Hello I m running a hot water boiler with two zones in a multi-unit residential building. Zone one is in the original part of the building and zone two is in the newer addition. Each zone is managed by a separate thermostat connected to a separate zone circulator through a its relay. Once either thermostat asks for heat the the zone relay triggers and the zone circulator is activated followed by the boiler. Both thermostats can trigger concurrently with each circulator deactivating after its zone demands are met and boiler turning off after the last zone demand or its high limit is met. This is how it works most of the time. On occasion one thermostat will remain triggered well beyond it temperature setting causing its zone to heat up excessively. I have tested the voltage at the thermostat and the relay and determined that thermostat is indeed calling for heat and its not just the case of a sticky relay. I have replaced the thermostat and the problem persists intermittently. What to do?
I should also mention that:
reducing the temp on the thermostat during the problem turns the circulator and boiler off
both thermostats share a transformer
zone one is larger using a larger pump and cast iron rads
zone two is smaller using a smaller pump and fin and tube baseboard
there seems to be little separation in the piping between the zone (manual zone valves), not sure it this typical in a circulator zone management setup
when one zone operates zone two heats up slightly
I am considering getting a circulator controller like the Azel SP-82 to isolate the two zones completely if this is cause of the problem, a second transformer might be a cheaper alternative.
Not sure how to resolve this problem, I can't have my boiler and circulator running endlessly. Any help is much appreciated.