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When it is freezing cold outside in Pennsylvania and suddenly your furnace stops working, this is nothing less than a headache. Many of us panic at that moment and overthink major repairs worth thousands of dollars. However, the actual issue is, most of the time, very easy to figure out and get fixed. A limit switch, which is a tiny device on the furnace, is often a culprit in these issues. In this blog, we will discover why the furnace keeps tripping the limit switch and how to fix such issues.
It is a tiny switch that sits inside the main cabinet on the back or side wall of the upper heat exchanger. The primary job of this switch is to ensure that the furnace keeps working at the correct temperature and prevents overheating. If the furnace temperature goes beyond the limit, this switch shuts the furnace off automatically and prevents damage. Sometimes, this switch trips frequently. The core cause behind this might be issues in any furnace component other than the switch itself.
There are several reasons a furnace trips the high-limit switch. From component problems to performance issues and associated electrical failures, causes are diverse. Here are some of the primary causes of this issue.
If the airflow is restricted, the system can’t distribute warm air evenly throughout the space. On the other hand, the ignition chamber burns the fuel at the same rate and produces heat energy continuously. This heat overheats the system and results in the high-limit switch turning off.
A clogged filter blocks the air from moving as it should. Dust and pet hair get caught in the air filters over time, and if you forget to swap them out, the furnace’s limit switch turns the system off. If you try again to turn the system on, the same happens until you clean the air filters or replace them to ensure proper airflow.
The heat produced in the ignition chamber goes to heat exchangers and then to the blower motor. The blower motor then distributes this warmth throughout the house. If this motor is faulty, the heat produced remains within the system. The result is that the system overheats and the limit switch turns the system off.
Not every time the switch trips due to other component issues. Sometimes, the switch is itself faulty. Just like other components, the furnace limit switch also experiences continuous wear and tear. If the switch is old, cracked, or has serious mechanical damage, it trips regardless of whether every other component is working correctly.
Though calling for a professional furnace repair service in PA is recommended, you can try quick fixes yourself in many scenarios to sort the issue out yourself. Here is what to check.
Replace dirty air filters with fresh ones every couple of months
Open up all supply and return vents across the house
Clear away furniture or rugs blocking warm air registers
Check blower intake grills for buildup of dust and lint
Though almost every furnace component can trigger limit switch tripping, three components are more directly related to such issues. The table below lists those three parts along with how they create limit switch problems.
Component |
Primary Function |
Failure Effect on Switch |
|---|---|---|
|
Air Filter |
Traps dust and debris |
Restricts airflow and causes rapid heat buildup |
|
Blower Motor |
Circulates air through ducts |
Allows internal heat to build without distribution |
|
Flame Sensor |
Detects presence of burner flame |
Irregular cycling that confuses system controls |
If the DIY steps fail, call Lehigh HVAC for expert, on-site assistance. Here are some scenarios when only a certified technician can fix the underlying cause and restore normal system performance.
Air filter replacement does not stop the switch from tripping repeatedly
The blower fan makes loud squeaking or grinding noises during operation
The furnace burns with a yellow or irregular flame instead of blue
The system shows signs of electrical burning, such as smells or strange burning odors
