Failsafe behaviour

[skwerl]

Does anyone know if failsafe kicks in if the batteries have died on a room stat?
I have a Y87RF that has dead batteries and wondering if that is causing heating to cycle on/off under failsafe condition. I'm away from home so can't see anything other that what's on TCC. Nothing in the Evohome manual about fail safe parameters, except a link to a page that doesn't exist

[Richard]

All depends on the failsafe condition set, but yep loss of comms with any device would result in it coming on. I always turn failsafe off personally.

[skwerl]

The failsafe setting on the controller is for the device that tells the boiler to heat. That’s typically a BDR91. If you turn failsafe on then, if the BDR91 loses comms with the controller it will start cycling the boiler in a 20%on/80%off cycle.

The issue I’m trying to understand is what happens if a thermostat loses comms. I have several Y87 stats controlling UFH zones. I went on holiday and the batteries in one of the stats leaked, causing the power to go and comms to fail. This caused that zone to start switching on and off. Honeywell say that if a zone stat loses comms then the controller will cycle heat on and off to that zone in an attempt to keep it above 21degC. I assume it knows roughly what cycling to use from its TPI data. Honeywell also say that if the zone is set to off then this behaviour doesn’t apply. I turned all my zones off before I left yet the failsafe still kicked in. It also seems there is no way to enable/disable this failsafe behaviour.