A heating furnace has many components that count for its performance and the high-limit switch is one to name. Along with the thermostat, the limit switch monitors the temperature of the air circulating in the house. The limit switch will shut off the furnace blowers and burners if the unit overheats. It’s a safety measure to prevent fire.
The limit switch will make the furnace lockout mode when unusual behavior occurs in the system. Most of the time, this happens when the limit switch doesn’t stop tripping or after you have tried, without success, to turn the furnace back on.
Unless you service the unit and manually reset the unit, the furnace won’t come out of the ignition lockout state. If the limit switch is faulty, resetting the unit won’t help. A worn-out or broken limit switch will typically trigger a false alarm and make the furnace lockout.
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What should you do before resetting the furnace’s limit switch?
Even if resetting the furnace’s limit switch is not complicated, it’s also essential to discover why the furnace has gone into lockout mode. Modern furnaces feature a visual display that shows an error code. Various models have various error codes and you need to check out the user manual for your furnace. For instance, a blinking LED light will reveal which component has issues—the user manual will give you the details.
You need to diagnose the problem and fix it. Typically, you need to call an HVAC technician for diagnosis and repairs. Some of the issues that cause the limit switch to flip come down below:
Airflow problems
Furnaces need a steady airflow to circulate the heat that the burners produce. When there’s not enough air, the furnace will no longer be able to blow heat fast enough. It’s only a matter of time until heat buildup makes the furnace overheat. As a result, the limit switch will stay open.
The fix
The furnace’s airflow needs improvement and can be done in various ways. First of all, you should include regular inspection of the furnace filter. If necessary, you need to replace it. Air filters have to trap debris, dust, and other forms of dirt and stop them from entering the furnace. If they pass, they will damage the furnace. Usually, air filters become clogged eventually.
You should see that the high-efficiency furnace’s vents are not blocked. Leaves, insects, and animals can block the air vents and alter the airflow into the furnace. Look at the air filter and remove any blockage in the air vents. You can reduce the risk of future blockage by installing a vent screen.
The dirty flame sensor rod
The flame sensor rod is a component of the limit switch and it’s located in the plenum to monitor temperatures. It can come in contact with flame and soot will build up on it. As a result, the furnace will cycle on and off quickly, taking it into lockout mode.
The fix
You have to identify the flame sensor rod and clean it. After you access the blower compartment, you must remove the screws that hold the rod on the plenum. Clean the rod with some steel wool or a scouring pad. Don’t use any liquid and screw the rod back into its position after you clean it.
Is the limit switch bad?
In time, a limit switch will wear out as all parts of the furnace. The limit switch will trip at low temperatures or fail to inform the blower fan to turn off whenever the furnace overheats. It’s not a situation to ignore as it poses a fire risk and carbon monoxide gas exposure. It would help if you managed to protect your family.
The burning smell at the vents and the loud humming sound from the furnace signal that the furnace is overheated. When the furnace doesn’t shut off on its own, you have to turn off the power at the circuit breaker and call the certified HVAC technician.
Reset the furnace limit switch—here are the steps to take!
Once you identify the problem causing the limit switch to trip and solve it (on your own or not), you have to reset the furnace so that it keeps doing its job. With most gas furnaces, you only need to shut off the power supply and wait for 20 seconds before you turn it back on.
In some models, a controller unlocks the furnace after hours. If your furnace isn’t this kind of model, you need to reset the furnace limit switch manually.
Step 1
Always begin by turning off the power to the furnace and closing the gas valve.
Step 2
Find the furnace’s heating duct. Commonly, the heating ducts are in the basement or utility closet.
Step 3
Look for the furnace’s access panel—it’s typically on the side of the furnace housing or the bottom, close to the duct or return air vent. Some furnaces come with push or twist locks, whereas others require unscrewing to open.
Step 4
Check out the panel and look for the limit switch. Commonly, it’s in a white or silver cover. Remove the cover to access the main body of the high-limit switch. The limit switch features two arms: one controls the low temperatures, and the other maintains the high temperatures. Another component between the two arms has to control the furnace’s fan.
Step 5
Adjust the temperature on the central component and set it to 110-115 F degrees. The step will inform the fan to switch on and off at the right moment.
Step 6
Continue setting the left arm to 90F degrees (give or take). This will inform the furnace that it shouldn’t blow cold air when it reaches the set temperature.
Step 7
You have to set the right arm of the switch at 200F degrees. It will inform the furnace to shut off when the burner reaches this temperature. The shut-off temperature varies according to the model of the furnace, so check out the instruction manual before you adjust the right arm.
Step 8
You have to take the furnace into automatic mode. Do it by pulling out the white or silver switch. Automatic mode means that the furnace will follow the current settings. Close back the panel. Should you push the switch in, you will clear all settings.
One last thought
The furnace features several sensors that improve its performance and safety. The limit switch is one of these sensors that makes sure to turn off the furnace the moment it overheats. It will put the furnace into lockout mode and you need to service it. A manual reset of the limit switch will be necessary and you might have to replace the limit switch if you don’t see results after resetting. If you don’t call the HVAC technician, you should at least carefully review the owner’s manual before manually resetting the switch.
