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HVAC Furnace Short Cycling: High Limit Switch Explained
Short cycling is a common problem with HVAC furnaces, and it can be caused by a variety of factors. One of the most common causes is a faulty high limit switch. This blog post will explain what a high limit switch is, how it works, and how to troubleshoot it if it is causing your furnace to short cycle.
What is a High Limit Switch?
A high limit switch is a safety device that prevents your furnace from overheating. It is typically located in the furnace's combustion chamber or heat exchanger. The high limit switch is connected to a thermostat that monitors the temperature of the furnace. When the temperature reaches a certain point, the high limit switch opens, interrupting the flow of electricity to the furnace and shutting it down. This prevents the furnace from overheating and potentially causing a fire.
How Does a High Limit Switch Work?
The high limit switch is a simple device that uses a bimetallic strip. A bimetallic strip is made of two different metals that expand and contract at different rates when heated. When the furnace gets too hot, the bimetallic strip bends, opening the switch and interrupting the flow of electricity.
Why Does a High Limit Switch Cause Short Cycling?
A high limit switch can cause short cycling if it is faulty or if it is not properly calibrated. If the high limit switch is faulty, it may open prematurely, shutting down the furnace before it has had a chance to heat the home properly. This will cause the furnace to cycle on and off frequently, leading to short cycling.
If the high limit switch is not properly calibrated, it may not open until the furnace reaches a dangerously high temperature. This can be a safety hazard and can also cause the furnace to short cycle.
Troubleshooting a High Limit Switch
If you suspect that your high limit switch is causing your furnace to short cycle, you can troubleshoot it by following these steps:
- Turn off the power to the furnace. This is important for safety reasons. You should never work on a live electrical system.
- Locate the high limit switch. The high limit switch is typically located in the furnace's combustion chamber or heat exchanger. It is usually a small, round device with two wires connected to it.
- Test the high limit switch. You can test the high limit switch with a multimeter. Set the multimeter to the ohms setting and touch the probes to the two terminals on the high limit switch. If the switch is working properly, the multimeter should read a low resistance. If the switch is faulty, the multimeter will read a high resistance or an open circuit.
- Replace the high limit switch. If the high limit switch is faulty, you will need to replace it. You can purchase a new high limit switch from a local HVAC supply store or online.
Other Causes of Short Cycling
While a faulty high limit switch is a common cause of short cycling, there are other factors that can contribute to this problem. These include:
- Dirty air filter. A dirty air filter can restrict airflow, causing the furnace to overheat and shut down.
- Blocked air vents. Blocked air vents can also restrict airflow, causing the furnace to overheat and shut down.
- Faulty thermostat. A faulty thermostat can cause the furnace to cycle on and off frequently, leading to short cycling.
- Low refrigerant levels. Low refrigerant levels can cause the furnace to overheat and shut down.
If you are experiencing short cycling, it is important to have a qualified HVAC technician diagnose the problem. They can determine the root cause of the problem and recommend the best course of action.
Conclusion
A high limit switch is an important safety device that prevents your furnace from overheating. If your furnace is short cycling, a faulty high limit switch could be the culprit. By understanding how a high limit switch works and how to troubleshoot it, you can help to keep your furnace running safely and efficiently.