Above images are two of the more common high limits found in residential and lite commercial furnaces
A high-limit switch is a bi-metal snap disc switch that warps when heated to break a heat cycle and used to protect the heat exchanger from overheating and burning out due to insufficient indoor air passing over it. A dirty filter or squirrel cage, undersized ductwork, or many supply air vents closed are among some of the reasons to set this switch off.
Once a high limit senses the heat exchanger is in danger of overheating, it breaks the circuit shutting off gas flow and runs the furnace blower to cool the heat exchanger. Once the temperature is safe for the heat exchanger, the heating process continues.
These high limit switches are almost always automat reset meaning the user does not have to reset the switch. The most common in furnaces today are snap disc limits like the one above. If the heat exchanger is heated beyond the rating of the high-limit, a small disc inside the switch will warp and break the electrical flow of the circuit.
The high-limit will usually be marked with a rating similar to 160-40°F meaning at 160°F the high-limit will trip and reset when it drops 40°F down to 120°F.
Many things can cause these to trip, but they will, for the most part, be related to indoor air moving across the heat exchanger.
Common causes for high-limit to trip
- A dirty filter will not allow the proper amount of air to absorb heat from the heat exchanger and move it to your living space
- Too small of return air or not enough return air installed in ducting
- A dirty blower wheel will reduce the air that the blower wheel can move across your heat exchanger
- Wrong blower speed will also move the wrong amount of air across the heat exchanger
- A broken belt will not allow for the proper movement of air on belt-driven motors. A broken belt will also make pretty loud banging noises
- A faulty capacitor
- A dirty evaporator coil
- A dirty secondary heat exchanger
- A cracked/damaged heat exchanger
- Incorrect gas pressure
- Incorrect burner orifices