The limit switch is one of several sensors allowing your furnace to heat your home safely and efficiently. Like many other sensors in your furnace, a faulty limit switch usually prevents the furnace from running. However, a faulty limit switch won't always completely shut your furnace down. Switches that are just beginning to fail may work intermittently or behave inconsistently. 

If you have reason to think your furnace's limit switch is failing, you might want to put off a repair for as long as possible. Unfortunately, trying to save money by deferring this essential repair may have more frustrating, costly, or even dangerous consequences. Instead of delaying the inevitable, here are three reasons to immediately replace your faulty limit switch.

1. Ice-Cold Drafts

The primary role of your limit switch is to detect the temperature around your heat exchanger, but this information serves more than one purpose. Your furnace's heat exchanger will initially be cold when your thermostat first calls for heat, and it will take some time to heat up after the burners turn on. If the house blower engaged immediately, you'd receive a blast of cold air from your vents.

The limit switch waits for the heat exchanger to warm up before turning on the blower, keeping room temperatures more comfortable and consistent. A faulty limit switch may fail to detect the proper temperature, allowing your furnace to start before the heat exchanger warms up, ultimately leading to icy drafts every time your furnace turns on.

2. Poor Efficiency

In addition to turning the blower on, the limit switch also waits to turn the blower off. When your thermostat stops calling for heat, your furnace's burner will shut off. However, some heat will remain in the exhaust stream, and the heat exchanger's metal won't cool down immediately. This extra heat would go to waste if the blower and burner stopped simultaneously.

Most furnaces will wait until the limit switch reports its lower limit temperature at the heat exchanger. This approach maximizes efficiency by extracting as much heat from the combustion process as possible. A faulty limit switch may allow the furnace to shut off too soon, wasting heat and lowering efficiency, and run the blower for too long, producing cold drafts.

3. Potential Heat Exchanger Damage

The final role of the limit switch is a protective one. As the heat exchanger extracts energy from the exhaust stream, the metal on the heat exchanger will heat up. Under normal circumstances, airflow across the heat exchanger will keep these temperatures under control. Unfortunately, certain situations can allow the heat exchanger to overheat, potentially leading to dangerous cracks.

The limit switch detects high temperatures (an "over-limit" condition) and shuts the burners down to prevent damage. A damaged or faulty limit switch may affect this critical safety feature, leaving your heat exchanger vulnerable to premature wear and failure.

Contact a local furnace repair service to learn more.