Priority domestic hot water PDHW

[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!