Testing the furnace limit switch by bypassing it is common to see if it has gone bad.
A furnace should only be run for a short while to test the switch.
The furnace limit switch helps protect the heat exchanger from becoming too hot and cracking.
If the heat exchanger cracks, it can let noxious fumes into a home which is dangerous.
Can You Bypass the Furnace Limit Switch?
When testing if a switch is good or bad, yes it can be bypassed for a short time.
This is a common method to see if the switch needs a replacement.
The furnace should Not be run for an extended time with the jumper as the limit switch is a necessary safety feature.
The limit switch turns the furnace off when it becomes too hot, which prevents the heat exchanger from becoming damaged.
If the heat exchanger becomes damaged, it can release noxious fumes, which can be toxic.
Be sure to have a working CO alarm on every level of the home, as running a furnace with the limit bypassed can overheat it and cause a fire or carbon monoxide leak.
When jumpering the wires, using a shielded wire as an open wire may ground out, causing damage to the mainboard.
Keep in mind, the power should be turned off before placing or removing the jumper, and the furnace should never be left running unattended with it in place.
Also, the rollout switch and flame sensor should never be jumpered, as they are separate safety controls.
How To Bypass the Furnace Limit Switch
- Locate the Limit Switch in the Furnace.
- Remove the two wires going to the Limit Switch.
- Insert a shielded jumper wire in-between the two wires.
- Once jumpered, test the furnace; if it works, the switch is bad and needs to be replaced.
Keep in mind, a limit switch that keeps tripping usually points to a real problem such as a dirty filter, closed vents, a weak blower or bad capacitor, or a cracked heat exchanger.
Summary
To quickly test the limit switch, it can be bypassed with a shielded wire.
This should be done only to test the switch and not as a permanent fix.
If the switch is found to be bad, it will need to be replaced with a good one that matches the OEM temperature rating.
The limit switch can be bought at many HVAC parts stores as well as online on Amazon or eBay.
As with any heating equipment, call a professional if you have no prior HVAC experience.