Is your furnace constantly shutting off unexpectedly, leaving you shivering in the cold? Or perhaps you’ve received a hefty energy bill due to inefficient heating? These could be signs of a malfunctioning furnace high limit switch. While this component might sound like something out of a sci-fi movie, it plays a critical role in keeping your home warm and safe.
To help you further understand the ins and outs of the furnace high limit switch, we’ve done all the leg work for you. Do read through this article to learn more about what a furnace high limit switch is, why it matters, and much more.
What Is A Furnace Limit Switch And How Does It Work?
A furnace limit switch keeps your furnace running safely by monitoring the air temperature. Strategically placed, it detects when the air passing through the furnace exceeds a preset temperature. At this point, the high-limit furnace switch activates, cutting power to the burners and stopping heat production. Once the temperature returns to normal, the switch resets automatically, allowing the furnace to resume operation. This cycle prevents the furnace from overheating, which could cause equipment failure or fire hazards.
How To Identify A Furnace Limit Switch
The furnace limit switch is crucial for the safety and function of your furnace. To identify it, look for a device with a long temperature sensor probe. This probe detects the air temperature inside the furnace. It is usually connected to a mounting plate that attaches the switch to the furnace’s air plenum, the main channel for heated air distribution.
The mounting plate typically has two or more terminals. These terminals connect the limit switch to the furnace’s control system, linking to the blower fan and gas valve wires. This connection allows the switch to perform its safety functions, shutting down the furnace if it overheats and restarting it when temperatures are safe.
How To Replace A Limit Switch?
Replacing a furnace limit switch is essential when the existing one fails or becomes unreliable. Here’s how you can replace the switch yourself to ensure your furnace operates safely and efficiently:
- Turn off the power to the furnace at the breaker box to avoid electrical hazards.
- Locate the limit switch on the furnace’s air plenum.
- Remove any panels or covers to access the limit switch. Carefully note the arrangement of wires before disconnecting them.
- Remove the old switch by unscrewing or unclipping it, depending on the furnace model.
- Install the new limit switch in the same position. Secure the new switch using screws or clips.
- Reconnect the wires according to your notes or the picture you took earlier. Ensure each wire is properly attached to the correct terminal.
- Restore the power to the furnace.
- Test the furnace to ensure it operates normally.
If the furnace doesn’t work as expected, consult a professional technician.
Causes Of Furnace Limit Switch Tripping
Understanding why your furnace limit switch trips can help prevent issues and ensure your system runs smoothly. Here are the common causes:
- Dirty Air Filter: A clogged filter restricts airflow, causing overheating and switch tripping.
- Blocked Air Vents: Closed or obstructed vents also reduce airflow and increase furnace temperature.
- Blower Motor Issues: A malfunctioning blower motor won’t circulate air properly, leading to heat buildup. If the blower doesn’t shut off, it can lead to continuous cycling and overheating.
- Cool Air Output: When air exiting the system is cool, it indicates insufficient heating, which can trigger the limit switch due to abnormal operation cycles.
- Oversized Furnace: If too large for the space, the furnace cycles off too quickly, overheating the heat exchanger.
- Short Cycling: Frequent on-and-off cycles put excessive stress on the furnace, causing overheating.
- Faulty Installation: Incorrectly installed ductwork or furnace components can restrict airflow.
- Worn Out Switch: An old or damaged switch may malfunction, leading to unnecessary trips.
Safety Features For Furnace’s Limit Switch
A furnace limit switch is a crucial safety component that prevents the furnace from overheating and causing damage. Here’s how to understand its function:
- Dirty Air Filter: A clogged filter limits airflow, causing overheating and tripping the switch.
- Blocked Air Vents: Closed or obstructed vents reduce airflow, raising the furnace temperature.
- Blower Motor Issues: A malfunctioning blower motor won’t circulate air properly, causing heat buildup and continuous cycling, leading to overheating.
- Cool Air Output: Cool air exiting the system signals insufficient heating, triggering the limit switch due to abnormal operation cycles.
- Oversized Furnace: An oversized furnace cycles off too quickly, overheating the heat exchanger.
- Short Cycling: Frequent on-and-off cycles stress the furnace, causing overheating.
- Faulty Installation: Incorrectly installed ductwork or furnace components can restrict airflow.
- Worn Out Switch: An old or damaged switch may malfunction, causing unnecessary trips.
FAQs:
Replacing a furnace high-limit switch usually costs between $150 and $300, including parts and labor. The price can vary depending on factors like geographic location, time of year, furnace type, and whether you choose to DIY or hire a professional. If you’re handy, the switch itself might cost just a few dollars, but professional installation can range from $100 to $1,000.
To reset a furnace limit switch:
* Turn off power: Locate and switch off the furnace’s power supply.
* Access control panel: Open the furnace control panel.
* Locate switch: Find the red or yellow limit switch button.
* Reset: Push and release the limit switch button.
* Restore power: Turn the power back on and test the furnace.
If this doesn’t work or the issue persists, contact a professional HVAC technician.
When the thermostat signals for heat, the furnace turns on, and once it heats up, the limit switch closes, activating the blowers. The burners shut off when the desired temperature is reached, but the blower continues until the furnace cools to the lower limit. This cycling, controlled by the thermostat and limit switch, enhances furnace efficiency and prevents overheating.
A malfunctioning furnace limit switch can cause issues like the furnace not turning on, frequent on-and-off cycles, no heat production, the blower running without heat, and overheating. If you suspect a faulty limit switch, check for a tripped reset button, inspect the switch for damage, and test it with a multimeter. If you’re unsure about these steps, it’s best to call a professional HVAC technician for diagnosis and repair.
Conclusion
The furnace limit switch plays a vital role in your heating system’s safe operation. It monitors and controls the temperature, preventing potential overheating and ensuring efficient furnace performance. Regular maintenance of this component helps avoid costly repairs and keeps your home safe. A working furnace limit switch is not just a regulatory requirement; it supports the overall longevity and reliability of your heating system.
