Why Does My Furnace Keep Shutting Off?


Have you noticed that your furnace keeps shutting off almost as soon as it turns on, leaving you in the cold? Are you tired of seeing the temperature jump up and down, and worried that one day it won’t come back on?

Short-cycling – or having a furnace turn off and on quickly and more often than you have programmed – is a big problem for homeowners. It’s not just incredibly annoying; heating bills can become ridiculously expensive if the inefficient heating pattern continues.

So what causes a furnace to keep shutting off? And how do you fix it? Here a few potential issues your furnace could be facing.

1. The Filter is Dirty and Clogged

This is the first thing you should check, because it’s the easiest problem to solve (not to mention the cheapest).

A dirty and clogged furnace air filter will not only prevent proper air filtration, but it will actually obstruct airflow to the heat exchanger. If the heat exchanger gets too hot from lack of airflow, a safety mechanism will shut the furnace off to protect it. While this is normally a good thing, it’s not what you want when the temperature outside is -20° or worse.

Luckily, you can fix a clogged filter with just a few simple steps. First, turn off the furnace for safety. Then simply remove the filter, and find out whether it is a disposable or reusable filter. If it’s disposable, all you have to do is throw it away and buy a new one. You can get them at your local hardware store.

A reusable furnace filter is of higher-quality and will last longer, but it takes a little more effort to clean. Depending on what kind it is, either gently rinse with water, or gently vacuum up the dirt and debris.

Tip: To keep your furnace running smoothly, and your air filtered, you should check your furnace filter every 30 days.

Keep Vents Open

Your furnace filter isn’t the only thing that could restrict airflow to the heat exchanger. Closed vents would also do the trick. While it’s understandable to have some vents closed if the room is too hot, make sure enough of your vents are open so that an ample amount of air gets back to the heat exchanger.

2. The Thermostat is Acting Up

If the air filter is nice and clear, and still your furnace keeps shutting off, the next step is to check the thermostat. There’s a chance that it could be malfunctioning.

First, make sure that your thermostat is switched on, and set to ‘heat’. Next, make sure that your temperature is properly programmed – the furnace might be switching off because someone set the thermostat to a lower temperature, and never changed it back.

Once all the settings are as they should be, you’ll want to enlist a partner to help you check whether the thermostat is communicating with the furnace. While you operate the furnace, get your helper to listen by the furnace.

If the thermostat is working properly, when you adjust the temperature from low to high, there should be a sound. If there is a sound, then it’s more likely a furnace problem. But if there’s no sound, your thermostat is most likely the culprit. In that case, you should call us to come do some further investigating.

Thermostat placement is also important. If it’s placed directly in sunlight, or near another heat source, it will register a warmer temperature, and turn your furnace off.

We can help you find the perfect place for your thermostat, and have a wide selection of models for you to choose from >

3. The Flame Sensor is Soot-Covered

Sometimes a furnace keeps shutting off because the flame sensor is corroded or caked in soot. When operating normally, the flame sensor detects flames, and turns off the gas valve when there isn’t any. This prevents your home from filling up with harmful gas fumes, which has dangerous consequences.

Normally, the flame sensor does exactly what it’s supposed to do to keep you safe and warm at the same time. But when it’s corroded or soot-covered, it becomes unable to sense flame at all – even if the furnace is on. So it shuts off the gas valve. Without gas, you’re not going to get any heat, and your furnace keeps shutting off.

This is a repair that needs to be handled by professionals. If you suspect your furnace keeps shutting off because of this, or another internal issue, book an appointment with us.

4. Your Furnace is Too Big

When you get a new furnace, it’s crucial that it be properly sized for your home. Otherwise you run into problems.

When your furnace is too big for the space, it heats your home quickly. Then its job is done, and the thermostat shuts if off. But without the consistent heating, your home will quickly cool down. Then the process starts again. And again. And again.

The only way to fix this issue is to get a new, properly-sized furnace. The experts at Knight will not only be able to give you a quote on the correct furnace size, but give you advice on which model will work best for you.

Knight Technicians Can Get Your Heat On, and Keep It That Way

If your furnace keeps shutting off, call Knight. Our licensed and experienced technicians will be able to get your heat on, and keep it on. Say goodbye to short cycling and cold snaps by requesting a quote!

Request a Quote