When Your Air Conditioner Won't Work

Learn Why Your Furnace Runs Non-Stop

15 January 2019 by Clara Fernandez

What's worse than having no heat? Having a furnace that won't stop putting out heat. A furnace that runs non-stop can easily drive you out of the house with unbearable temperatures, not to mention that non-stop operation isn't good for your furnace's health – or your next energy bill.

A furnace that won't shut off when it's supposed to is likely suffering from one of the problems listed below. Read on for a quick explanation and what can be done to give your furnace a rest.

1. Malfunctioning Thermostat

Your furnace may be the heart of your home's heating system, but it's the thermostat that's the brains of the operation. Unfortunately, that makes thermostats vulnerable to the occasional brain fart, whether due to installer error, old age or an electrical glitch that prevents it from working properly.

If your thermostat refuses to respond to your commands, check for a dead or dying battery first. A dying or corroded battery can cause thermostats to behave strangely, in some cases. If the battery is fine, make sure the thermostat is properly wired to your furnace. Frayed, burnt or cut wiring can also cause some unexpected issues.

2. Faulty Fan Limit Switch

The thermostat isn't the only device that tells your furnace when to do its thing. There's also the fan limit switch, a device that commands the furnace's blower fan to kick into action once the burners are working and the air inside the furnace's air plenum is at the proper temperature. This prevents the blower fan from blowing chilly air throughout your home before your furnace is ready to deliver heat.

Fan limit switches are also a safety feature. Not only is there a low limit setting to prevent cold air from being circulated, but there's also a high limit setting to prevent overheating. The high limit setting keeps the heat exchanger and other crucial components from being damaged if furnace temperatures get out of hand.

There are plenty of ways that fan limit switches can break. Physical damage from accidental contact can cause these switches to fail. Electrical surges and shorts can also damage the fan limit switch. Debris buildup can also prevent the switch from properly sensing temperatures, allowing the furnace to run in excess of what's needed to cool your home.

3. Ductwork Leakage

Ducts with large gaps, holes or tears can send heated air into places where it shouldn't go, leaving other areas of your home out in the cold. Leaky supply ducts can also cause your furnace to run non-stop in a futile attempt to reach your desired temperatures.

Leaky ducts can make themselves known by strange noises and cold spots in certain areas of your home. If you suspect duct damage, have a furnace repair professional perform an in-depth inspection and make repairs as needed.

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