Priority domestic hot water PDHW

[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! 😡

 

[EvilDrPorkChop]

As requested attached is my sketch of the HW Priority using either a SP/DP Contactor. If you like you can add in a switch between the Cylinder Stat and the contactor so the HW Priority can be turned on and off as required. Using the contactor will allow your customer to still have independant control of the HW & CH; so the CH can be still be selected without HW. The only difference is that when both are on together that the HW will reheat first before the CH will heat.

SP or DP Contactors are fine. You will require DP (Or referred to as a 2 Pole contactor) if you have more than one heating Circuit - for instance upstairs and Downstairs.

I keep these in the van as they give you two options.- They have a NC and NO on the contactor so can be used for a variety of uses. - 20A 2 Pole Contactor 230V AC 1 Normally Open N/O and 1 Closed N/C Modular for DIN Rail Mounting 20 Amp: Amazon.co.uk: DIY & Tools - https://www.amazon.co.uk/gp/product/B07Q9QT9DH/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1

You will need a DIN railed enclosure for them too - Wylex ESE2 IP40 2-Module Unpopulated DIN Enclosure - https://www.NoLinkingToThis/p/wylex-ese2-ip40-2-module-unpopulated-din-enclosure/32311

@fixitflav Coming back to your solution - The two issues here are one; You have to have HW & CH 'ON' together. Secondly; On your diagram you have the Cylinder Stat '2' port coming back to the clock and the clock switching this live feed. The issue here is that on most Two Channel Programmers the Live to the CH 'ON' on the back plate is supplied by the clock and not by a link. So the only way to turn the CH off would be turning the Roomstat down.

 

[SJB060685]

Thanks Dr. Thats easy enough

 

[fixitflav]

Using the contactor will allow your customer to still have independant control of the HW & CH

Agreed, my scheme does not allow independent control of the HW & CH, it behaves as system W. If going for PDHW I assume that’s the main intention.

I never saw much advantage in Y or S-plan. I had W-plan in 70s/80s, with 3 young kids and an h&c fill washing machine so plenty of HW demand, and didn’t have any problem. And that was with a pretty basic HW cylinder. With a modern cylinder like your 15kW coil even less problem. I estimate my current cylinder, probably well scaled up, transfers about 3kW (30 mins recovery time) and it isn’t a problem, though I do live on my own now.

With PDHW it seems things have gone full circle. And necessary if weather compensation is used, as discussed.

If you like you can add in a switch between the Cylinder Stat and the contactor so the HW Priority can be turned on and off as required.

If I understand you right, putting the (NC) switch there, when opened closes the relay contacts allowing CH, but stops HW. I would put a NO switch in parallel with the relay contacts.

SP or DP Contactors are fine. You will require DP (Or referred to as a 2 Pole contactor) if you have more than one heating Circuit - for instance upstairs and Downstairs.

With my scheme you can add extra roomstats and zone valves (not limited to 2 total). Updated sketch attached.

Coming back to your solution - The two issues here are one; You have to have HW & CH 'ON' together.

Agreed, comments above.

Secondly; On your diagram you have the Cylinder Stat '2' port coming back to the clock and the clock switching this live feed. The issue here is that on most Two Channel Programmers the Live to the CH 'ON' on the back plate is supplied by the clock and not by a link. So the only way to turn the CH off would be turning the Roomstat down.

Agreed, it depends on the programmer. I have a programmer leaflet which shows both HW and CH with volt-free DT contacts, so OK for my scheme. If on a new installation need to use that type or similar (DT not required). If an existing installation and it’s not that type, it’s clearly easier and cheaper to do it your way than change the programmer.

Interesting discussion, we may have kicked this to death. Have a good day!

 

[EvilDrPorkChop]

If I understand you right, putting the (NC) switch there, when opened closes the relay contacts allowing CH, but stops HW. I would put a NO switch in parallel with the relay contacts.

No sorry, I think you misunderstood. I was meaning if you put a Switch between the Cylinder Stat and the Live to the Contactor this allows the HW priority feature to be turned On and Off as required.

Hope i've explained it a bit better this time!

 

[fixitflav]

No sorry, I think you misunderstood. I was meaning if you put a Switch between the Cylinder Stat and the Live to the Contactor this allows the HW priority feature to be turned On and Off as required.

Hope i've explained it a bit better this time!

OK thanks.
(Thought I'd sent this yesterday but getting a message saying it wants more than 20 characters.)

 

[John.g]

Is this another form of PDHW and what plan? W?

 

[fezster]

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?)

Replying to a slightly old post, but this is exactly what I did recently and it works fine. I used a 2NC contactor (as I have 2 CH zones) and a 3 module wylex enclosure, as the 2 module one linked to in this thread doesnt have the required depth to house the contactor.

When DHW fires, it shuts the CH zone valves.

All in all, cost under £20 and a bit of wiring.

 

[SJB060685]

@John.g so the relay contacts will be connecting HTG ON at programmer to terminal 4 in wiring centre? That would work and for a cheap price but the same can be achieved with a simple diverter valve, albeit at more expense.

 

[ShaunCorbs]

@John.g so the relay contacts will be connecting HTG ON at programmer to terminal 4 in wiring centre? That would work and for a cheap price but the same can be achieved with a simple diverter valve, albeit at more expense.

or a 28mm port valve with the no and nc contacts

 

[SJB060685]

or a 28mm port valve with the no and nc contacts

I would need to see the wiring of this mate, although I don't doubt you 👍

 

[David Heath]

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.

Is there any reason that a combination boiler cannot be used for this? instead of the combi being connected to the incoming cold mains and out to the hot water tap, they could be connected on a closed circuit to the flow and return of a cylinder coil with a circulation pump that runs through a relay, the flow switch feed being substituted with the cylinder thermistor. easiest way to set up the diverter is to use a 3 port valve, installed so htg does dhw and vice versa, so that it is only energised to pdhw and high burn/flow when there is a dhw demand? then is it is de-energised the boiler will go back to heating on the open way through the valve (originally dhw, but now htg) so that the boiler can do heating on open them or similar modulation.

 

[mitchellmoxo]

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.

Hi fixitflav thanks for this wiring diagram. For this to work does it mean that I need to find one of those programmers which have a common for each channel, rather than a single shared live like the standard drayton, hive etc programmers