If the high limit switch in a furnace keeps tripping, the most common cause is a lack of airflow.
When the air can not flow through the furnace, it heats up and trips the limits switch off.
Why Does the High Limit Switch Shut Off the Furnace?
The high limit switch is a safety feature, created so the heat exchanger does not overheat and crack.
The heat exchanger is a metal chamber where the flames burn heating and keep the gases separate from the air going into a home.
A limit switch will shut down a furnace at roughly 160°F, which will cause the burners to turn off.
Air flows outside the heat exchanger while the flames burn inside and the gases are vented.
If the heat exchanger cracks, it can be a huge safety problem and allow harmful gases to enter a home.
The high limit switch detects when the inside of the heat exchanger becomes too hot and needs to shut down before damage occurs.
Without the safety switch, the furnace could become damaged, causing a large repair bill and possibly toxic gases to enter a home.
Can a High Limit Switch Be Bypassed?
The switch should only be bypassed temporarily for testing and troubleshooting.
While it can be bypassed if the furnace is run without the switch, it can cause damage and other safety issues.
A furnace should not be run without the safety switch, and the reason the furnace is overheating should be solved.
Furnace High Limit Switch Keeps Tripping Causes and Fixes
- Air Filter
- Blocked Register
- Exhaust Vent
- “A” Coil Clogged with Debris
- Blower Motor
- Bad Inducer Motor
- Bad Furnace Limit Switch
A dirty air filter is the most common cause of a limit switch that keeps tripping off.
When a filter becomes dirty, it blocks the airflow through the furnace, which causes the heat exchanger to become too hot and trips the switch.
Also, be sure the arrows on the filter are pointed in the right direction going with the airflow and not against it.
The registers in a home let the airflow out of a furnace, heating up the space.
If a register is blocked, it will also stop the air from flowing.
Check all the vents for anything blocking them, which will cause the air to pass through.
After the flames burn, the gases they let off, need to be vented outside.
This is done through a vent that should not be blocked.
Check the vent for anything that may block the gases from going outside.
An A-coil is used inside many furnaces with a split system air conditioner to cool a home in the summer.
If A-coil becomes clogged with debris, it can stop the airflow from passing through the system.
The coil can be cleaned with water and a brush to get out any build-up that may have occurred.
If the blower motor is not working correctly, and not blowing the air fast enough through the system, the heat exchanger may overheat.
Check the motor to see if the air is blowing well past the filter and out the registers.
The inducer motor helps to vent out toxic gasses through the vent.
IF it goes bad the furnace will usually not start but if it having an intermittent problem it may be the cause.
It is possible the furnace lite switch has gone bad and not working correctly.
Replacing the switch will be the best option to see if it has gone bad and shutting the furnace down at the wrong temperature.
Summary
If the high limit switch in your furnace keeps tripping, there is a problem with the airflow.
A dirty filter is the most common issue nut also a blocked register can block the air from passing through the system.
Other causes can be a blocked vent, dirty A-coil, and blower motor that has gone bad.
It is also possible the limit switch has gone bad and needs to be replaced.
Have you had issues with a furnace high limit switch? Let us know your thoughts below.
What ended up to be the cause of the problem? I’ve heard that the more expensive filters can restrict air flow, but you said the problem persisted even though you put the inexpensive one back in?
I change my furnace filters every 3 months. I usually use the inexpensive ones but decided to use a 3M expensive one a month ago. I turned my thermostat to 72 degrees and it won’t heat above 68 degrees. It’s just blowing cool air out, and I’m hearing something turning off and on inside the furnace. I did put in a inexpensive filter thinking the expensive one wasn’t letting air flow through. What should I check next? I did call a company who can come tomorrow with a $90 estimate fee. I am retired and was wondering what should I expect to pay for it to be fixed without being scammed. Please help