boilerthermostat
Our flat uses a communal CHP system, with a Gledhill Boilermate 2000 in our flat (i.e. we don't have direct access to a gas boiler).
We're looking to set up a Honeywell Evohome system consisting of a smart thermostat and radiator TRVs. We were wondering if our Boilermate can work with this, or if it's a non-starter.
There's an optional ATF500DHW Hot Water Kit available from Honeywell too. Would that work with the Boilermate too?
In theory this should be easy -- have the second thermostat connect the R and G wires when it calls for cooling, and the fan will run.
Here's one reason it might NOT work, and it's subtle/non-intuitive so I thought it was worth mentioning: Some thermostats short the Y (cooling) and G (fan) wires together whenever they're in cooling mode. They do this so that they can use one relay to turn on both fan and A/C during a call for cooling. (Relays are relatively expensive components.) This shorting happens whenever the thermostat is in "cool" mode, whether it's actually calling for cooling or not. (Although if you set such a thermostat's Fan switch to "ON" instead of "AUTO", the short is removed so as to run the fan only.)
Where there's only one thermostat in the system, this shorting behavior does no harm. But if a second thermostat is added and it calls for "fan", the power will go through the short in the first thermostat and turn on the A/C as well. I have this exact arrangement in the house I just bought, and I cannot run the fan independently from either thermostat -- it will always run the A/C as well. If I unwire either thermostat (or set it to "heat" mode), I can then run the fan without the A/C.
So you might try shorting R (or Rc) and G with your existing setup to make sure that the A/C does not come on as well.
Source: personal observation confirmed by isolated testing on Robertshaw 9600 programmable thermostat
I think you're out of luck, since the G wire passes through the ventilation control system. You have to leave the G wire as a G wire, or remove/bypass the ventilation system. The easiest solution, would be to run a new 5 wire cable. Or don't use a fancy thermostat.
If you follow the G wire through the system, you can get a better picture of what's going on.
When the thermostat energizes the G terminal, the Gt terminal on the ventilation control receives the signal. The ventilation control does whatever it does, and eventually energizes the Gf terminal. When the Gf terminal is energized on the ventilation control, the furnace receives the signal and starts the blower.
From what I can tell based on the images, this is what the whole system looks like.
On a side note. The installer that left that rats nest of wiring in the furnace, should be ashamed of himself.
Best Answer
The thermostat and the radiator units worked fine. The hot water kit was not used.