Priority domestic hot water PDHW

[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