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[Updated for 2023] Why Does My Furnace Keep Shutting Off?

Furnace keeps shutting off

If your furnace won’t stay on or is turning on and off frequently- you have a problem. But we don’t have to tell you that— your unstable household temperature is likely already alerting you.

Have you noticed that your furnace shuts 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; a short cycling furnace will mean heating bills can become ridiculously expensive if the inefficient heating pattern continues.

Sometimes, an aged furnace is simply ready for an upgrade.

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

The Filter is Dirty and Clogged

When your heating system feels more like air conditioning, check for dirty air filters. 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, or Knight can help you find the right air filter, and install for you while running a check-up.

A reusable furnace air 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 to switch out or clean your dirty air filters.

How Do You Fix a Dirty and Clogged Filter?

Keep Vents Open

Your furnace may be shutting off due to a blocked vent. 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 air vents are open so that an ample amount of air gets back to the furnace’s heat exchanger. A gas furnace will also have a vent for combustion and most furnaces are equipped with a safety device designed to shut down if there isn’t enough air circulation.

Check your Blower Wheel

A dirty blower wheel will slow down the blower motor and cause it to work much harder to keep your home at the desired temperature. If you have a dirty filter, make sure to check the blower wheel as well for any dirt, dust, or grease that might be weighing it down. This is seen commonly in an older furnace or if the maintenance wasn’t done regularly. If your blower motor is being weighed down, you’ll have less air coming through the vents and your motor will wear quickly as heat builds and it works harder to maintain air flow.

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 the temperature setting is as it 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 faulty 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 in direct sunlight, or near another heat source, it will register a warmer temperature, give you a faulty temperature reading and turn your furnace off.

We have a wide selection of thermostat models to choose from, including smart thermostat options. Plus, we can help you find the perfect place for your thermostat. No more being left in the cold.

The Flame Sensor is Soot-Covered

Your furnace might be shutting off because of a dirty flame sensor. This is common but it is not a DIY fix. 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.

Your Furnace is Too Big

Your furnace’s short cycling could be directly related to the size of your furnace. 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 it off. But without consistent heating, your home will quickly cool down. Then the process starts again. And again. And again. This is not the furnace cycle it was built for and it will cost you more in the long run.

The only way to fix this issue is to get a new, properly-sized furnace. An oversized furnace can only be fixed by getting a bigger house- not the cheapest fix!

The friendly experts at Knight will not only be able to give you a quote on the correct furnace size, but give you advice and comparisons on which furnace 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!

Need a new furnace? Check out our buyer’s guide for the best furnace to install in your home.