The Efficient Home Forum

In the event the heating zone valve fails, if I understand correctly. The controller will call for heat, activating the heating BDR91 which will respond. And Evohome will think all is well and the boiler has been activated. When in fact this is not the case, the boiler will be sitting idle. The pump will keep running and the controller will continue calling for heat.


I see that HR92 is replacing the heating zone valve, the HR92 now controls flow not the heating zone valve. That's the (only) installation difference between "S Plan two-port valves" and "Stored hot water and zoned heating". So yes I see how it's easy to just manually fix the heating zone valve open.

I just can't understand why it makes a difference, or wouldn't work properly.

Either the BDR91 tells to boiler to switch on/off, or it tells the zone valve to switch on/off (which in turn tells the boiler to switch on/off). So same result either way.

In one setup the heating zone valve opens every time hot water is called for, in the other setup there is no heating zone valve. Same end result either way.


One other thing I noticed. In both cases, any radiator that does not have a HR92 fitted will heat up every time hot water is called for. In my house only the bathroom doesn't have a HR92, but it does have a normal "dumb" TRV fitted.

Can you not just bind one BDR91 as Hot water valve, and the other as Heating valve instead of boiler control ? Hot water priority won't even work without two valves.

Ben565 wrote:In the event the heating zone valve fails, if I understand correctly. The controller will call for heat, activating the heating BDR91 which will respond. And Evohome will think all is well and the boiler has been activated. When in fact this is not the case, the boiler will be sitting idle. The pump will keep running and the controller will continue calling for heat.


I see that HR92 is replacing the heating zone valve, the HR92 now controls flow not the heating zone valve. That's the (only) installation difference between "S Plan two-port valves" and "Stored hot water and zoned heating". So yes I see how it's easy to just manually fix the heating zone valve open.

I just can't understand why it makes a difference, or wouldn't work properly.

Either the BDR91 tells to boiler to switch on/off, or it tells the zone valve to switch on/off (which in turn tells the boiler to switch on/off). So same result either way.

In one setup the heating zone valve opens every time hot water is called for, in the other setup there is no heating zone valve. Same end result either way.


One other thing I noticed. In both cases, any radiator that does not have a HR92 fitted will heat up every time hot water is called for. In my house only the bathroom doesn't have a HR92, but it does have a normal "dumb" TRV fitted.

It makes a difference because the zone valve is not necessary and its a failure point evohome doesn't know about. evohome was designed for HR91's/HR92's on all radiators and no heating zone valve when the heating system runs a high temperature strategy (oil, biomass and gas boilers without OpenTherm or Weather Comp). If it runs OpenTherm or WC with two flow temperatures you need a valve on the heating to create a priority demand for DHW but we do this differently verses what Resideo do (X Plan). I'm not here to teach people heating design with evohome though.

S Plan and Y Plan systems are bad heating strategies for gas condensing heat only and system boilers. Maybe have a read of this if you are interested - https://theintergasshop.co.uk/content/1 ... -be-banned