If the bellows of a thermostat are fully expanded, what should be checked before energizing the equipment with a new thermostat?

When the bellows of a thermostat are fully expanded, it indicates that the thermostat is calling for heat or cooling, depending on the system. Before energizing equipment with a new thermostat, it's crucial to ensure that all components in the control circuit are functioning correctly.

Checking for a stuck contactor or gas valve is particularly important because if these components are unable to respond properly, they could lead to equipment malfunction or unsafe conditions. A stuck contactor, for instance, may remain closed, continuously powering equipment even when not needed, whereas a gas valve that does not open could prevent the system from functioning altogether. Ensuring these components are operational confirms that when the thermostat signals a demand, the system can respond appropriately.

Other possibilities, such as a faulty fuse, a broken sensor, or a loose connection, are all valid points to consider during maintenance, but ensuring a stuck contactor or gas valve is addressed first is essential to effectively and safely manage the system as it engages. If the thermostat activates an equipment malfunction due to a stuck component, it can lead to further complications that could have been avoided with prior checks.