Is your furnace making a banging noise repeatedly? Here’s the thing: a furnace should make some noise when it starts. You will normally hear the inducer come on, the burners ignite, and then the blower starts moving air through the house.
What you should not hear is a loud bang, boom or repeated knocking sound.
When our heating and cooling technicians at Hamilton Home Comfort get a furnace noise call, one of the first things we try to figure out is exactly when the bang happens. That usually gives us a much better idea of where to start looking.
A bang when the burners light is very different from a bang coming from the ductwork after the furnace shuts off. Let’s explore some of the top reasons why you may be hearing a loud furnace bang.
You Hear A Furnace Banging Noise When the Burners Light is Delayed Ignition
If you hear a noticeable boom right when the furnace lights, this is the one we would not ignore.
Your furnace burners should ignite smoothly.
If ignition is delayed, gas can enter the burner area before it fully lights. When it finally ignites, you may hear a bang or small boom.
There are several reasons this can happen, including dirty burners, ignition problems, burner alignment issues or improper combustion.
A furnace should not boom every time it starts.
If the sound is clearly happening at ignition, we would recommend shutting the furnace down and having it looked at rather than continuing to run it and hoping the noise goes away.
Sometimes the Furnace Making a Banging Noise Is Actually the Ductwork
Your loud furnace bang can have a simple reason. Sheet metal ductwork expands when hot air moves through it and contracts again as it cools.
That movement can create a pop or bang that sounds like it is coming directly from the furnace, even when it is actually happening several feet away in the duct system.
We tend to see this more in homes where the ductwork moves under pressure or where there are larger flat sections of sheet metal that flex when the blower starts.
In some older Hamilton homes, the ductwork has also been changed or added onto over the years. That can sometimes create sections that are more prone to movement and noise.
A single pop as the duct heats up is generally very different from a violent bang coming from the burner compartment.
Furnace Banging Noise May Be Airflow Within Ductwork
If the duct system is under too much pressure, you may hear the sheet metal flex when the blower starts or stops.
A dirty filter can contribute to this, but so can closed vents, restricted return air, an incorrect blower setting or ductwork that is simply too small for the amount of air the furnace is trying to move.
This is why we do not automatically assume a banging noise means something inside the furnace has failed.
Sometimes the furnace is exposing an airflow problem somewhere else in the system.
A Repetitive Loud Furnace Bang Can Point to the Blower
The type of noise matters.
If you hear one pop when the furnace starts, ductwork may be involved.
If you hear a repeated banging, knocking or thumping sound while the blower is running, we would be more suspicious of the blower assembly.
A blower wheel can become loose, damaged or unbalanced. In some cases, it can begin contacting the housing as it rotates.
Something loose inside the blower compartment can also create a repetitive noise.
This is not something we would recommend letting run indefinitely. A relatively small blower issue can turn into a much more expensive repair if the wheel or motor becomes damaged.
A Furnace Banging Noise Can Be Something Simple
Not every furnace noise is serious.
We have seen access panels vibrate, sheet metal move, pipes touch ductwork, and other small issues create surprisingly loud noises.
That is why actually listening to the furnace and identifying where the sound originates matters.
Replacing parts based on the description “my furnace is banging” would be guessing.
A technician should be able to reproduce the noise, determine when it happens and work backward from there.
Pay Attention to When the Noise Happens
This is probably the most useful thing you can do before calling for service.
If the bang happens the instant the burners ignite, tell the technician.
If it happens when the blower starts, mention that.
If you hear it after the furnace shuts off, it may be the ductwork contracting as it cools.
If it is continuous or repetitive while the furnace is running, there may be a mechanical component moving or contacting something it should not.
Those details can save a lot of time during diagnosis.
When Is a Furnace Banging Noise Serious?
A loud boom at ignition should be taken seriously.
You should also stop operating the furnace if you smell natural gas, notice unusual burner behaviour, smell something that seems related to combustion or have a carbon monoxide alarm activate.
A small duct pop is one thing.
A furnace that sounds like a small explosion every time it lights is something completely different.
Is Your Furnace Making a Banging Noise Continuously?
Start by checking the simple things.
Look at your furnace filter. Make sure your supply vents and return air grilles are open. Listen carefully and try to determine exactly where the sound is coming from and at what point in the heating cycle it happens.
If the noise continues, have someone inspect it rather than trying to diagnose a gas furnace by sound alone.
At Hamilton Home Comfort, our technicians work on furnaces throughout Hamilton, Stoney Creek, Ancaster, Dundas, Binbrook and the surrounding area, and unusual furnace noises are something we diagnose regularly during the heating season.
The important part is not simply making the noise disappear.
It is figuring out why the furnace is making a banging noise in the first place and whether it is a combustion issue, an airflow problem, moving ductwork or a mechanical component inside the furnace.
If your furnace has suddenly started banging, booming or knocking, that change in sound is worth paying attention to.
Call Hamilton Home Comfort and one of our licensed HVAC technicians can inspect the system, diagnose the problem and explain what is causing it before recommending a repair.
Call Hamilton Home Comfort at 905-643-098
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