Honeywell CMT927 workings

[BobM]

Anyone any idea how to wire the CMT927/R6660D to create multiple heating zones?

[BobM]

Anyone any idea how to wire the CMT927 and R6660D to create multiple heating zones?

[John Stumbles]

Yes, use one for each zone. What aspect of the wiring are you having difficulty with?

[BobM]

Does it use a unique channel for each zone? How do you wire two or more to work together and synchronise effectively, especially with respect to pump overrun?

[John Stumbles]

You'd wire the boiler units (or whatever they're called: the receivers) in a standard S plan circuit with a 2-port valve for each zone. Pump overrun would be controlled by the boiler, with a pressure-operated bypass valve allowing circulation when all valves are closed.

Does this help?

[Roger Mills]

Yes.

Wire it up more or less as an S-Plan Plus system, as shown at

The CM927s replace the mechanical room stats, so that each controls one zone valve. The zone valves, in turn, control the boiler and pump via the secondary contacts which close when the valve is open. If the boiler requires pump over-run, it will provide the logic for controlling the pump - which must be connected to the boiler's 'pump' terminals rather than just being in parallel with the boiler as shown in the S-Plan diagram. Not directly related to the programmable stats - but you will also need a (perferably automatic) by-pass valve to provide a flow path during the pump over-run phase when all zone valves are closed.

[BobM]

Thanks Roger. This is the best info I have had on wiring up the CMT927 yet. Regards, Bob

[BobM]

Hi Roger, I tried emailing you directly, but I''m not yet up to speed with Google news groups so it probably disappeared into hyperspace. The S Plan and other variations are fine and about what I would have expected without the pump overrun and synchronisation issues relating to the CMT927 and it's relay box(es). I would guess that it makes best sense if the relay box was wired directly to the zone valve for it's particular zone. I would also guess that the 'undocumented' terminals on the relay box are used to switch on the zone valve and remain open for the required overrun time

- but I can't get the wiring diagram for multi-zone operation to prove it. Also, I need some information about how to bind individual CMT927's to their relay box and how to then synchronise the CMT927's. Can you (or anyone else) help please? Bob

[Roger Mills]

As stated in my signature, I rarely look for emails sent to my Googlemail address, and hadn't seen your email - but now you've prompted me, it *is* there.

I've no idea what the 'undocumented' terminals are for. But I can't see how what you're suggesting could possibly work. In an S-Plan system, whenever a zone valve is open it's secondary contacts tell the boiler to fire. The whole point about pump over-run is that the pump needs to keep running after the boiler demand has been removed. So if you keep the valve open, the boiler will continue to fire!

I've personally never used a wireless stat, but you can find the installation instructions on the Honeywell site. My understanding is that the relay box and wireless bit are set up to work together before they leave the factory, but that there *is* a procedure for re-registering them if, for example, you need to replace just one component. With a bit of luck, if you buy two, they'll be factory set to different channels but if not, you'll have to change one of them manually.

Your private email seems to be suggesting that you have a combi boiler. My earlier reply about how pump over-run normally works was based on the assumption that it was a convenional boiler. I don't know how combis normally do it - maybe someone else does?

[John Stumbles]

maybe that wasn't such a good idea:

this (and any other) news group is not a part of the Google empire. They just provide a web based way to read and respond to posts, and an archive (which they took over from Deja News)

As suggested by another poster in thsi thread

I don't have one to hand but if there are any undocumented terminals I'd leve them alone

Are there not instructions for this in Honeywell's manual that came with the unit? Again I don't have one to habd but they certainly used to give binding directions for the earlier CM67RF units.

You don't need to synchronise them if you're using one for each zone. Synchronising is for where you have an extra relay box driving the boiler directly (doing the wired-or function AFAICT)

[BobM]

Thanks for your replies, they are appreciated. With a combi boiler, things are a bit more complicated. In particular, you don't have access to the pump wiring. It is essential for a combi to keep the current zone valve open for the overrun period to allow the boiler to expel very hot water and avoid damage. This means the time the zone valve needs to be open is different (longer) than the period of demand for heat from a particular zone. Also, demand may overlap in two or more zones. Keeping a zone open to deal with pump overrun becomes counter-productive if there is already demand from another zone and that zone valve is open. Bob.

[Roger Mills]

So what's wrong with having an automatic by-pass valve which only opens when there is sufficient pressure - as will happen when the zone valves are shut but the pump is still running - thus providing a flow path to allow the boiler to dump its residual heat? This is standard practice with S-Plan systems using conventional boilers. Why should combis be any different in this respect. Since you cant get at the pump wiring, I assume that the boiler's internal logic decides when to run the pump, so controlling the over-run isn't a problem. You've simply got to provide a flow path. You can't open a zone valve because that would keep the boiler running.

How far are the zone valves from the boiler (in pipe-run terms)? As long as they're at least a few feet away, and as long as you put the by-pass valve immediately before the zone valves (and connect its output into the return line) you should have sufficient water in the by-pass loop to do the business.

[Tony Bryer]

It depends on the combi: mine, a Glow-worm 24Cxi, has an automatic bypass built in.

[John Stumbles]

Some Poxi-Battertons also have this with boiler-shutdown so that you then don't need interlock: you can fit all TRVs and no roomstat and the boiler will sort it out. Presumably for the sort of cheapskates who'd fit Poxis anyway :-)