, the radiators would also receive that higher temperature whilst the DHW was on. That might suit some people's situation better than PDHW. We have a large old cylinder with quite a small coil. It might be better for us to have the CH stay on whilst the cylinder reheats, especially in very cold weather.

 

[Mister Banks]

What happens if you plug the cylinder thermostat into X4 but don't have D/X-Plan set up? It looks like that would trigger the higher DHW flow temperature (parameter n). And if there was also a CH demand at the same time, the radiators would also receive that higher temperature whilst the DHW was on. That might suit some people's situation better than PDHW. We have a large old cylinder with quite a small coil. It might be better for us to have the CH stay on whilst the cylinder reheats, especially in very cold weather.

You don’t want 70c water flowing around your CH even for short periods so IMO sticking with an S or Y plan with 55c flow temp for both HW and CH would be a better situation than what you are proposing.

 

[MNW67]

You don’t want 70c water flowing around your CH even for short periods so IMO sticking with an S or Y plan with 55c flow temp for both HW and CH would be a better situation than what you are proposing.

We currently have 70C water flowing around our CH all the time! It doesn't cause any sort of problem or discomfort.

 

[Mister Banks]

We currently have 70C water flowing around our CH all the time! It doesn't cause any sort of problem or discomfort.

That’s good but it is costing you more in energy which is the opposite to what most people want.

 

[MNW67]

That’s good but it is costing you more in energy which is the opposite to what most people want.

I see what you mean now! We currently have a very old boiler which needs to run hot because it has a cast iron heat exchanger. I'm looking to change it in the Spring. If my idea above works, then it sounds a good compromise to me. I hope that 40 minutes or an hour a day at 70C won't cost too much extra and it saves the hassle of getting a new high performance cylinder.

 

[dilalio]

I see what you mean now! We currently have a very old boiler which needs to run hot because it has a cast iron heat exchanger. I'm looking to change it in the Spring. If my idea above works, then it sounds a good compromise to me. I hope that 40 minutes or an hour a day at 70C won't cost too much extra and it saves the hassle of getting a new high performance cylinder.

https://www.johnguest.com/gb/en/resources/blog/plumbing-and-heating-regulations-2023#:~:text=Heating%20engineers%20best%20pay%20attention,below%20this%20temperature%20if%20possible.

 

[MNW67]

@dilalio

Oh, that's a shame I've really noticed this winter when the temperature drops if the CH goes off. Is there any wiggle room for a rogue RGI?