Gas boiler relays exist to perform a simple function. When the thermostat is engaged, sending a signal to start heating the home, the gas boiler relay intercepts this signal and acts based on whether the boiler already contains hot water. If the water within the boiler is still steaming hot, the relay will engage the water pump to circulate the hot water, heating the house without having turned on the heating elements of the boiler. This is done to save energy, preventing the boiler from being turned on needlessly every time the thermostat is set to "heat."
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Water pump engages but boiler doesn't turn on
As already explained, this may simply be the relay trying to save energy. It becomes a problem, however, when the water in the boiler is no longer hot. If this is the case, then the boiler relay may be broken. To check this, find the gas boiler relay, a plastic box bolted to the side of the boiler's housing. There should be two lights on it. If they are not lit, despite the thermostat having been set to "heat," the relay is broken and needs to be replaced.
Boiler engages intermittently or not at all
Often, a boiler turning itself on and off is the result of a loose connection. Check the gas relay box again. If the lights turn on, then off rapidly, this is the case. Make sure the wires leading to the box are secure, and then follow them to their insertion point within the boiler. Should the wires be melted or broken, they will have to be replaced. Likewise, there may be corrosion on the terminals within the boiler. Unplug the boiler before cleaning the wires, and then make sure they make firm contact with the boiler's control terminals before starting the system up again.
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