Hi Kevin

I've recently installed Evohome with Opentherm control on a S Plan setup. I've spotted the opening and closing of the CH valve periodically when under low load, why would it being doing this? I thought the purpose of Opentherm was to keep radiators at a low temp, in my case I've temporarily latched open the CH valve as my radiators weren't getting warm during this low temp cycling and thus the room wasn't getting fully up to temp.

Quote Originally Posted by kevinsmart View Post
What boiler do you have? EvoHome does employ on/off control with OpenTherm under low load situations, but for me it is always at lower temperatures, typically under 40C.

Have you connected an OpenTherm gateway to monitor the flow temperature request? It could be that what you are observing is the boiler firing with its startup fan rpm at say 40% before modulating down, but the requested flow temperature is soon overshot by 5C resulting in the boiler turning off, starting its anti-cycle.

You may be able to tweak some of your boiler parameters to improve e.g. if you have Intergas, but fundamentally your boiler probably can’t modulate down low enough to satisfy the kW output needed, so you end up in this on/off situation with the boiler firing at higher output for short intervals.