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zone valve buzz or hum.

i have a large commercial steam boiler. it has a outside temperature switch when the temp drops the boiler comes on. it has two zones controlled by thermostats. the thermostats don't control the boiler, the boiler continues to come on and off with the outside air temp. i added a 24 volt relay in line with the thermostat wire. the 120 section of the relay i removed the outside temperature switch and connected it thru the relay. but now one of the 24 volt zone valves hums, why the hum? can it damage the valve?