Your furnace relies on feedback from a few different sources to control both its blower and its burner. The thermostats scattered around your home are the most visible control sources, but the limit switch and sensor are also crucial components. Limit switch problems can often appear similar to blower or thermostat problems, making them frustrating to recognize.

While many homeowners may not be familiar with this component, it is not a difficult one to understand. The limit switch measures air temperature near the heat exchanger for two purposes: to determine when the blower should run and to protect the heat exchanger from overheating. Both functions are critical to the proper operation and safety of your heating system.

Understanding How the Limit Switch Operates

Under normal circumstances, your furnace's burner turns on when the temperature measured at a thermostat falls below the setpoint. The limit switch then waits for the air near the heat exchanger to reach a proper temperature before engaging the blower motor. This method of operation allows time for the heat exchanger to warm up before pulling hot air away.

In addition to determining the proper time to engage the blower, the limit switch also shuts the burner off if the heat exchanger becomes too warm. An overheating heat exchanger can crack, leading to a costly repair at best and the release of dangerous combustion gases at worst. As long as the limit switch is operating correctly, the temperature at the exchanger will remain low enough to prevent damage.

Recognizing the Signs of Failure

A faulty limit switch has two modes of failure: open or closed. When the limit switch fails in the closed position, it allows the blower motor to run continuously. During the winter, this means that you may find your vents alternating between warm and cold air. The air will stay hot while the burning is running, but the blower will not shut off when combustion in your furnace stops.

On the other hand, a limit switch stuck in the open position won't allow your furnace to turn on at all. When the limit control is in this state, your furnace will believe that it is overheating and keep the burner shut down. Some furnaces may lock the limit switch into an open state if it triggers enough times to prevent possible overheating damage to the heat exchanger.

Fixing a Bad Limit Switch

Your limit switch is a replaceable part, so a technician should usually be able to repair a furnace experiencing this type of failure. Some furnaces may have replaceable temperature sensors at the heat exchanger, allowing you to replace only part of the limit mechanism. Always rely on an HVAC repair professional to perform this job since working near the heat exchanger can be challenging.