Reasons Your Furnace High Limit Switch Is Tripping Excessively

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A furnace is an indispensable appliance in most households during winter. This system comprises safety features to help keep your family safe. For example, a high-limit switch controls the heater's operation within a particular temperature range. If the heater exceeds this range, the limit switch will trip, powering the system down. This may create an uncomfortable atmosphere because the unit disengages before it has attained the thermostat setting. Therefore, it is important that you hire a heating professional to assess and repair your unit for peak functionality. Below are four reasons your high-limit switch is tripping excessively:

Faulty Limit Switch

Over time, your limit switch may deteriorate, causing it to malfunction. As a result, the switch fails to regulate your heater's temperature. Consequently, the defective switch will stick in the open circuit position, restricting the heater's operation. A faulty switch may also keep the fan running after the burners stop. This may lead to the unit expending more energy as it runs in overdrive, spiking your electric bills.

Dirty Flame Sensor

A flame sensor is a metallic rod that generates a current of electricity to detect the presence of heat from a flame. However, the flame sensor may become dirty with time and malfunction. This is due to prolonged exposure to the moisture that burning fuel generates. Since the sensor is metallic, these conditions cause rusting, restricting proper flame detection. Accordingly, this may trigger the limit switch as the sensor fails to relay the right signal to the control board.

Short Cycling

If you mount your thermostat in an unsuitable location, such as a window, the heater may short cycle. This is due to the thermostat detecting the temperatures on these surfaces, leading to the unit powering on and off intermittently. The resulting short cycling exerts additional strain on the heater, triggering the limit switch. Thus, contact a furnace technician to help situate your thermostat for effective temperature detection.


Air filters may clog with airborne contaminants over time, restricting airflow to the system. This may cause the unit to strain in drawing sufficient air for heating, resulting in overheating. In addition, a blockage in the ducts and registers can impede heated air from flowing out of the system, causing it to overheat. Consequently, the limit switch will trip, rendering the system inactive.

Resetting the limit switch, in and of itself, may not remedy the underlying malfunction. Employ the services of a heating contractor to inspect your unit and ensure it operates optimally.