Look at the furnace date plate on the side of the air handler. This will show the maximum temperature that heated air should be when it enters a room through the supply vents.
Block off the return air vent(s). Place a piece of cardboard over the top of the vent and tape it in place.
Stick a temperature probe into the supply air plenum. The supply air plenum is the tube or duct that comes out of the top of the inside unit, also called the air handler.
Turn on the heat.
Wait for the temperature in the air plenum to reach the maximum supply air temperature. Listen to hear the furnace shut off the burners. Replace the high limit switch if the furnace does not shut off or get near the maximum supply air temperature.
Things You Will Need
- Temperature probe
- The temperature probe should have a needle-like tip and be able to puncture a small hole in the plenum. If it is not sharp enough, use a small screwdriver to make a hole.