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Relay problem or something else?
One of the three zones in my gas-fired hydronic heating system, which is roughly 8 years old, has begun to work erratically. It will heat properly and cycle when set manually to hold a temperature, but in program mode it now pretty invariably fails to start. When this happens, the 24V mechanical relay in the system becomes very hot.
Components: Taco SR503 3 zone switching relay, Totaline 24V relay, Honeywell RTH6400 series programmable thermostat, First 36 MBXR 3HW heat exchanger/air handler.
Diagnostics already done:
1) replaced Totaline 24V relay with identical component (P2830340)
2) replaced previous programmable Honeywell thermostat with current model (RTH6400D). Also, air handler fan starts immediately when set to run continuously at thermostat.
3) exchanged electronic Taco relay modules between working (zone 1, hot water heater or zone 2, second floor heating) and non-working zones (zone 3, 1st floor heating). There are no obviously fried components in the relay box, though I haven't removed the circuit board to look its backside.
Any suggestions where to go next?