Furnace Makes Banging Noise When It Shuts Off? (What It Means)

Ryan Womeldorf
by Ryan Womeldorf

It is normal for a furnace or air conditioner to make noises, but it can be disconcerting. Banging sounds that come from your furnace are common, but that doesn’t mean that it’s a good thing. So, why is it that your furnace makes a banging noise when it shuts off?

A furnace makes a banging noise when it shuts off if the ductwork is dirty or clogged. Clogged ducts or a dirty air filter cause hot air to back up which increases the pressure within the system. Temperature changes, dirty fan blowers, and a loose actuator can cause your furnace to make banging noises.

It is important to clean your furnace filter 3-4 times per year so that your furnace will function as intended. Banging noises when your furnace shuts off don’t always indicate a serious problem, but it’s important to inspect several components. Follow along as we explore what it means when your furnace makes a banging noise when it shuts off.

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Before We Begin

Before we get down to the potential causes of that banging noise in your furnace, there is one thing to remember. No matter what the cause may be, it is imperative that you do not ignore that noise.

There are too many things that could be wrong with your furnace and ignoring the issue will generally only lead to more expensive repairs down the line. Even if you decide to inspect the issue yourself and don’t call in a technician, it is better than simply ignoring the problem.

Furnace Makes Banging Noise When It Shuts Off

It is important to troubleshoot several options if your furnace makes a banging noise when it shuts off. The problem is typically due to an increase in pressure within the system due to dirty ductwork or a clogged filter. Let’s take a look at the most common causes and solutions.

1. Loose Actuator

The actuator is an important part of your furnace that helps push air through the system. It is common for a furnace actuator to come loose during operation. You will hear it bang around when your furnace shuts off if it is loose.

Luckily, all that you need to do is adjust your actuator if it is loose. The actuator is located above or below the coil depending on your furnace. Tighten the nut on the actuator dampers with a pair of pliers so that it doesn’t knock and bang around.

2. Clogged Ductwork

Clogged ductwork is unhealthy for an HVAC system and can explain why your furnace makes a banging noise when it shuts off. Hot air can back up and cause the pressure to increase within your ductwork if there is a lot of debris. The combination of heat and pressure can create a banging noise when your furnace shuts off.

First, you must remove and clean the vent covers so that you can access the ducts. Knock the dust loose with a long tool such as a broom handle or even a drain snake. Vacuum the rest of the dust and debris with as long of a vacuum hose as you can find.

This may not be doable for every homeowner, and that’s why it’s a good idea to hire a professional. Clogged ductwork can increase the risk of a fire hazard and worsen the air quality in your home.

3. Dirty Fan Blower

Your furnace’s fan blower can get clogged just like the ductwork and air filter. This blower is important because it circulates air for your furnace and air conditioner. HVAC professionals recommend that you clean the blower fan every 2 years so that it can function properly.

Otherwise, you will hear it bang and knock when your furnace or air conditioner shuts off. Make sure to shut off power to the system before your clean your fan blower so that it is safe to work with. You must remove the panel and pull the blower out so that you can clean it.

Avoid disconnecting the wires so that you can easily put the blower back where it was. Clean the blades with a small brush and clean the blower housing with a cloth. You can clean the blower housing with a vacuum if it is particularly dirty, and your furnace should stop making a banging noise when it shuts off.

4. Old Furnace Filter

A dirty filter can cause your furnace’s performance to suffer in many ways. Dirty filters can even cause your furnace to make banging noises when it shuts off. Filters can easily clog when they become dirty which can block the airflow.

The backup of air from a clogged filter can increase pressure within your furnace’s ductwork. Eventually, the increased pressure and temperature will create knocks and bangs that ring out throughout the furnace. Change your furnace filter 3-4 times per year to avoid HVAC problems.

Ideally, you should change your furnace filter at the beginning of each season or sooner if it is dirty. Dirty filters force your furnace motor to work extra hard to make up for the clog.

5. Temperature Changes

Temperature changes can cause several components, such as the ductwork, to expand and contract. Pressure within your furnace and ducts will fluctuate when the temperature changes. You will notice buzzing, humming, and banging noises when your furnace shuts off or runs in many cases.

Banging noises are common when your furnace shuts off if you live in a hot and humid climate. The spring and summer are known for rapid temperature changes, especially if you live in a hot or tropical zone. In this case, there is nothing wrong with your furnace if it makes a banging noise when it shuts off.

You can narrow the banging sound down to temperature changes if you inspect your HVAC system. Temperature changes are to blame if your furnace filter and ductwork are clean and the actuator dampers are in the proper position.

Other Loud Noises to be Aware of

Banging noises are certainly not the only kind of loud, abnormal noises that you may hear coming from your furnace. Each noise could be indicative of a different problem and will require a different repair. Being able to identify the various noises coming from your furnace can allow you to get the jump on the necessary repairs.

  • Thumping. Though there could be other reasons, the most likely cause for thumping is due to a blower motor or wheel that has become unbalanced. This can be a relatively easy fix, especially if you have experience with furnaces.
  • Scraping. The most likely reason that you are hearing scraping coming from your furnace is because of a loose or broken component. The most likely components impacted will be a blower wheel or a motor mount, though it could be a loose connector as well.
  • Humming. It is okay if the furnace hums lightly while in use. When that humming gets louder, it could be the result of a loud transformer. A persistent humming can be the result of a bad capacitor or fan and may require replacement.
  • Rumbling. If you are hearing rumbling coming from your furnace, shut the burner off immediately. Rumbling can be serious and should be handled by an HVAC professional. Do not attempt to resolve rumbling issues on your own.
  • Squealing: A squealing noise coming from your furnace is usually an indication that the blower belt is not lubricated enough or the fan belt is loose. In either case, you should contact an HVAC professional to diagnose and fix the problem.

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Related Questions

Why does my furnace make a banging noise when it turns on?

We now know why there may be a banging sound when the furnace turns off, but what about when it starts up? When there is a delayed ignition in the furnace, it can cause a sharp boom or bang to be heard as the burner cycles itself on.When running normally, the electric igniter or pilot flame should ignite the gas as it flows through the open gas valve. If it takes a moment to produce, the excess gas can result in a loud banging sound inside of the furnace cabinet.

Can a gas furnace explode?

While it is possible for a gas furnace to catch fire or explode, it is very unlikely. If there is any danger than an explosion might occur, the furnace will typically turn off automatically, as they are designed to do so. If you have a quality furnace and always perform the necessary routine maintenance, you should never have to worry about explosions or fires.

Ryan Womeldorf
Ryan Womeldorf

Ryan Womeldorf has more than a decade of experience writing. He loves to blog about construction, plumbing, and other home topics. Ryan also loves hockey and a lifelong Buffalo sports fan.

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