I have a problem I have not met before. The system consists of a system boiler, hwc and rads. The two zones are controlled by a wireless programmable thermostat (Heatmiser) with an on/off channel for the HW. The pipework is an 'S' system. The boiler will fire when there is a demand for heat for either zone but shuts off if both zones are calling for heat. It doesn't matter which zone calls for heat first; shutoff happens with a dual call. The zone lights stay on for both, by the way so the calls are acknowledgeđ. Our work around is sequential but separated heating phases which is fine but no option for boost without shutting off the running phase. The wiring looks fine and my thought is a problem in the zone receiver.
Any thoughts?