How to Fix an HVAC Thermostat

First Things First

• Ensure the thermostat is set correctly. The switch or slider control marked “Heat” and “Cool” must be aligned with the system function required. So if the environment is cold and the HVAC system is being asked to warm it up, the device must be switched over to “Heat.” Further, the integral control circuits will not allow the system to power up if the air mover cannot distribute the treated air; check the fan switch is set to “Auto.”

Power Supply

• If the entire system refuses to run, first ensure there is power to the thermostat. Some thermostats use a battery to operate; exchange the battery for a new one. Even if the digital display appears functional, there may not be enough power to operate switches.

  Thermostats seldom have integral fuses but rather are protected upstream by an external fuse or circuit breaker; these devices are intended to open if a fault in the system -- such as a dead short where a loose hot wire comes into contact with a grounded metal component -- creates a massive power surge. Try replacing the fuse or resetting the breaker. If the device immediately reopens, have a certified professional diagnose the problem.

Physical Considerations

• Even modern digital thermostats initialize the HVAC system in response to physical conditions and can be fooled if these are changed. Confirm that the thermostat is absolutely level, because the mercury trip switch inside the housing is extremely sensitive to changes in angle.

  Ensure the thermostat is not in a draft. This is particularly important when the system is set to cool; if the thermostat is in a draft that is cooler than the rest of the environment, it is fooled into thinking the entire space is as cool as the drafty area. In turn, it will not initialize.

Replacing a Failed Thermostat

• Disconnect the power supply at the fuse or circuit breaker that protects the entire HVAC system. Draw a plan that details the combinations of color-coded wires and terminal numbers inside the thermostat; consider taking a picture with a digital camera for reference. Remove the failed thermostat from the wall by retracting the fasteners that pass through the back of the case into the wall. Take the unit to a plumbers’ merchant or a home improvement warehouse and ask for a compatible replacement; consider purchasing a programmable unit. Although initially more expensive, these devices will save money in the long run. Install and program it according to the manufacturer’s instructions.