It’s terrifying. Your gas furnace, which normally works fine, kicked on with a scary BANG or BOOM noise. It’s not only disturbing, but it can wake light sleepers up in the middle of the night.

What the heck is going on?

There are 2 common causes of this bang noise:

  • Delayed ignition
  • Expanding air ducts

To know which is the problem, determine where the loud noise originates from. Stand near your furnace as it’s about to turn on and listen:

  • If the furnace made a clicking noise trying to turn on and then makes the BANG noise at the furnace, it’s most likely a delayed ignition.
  • If the noise originates a few feet away from the furnace when the blower turns on, the problem may be expanding air ducts.

Let’s go into detail about these 2 issues.

Delayed ignition

Imagine a room full of gas and then someone decides to light a match. Big problem, right?

Well, that’s what’s happening in your furnace if the ignition of gas is delayed.

You see, this is what normally happens when your thermostat calls for heat from the furnace:

  • The gas valve opens and sends gas to the burners where it’s then ignited by the ignitor (or pilot light for older furnaces).
  • Heat from the burner flames heats the heat exchanger
  • Your blower blows air over the heat exchanger and then pushes that heated air throughout your home.

(See how a furnace works.)

But a variety of issues can delay the combustion of that gas. So, the gas builds up in the furnace until—BOOM—it ignites, causing a mini-explosion.

This can harm the heat exchanger, which is very expensive part to replace.

Some causes of delayed ignition includes:

  • Dirty burners
  • Flames are lifting away from the burner outlet due to too much primary air
  • Too much gas

Expanding air ducts

It’s no mystery that expanding metal ducts would cause a loud bang noise.

But what causes it?

Usually, too much static pressure in the ductwork. Static pressure is stationary pressure in a confined space (it’s just like blowing air in a balloon). In this case, there’s too much pressure pushing on your ductwork, causing it to expand and make a BANG noise.

Causes of high static pressure include:

  • Undersized ducts
  • Closed supply vents
  • Dirty air filter—especially dirty, 1-inch pleated filters

So, what now?

If the noise came from your ducts, change the air filter (if it’s dirty). Also, open your supply vents and make sure they’re not blocked by furniture or curtains.

If that didn’t work, call a professional heating contractor to find the issue and fix it.

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Ragsdale Heating, Air, Plumbing & Electrical has been serving the metro Atlanta area for over 25 years. 

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