A gas furnace features the fan limit switch and the high limit switch. They can come separately or in just one package, known as the combination switch.
The high-limit switch has to turn off the gas to the furnace when the temperature gets too high. In contrast, the fan control limit switch detects the temperature inside the furnace and turns the blower motor on when the temperature gets warm enough. When the blower motor stops running, no air moves through the furnace. The flames have to increase the temperature inside the furnace and have no place to get hotter. For safety reasons, the high limit switch will turn the gas off if the temperature gets too high.
The fan control switch makes sure that no cold air is blown into the house. It will wait for the gas flames to heat up before turning on the blower. Some furnaces will come with two and even three limit switches wired in series to control the temperature at various points in the furnace.
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Bypassing the limit switch
When your furnace doesn’t operate as it should, bypassing the limit switch is one of the solutions to try. You can only bypass the furnace limit switch to test it and not for other reasons. Bypassing the limit switch can be dangerous and poses a risk of fire. We strongly recommend you ask an HVAC technician to come and bypass the limit switch for testing.
What’s the role of bypassing the limit switch
You should bypass the limit switch only when you need to decide if the limit switch needs replacing or not. While testing, you have to leave the furnace open for a short time. You are using the furnace while testing has fire risks and can cause accidents.
The steps to take on when bypassing the limit switch
If you decide to bypass the limit switch on your own, here’s what you need to do:
- Start with switching off the furnace
- Remove the furnace cover
- Look for the furnace limit switch
- Use pliers to pluck the wires
- Get a shielded wire to connect the limit wires
- Put the furnace cover back on
- Turn on the furnace
Testing the limit switch
It’s common for homeowners to keep servicing the limit switches. Truth be told, many furnace owners aren’t familiar with the limit switch. However, the limit switch is an essential safety feature for the furnace because it monitors the air and turns it off when it reaches the set temperatures.
Reduce the risk of furnace damage and keep the house nice and warm by paying attention to the limit switch. Don’t skip testing, repairing, cleaning, or replacing the limit switch. Here’s when you should run a test for your limit switch:
- The furnace keeps operating
- The furnace fan doesn’t run at all
Once you notice any of the two scenarios, don’t postpone testing the limit switch in your furnace.
Safety concerns
When you test the limit switch, don’t leave the furnace running for a long time. Should you let it run, you must have someone look over the furnace. The limit switch has to turn the furnace fan on and off whenever necessary. The limit switch will turn off if the furnace gets too hot.
Prolonged use while you troubleshoot the limit switch may cause overheating and, from that point onward, damage to the heat exchanger. Fires can also take place due to the release of toxic fumes. If inhaled, the poisonous fumes will generate even severe health issues.
Please contact an HVAC technician if you aren’t sure how to bypass the limit switch.
Repair or replacement
If testing the limit switch reveals defective, you should replace it. Test the limit switch’s lowest Ohms of resistance with a multimeter set. The controller is probably broken if there’s no continuity and replacement is the only solution. When there is continuity, but the furnace still shuts down before reaching the desired temperature, you must replace the limit switch.
According to the brand, you will pay anywhere between $5 and $20 to replace a furnace limit switch. Ideally, you want to buy the replacement limit switch from the furnace’s manufacturer. Universal furnace limit switches are also available and they are more affordable. Replacing the furnace limit switch doesn’t require attention from professionals. If you call an HVAC technician, expect to pay between $150 to $300 to replace the furnace switches.
Can the furnace perform without the limit switch?
You cannot use the furnace without the limit switch. Even when bypassing the limit switch, you should only do it for a short time. Limit switches match the furnace’s temperature to the thermostat. After the system reaches the set temperature, the furnace fan will automatically turn off.
If there is no limit switch, you must manually turn off the furnace after it reaches the desired temperature. There will be a fire risk due to releasing harmful gases and the furnace might overheat and even break. Let’s not forget the high fire risk if there’s no limit switch.
How to prevent limit switch problems?
You may avoid furnace limit switch issues with regular maintenance and unit inspection, especially if the furnace is old. Here’s what you need to do when checking out the furnace:
- See if the sensor needs cleaning. If it does, carefully remove grime, dirt, and debris, as they can alter the efficient performance of the limit switch.
- Examine the air filter and see if it’s clogged or not. The furnace plenum will overheat and break because of a clogged air filter. The furnace plenum gathers heated or cooled air from the furnace and sends it to the ducts; they will then spread it evenly inside the house. To avoid clogging, clean the filter every two weeks.
- Check out the ducts and the air vents. Make sure there is no blockage and they can freely blow air.
