Furnace Limit Switch: What Tripping It Means

Airflow, in the order it is usually wrong

The filter. Every time, first. A clogged one starves the furnace of the air it needs to carry the heat away, and Carrier’s manual lists it first for a limit fault. While you have it out, check it is the right size and the right way round: which way the arrow goes.

Closed or blocked registers. Vents shut in spare rooms, a sofa across one, a rug over another. Every one you close raises the pressure and cuts the flow: closing vents in unused rooms.

The return. Furniture in front of the return grille, or a return that was always too small for the furnace fitted to it.

The blower wheel. A drum of small blades that fills with dust until it is spinning fast and moving very little: what a packed blower wheel looks like.

The blower speed. If somebody changed it during a repair and it never went back, the furnace has been running hotter ever since.

Find it before you touch it

The limit. On the cabinet above the heat exchanger, often on the wall of the blower compartment, usually a small round disc with two wires. Most are automatic, so they reset themselves once things cool down, which is why the furnace starts again a few minutes later and then does the same thing.

The manual kind. Some have a small button you press. Pressing it once is a test. Pressing it a second time is feeding a fault that nobody has found: what a reset button really is.

The rollout switches. Down at the burner end, usually with a red button, sometimes two or three of them. These open when flame rolls out of the burner box instead of going up the flue, which points at a blocked heat exchanger, a blocked flue or a cracked one. Do not reset one and walk away.

If it lights, runs, quits and repeats, that is the whole picture and it has a page of its own: time the burners and read it off the clock.

How to know it is genuinely the part

Limit switches do wear out, and after years of wear they can open below the temperature they are supposed to. The honest way to tell is to rule out everything else first.

Measure the temperature rise. A thermometer in the return and another in a supply vent, subtracted, compared with the band on the plate inside the door. If the rise is inside the band and the limit still trips, the switch is suspect: how to take that measurement.

Check it is the right part. Limits are rated for a specific temperature, printed on the body. A previous repair fitted with whatever was on the van will trip early forever.

And rule out the cheap stuff twice. A new filter, every register open and a clear return, for a week. If it stops tripping, you never needed a part at all.