Central Heating Control Question

I've just had to replace the room thermostat on MIL's CH system, and have installed a Honeywell CM927 programmable wireless stat.

The old stat had live, switched live, neutral and earth connections, so I just linked the live to one side of the switch in the Relay Box, with the switched live on the otherside of the switch. All works ok except........

The live to the stat is actually switched by the old mechanical timer, so the power to the relay box gets disconnected when the timer goes off. The wireless stat seems to resync ok, but after a delay, and it might lead to some unreliability later on.

As a temporary work around I've left the mechanical timer permanently on, and relying on the HW cylinder stat to switch off the hot water when it gets up to around 45C.

It's going to be quite a job getting a permanent live to the relay box, even if I move it nearer to the airing cupboard where all the rest of the gubbins is, and the distance (plus walls) from it to the new room stat might affect the wireless ability.

An option I'd considered is to remove the live feed to the relay box from the mechanical timer and connect it to the switched live from the FCU for the system, thereby bypassing the timer. This would mean that the hot water only is controlled by the timer, and the CH is only controlled by the new stat (which of course has its own timer anyway). I'd put a note somewhere prominent to warn of this.

Anyone see any problems with this?

Reply to
Davidm
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I have the same stat, connected the same way you have at the moment at mu mum's house, and it isn't a problem at all - been like it for several years.

I have set the timer to be on 24/7 for the heating (so its controlled by the CM927), and timed for the DHW.

Reply to
Toby

Connect but not operated the same way.

So the receiver has permenant mains. The OP has an old mechanical timer it may not be possible to have the CH on "constant" and the HW on "timed".

It should be OK to do what the OP suggests, connect the timed CH wire dierct to the mains but it does depend a bit on how the rest of the system is wired. It's quite possible that the timeswitch feeds both the pump and room stat, the pump rund when ever the time switch says CH on, the room stat then controls the boiler. Making the "CH timed" wire permenantly live would mean the pump runs 24/7...

Reply to
Dave Liquorice

It would be normal practice to take the live feed for all stats and timers from the FCU. Then you have a single point at which you can isolate the system.

Reply to
dennis

You are correct, but I AIUI that is what the OP is intending to do.

Reply to
ARW

It sounds ok in principle, but doesn't the timer have separate channels for HW and CH which would enable you to have CH on constant and HW on timed? [The timer must have separate outputs for HW and CH - otherwise you wouldn't be able to bypass it for CH but not HW]

What is the rest of the control system - I assume that it's fully pumped rather than pumped CH and gravity HW? Is it a Y-Plan system with a

3-port mid-position valve or an S-Plan with two 2-port valves? What does the wireless thermostat actually switch?
Reply to
Roger Mills

Yes, I was confirming that its normal.

Reply to
dennis

As this is the MIL's place and has a mechanical timer I'd go for gravity HW, pumped CH and no indenpendant time control for HW CH. CH probably has off, on , timed. The HW will get heated whenever the CH demands heat, something similar to:

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Reply to
Dave Liquorice

All the ones I have ever come across so far the CH output controls just the pump and the HW output controls just the boiler. On a gravity feed the boiler will satisfy hot water demand whenever it is needed.

Old ones tend to be Off, Once daily, Twice Daily, On. (or dial settable by the quarter hour with pins)

To make mine control the CH so that either CH or Woodburner or Manual could run the CH pump I had to add one mains relay to make the CH live output available to a pair of NO contacts for pump and boiler.

This highlighted a switched live vs permanent live issue with the original boiler wiring leading to inelegant thermostat shutdowns.

Seems odd now that the devices all contain a micro that they don't offer a facility to gate the requests together inside the controllers.

(and have driven outputs called "boiler" and "pump")

That would do fine for most simple domestic setups.

Reply to
Martin Brown

Except that the OP mentions a cylinder stat, which you wouldn't normally have in a gravity HW system (Unless it's a C-Plan).

Reply to
Roger Mills

It's a Y-Plan, with 3 port mid position valve. The wiring is a bit of a challenge as all the connections have been crammed into a single width surface box (what you'd normally use for a 13a socket) with choc blocks, rather than use a proper wiring centre box.

The mechanical timer has on/of switches for CH and HW, and then a single switch of timed/continuous, so the option of HW timed + CH continuous isn't provided for.

I'll have to check how the pump is fed, I'd don't know if it comes via the boiler (Netaheat Electronic) on an over-run circuit.

Reply to
Davidm

That tells me all I need to know - don't worry about exactly how the pump is controlled - it will either be in parallel with the boiler, or controlled by it if it needs to over-run.

What you are proposing will be fine. Disconnect the programmer's CH ON wire and feed it (the wire) from a permanent live. Then use the programmer just for the HW. As long as the permanent live comes from the same FCU as the rest of the system, you will still be able isolate the lot when necessary. The only thing to be aware of when using the CM927 as the only timer for the CH, you will no longer have an absolute OFF because - even in the Off position - it will still act as a frost stat. That's not normally a problem.

Reply to
Roger Mills

Excellent, thanks Roger. Now I just need to get the test meter out and locate the correct wires in the "rats nest" connection box!

Reply to
Davidm

Are you sure it is a Y plan and not a W plan? They LOOK very similar but the W plan must always heat the HW. Double check the number on the "3 port valve" it may well turn out to be a 3 way diverter port.

Reply to
ARW

Good point. And the wiring is different, so the OP needs to know which it is when trying to decipher the rat's nest wiring.

The proposed solution - of using the original timer to time the HW and the new programmable stat to time the CH - would sort of work, except that you wouldn't get any CH until the HW demand was satisfied (even if HW is OFF at the programmer!) The only way to have CH without HW is to turn the cylinder stat right down so that it is satisfied even when the tank is cold.

Reply to
Roger Mills

We never did come up with a solution using the HW off connection on the programmer to overcome this. What would happen if the HW OFF was connected to the brown on the divertor valve - I know it would not work:-)

Feel free to email me if you want to chat off group with suggestions and you are interested - I no longer have your email address

Cheers

Reply to
ARW

The email address in my SIG works, even though it's not my regular one.

I've just had a look at a W-Plan diagram, and I reckon that connecting HW-OFF to the diverter valve *would* work. That would force the valve to the CH position and the room stat would control the boiler and pump. The state of the tank stat would be immaterial.

Reply to
Roger Mills

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