Can an Evohome system be introduced in stages?

[haggis999]

Quick search for an installation manual gave me the following link: http://www.free-instruction-manuals....f/p4734564.pdf

Seems like your setup is best described by #4e. Installing the BDR91 is practically the same as described earlier. In your drawing:
N = N
L = L/7
A = L/7
B = 5

Are you suggesting that I could do the following as a basic first step, or have I misunderstood the BDR91 wiring?

• Remove the CM907 and its wiring to my junction box
• Install a mains-connected BDR91 beside the junction box and wire its switched output to terminal 5 in that box
• Control the central heating via an Evohome controller, which will be plugged into a wall socket in a different location to the CM907
• Leave the hot water control as it is for the time being

[gordonb3]

Correct. Might have been somewhat clearer: left column are the connectors inside the BDR91, right column the connectors from your junction box.

I suggest you leave the wiring in place though, just disconnect them. You can use these later to power an ATF600 wall mount.

[haggis999]

Correct. Might have been somewhat clearer: left column are the connectors inside the BDR91, right column the connectors from your junction box.

I suggest you leave the wiring in place though, just disconnect them. You can use these later to power an ATF600 wall mount.

It's possible I might choose to place an ATF600 wall mount where the CM907 is currently located, so I will leave the cable in place as you suggest.

Does the Evohome controller contain a thermostat? If not, then simply replacing the CM907 with the controller wouldn't be enough and I'd need to install at least one HR92.

[gordonb3]

The controller does contain a thermostat, as far as I know it won't serve as a zone actuator though. The concept is a bit fuzzy, but the controller is more like a smart remote to the HR91/92 units that actually control the BDR91 boiler module. So yes you will need to have at least one HR92 valve module (or UFH module/second BDR) to create a zone. It will work just fine if you leave that unit sitting on/inside a cabinet though - it doesn't need to be actually mounted to a radiator. After three years that is still how I operate one of my zones as I am unable to install the unit directly to the heating device and need to insert an additional valve in the feeding line.

[haggis999]

How certain are you that the Evohome controller contains a thermostat? None of the online sources I've seen so far mention this. I'm also struggling to understand what role a thermostat in a portable controller would play in a fully installed Evohome system.

In the very limited scenario we have been discussing, you say that I must have at least one HR92 (or similar device). Is it not more likely that the thermostat in that HR92 would be what controls when the boiler should be switched on?

[therealfronty]

Evohome controller definitely contains a temperature sensor, I have mine wall mounted in the living room but as it's near a big plasma TV that gives off lots of heat it was inaccurately reading the room temperature, so I added a Y87RF2033 in a different part of the room and it maintains the temperature perfectly now. I have two radiators in that room with HR92's but it uses the remote sensor to read the temperature. Works a treat. I also installed mine in stages, just started off with a few HR92's then added the heating BDR, left the hot water on the old programmer, finally added the hot water kit and second BDR91, left the old programmer in place but just set it all to "constant". I have Y-plan too, and wired the BDRs across the roomstat and cylinder stat contacts in my wiring centre.

[haggis999]

That's interesting. The fact that you started off in your living room by using the thermometer in the controller and then moved to using the Y87RF2033 single zone thermostat suggests that the controller lets you choose which thermometer to use for any given Evohome zone. Did you ever consider just leaving the two HR92s to do their own thing, which I have been assuming is the default installation scenario?

EDIT: When I said "leaving the two HR92s to do their own thing" what I meant was that they would each try to make their respective radiators run at the living room zone temperature specified in the controller, but with the controller's own stat playing no part in the process.

[therealfronty]

For the zone that the controller is in you can choose whether to use the built in sensor or an HR92, or a separate sensor. You can have remote sensors in every room if you want, I've got another one in my conservatory. If you have multiple HR92's in a zone then only one is used for temperature sensing, I think it's the first one you bind, any others in the same zone are ignored.

[therealfronty]

So placement and/or binding order of the HR92's can have quite a big impact, in my living room the controller is near a telly so can't use it's built-in sensor, on one radiator the HR92 is next to a door way so that's no good, and on the other it's tucked away behind a sofa, so neither are in the best place to measure an "average" room temperature, so I have a remote sensor on top of the sideboard and in the conservatory I have another one on the wall (again both radiators are behind sofas).

[haggis999]

Thanks for that useful clarification. This thread has proved to be very helpful and I suspect I will be making my first purchase of Evohome kit very soon.