My Furnace Makes a Boom/Bang Sound When Turning On and Off

March 28, 2017

Wondering what’s causing your furnace to make that scary boom/bang noise? Well, it all depends on where you hear the sound:

  • If the boom/bang happens at the furnace itself, it’s likely delayed ignition.

  • If the boom/bang sounds farther away from the furnace, it’s likely your air ducts expanding/contracting.

We’ll explain each of these issues in detail and show you how you can fix the problem. But first, let’s do some quick troubleshooting to confirm the cause behind those scary furnace noises.

Need professional repair ASAP? Just contact us and we’ll send over a tech to diagnose and repair your furnace.

Is it delayed ignition or air ducts? Here’s how to tell...

To troubleshoot the cause of those boom/bang sounds, follow these steps:

Step 1: Stand at least 5 feet from the furnace while a helper raises the set temperature on the thermostat to call for heat.

Step 2: Keep an eye on the furnace burners and continue on to Step 3.

Screen Shot 2017 03 28 At 1 44 49 Pm

Step 3: Determine whether the burners light immediately or not (they’ll appear blue when lit).

  • If the burners don’t light immediately and you hear several clicks followed by a small explosion at the burners, skip to the “Delayed ignition” section.

  • If the burners light immediately without any explosion and the sound happens farther away from the furnace, skip to the section titled “Popping ductwork”.

Possible cause #1: Delayed ignition

What is “delayed ignition”?

Delayed ignition is a very dangerous situation that can happen with gas furnaces.

You see, when your gas furnace gets the call for heating, a gas valve sends gas to your furnace’s burners to be ignited. But “delayed ignition” occurs when that gas isn’t immediately ignited.

Instead, the gas valve sends more and more gas, allowing gas levels to quickly build up in your furnace burner. And by the time the gas is ignited, it produces a mini-explosion (that’s the BOOM you hear).

During delayed ignition, these burners won’t ignite immediately. Source

Othersigns of delayed ignition include:

  • The boom/bang only happens when your furnace is turning on, not off

  • Clicking noises right before the BANG/BOOM noise (that’s the furnace trying unsuccessfully to ignite the burners)

What causes it?

The most common causes of delayed ignition include:

  • Not enough gas

  • Not enough surrounding air

  • A faulty pilot light

  • Gas burners that are dirty, blocked by debris or misaligned

What can I do to fix it?

Have a professional examine your furnace as soon as possible.

Delayed ignition can cause costly furnace damage. For example, delayed ignition can eventually lead to a cracked heat exchanger, which costs upwards of $3,000.

But a cracked heat exchanger isn’t just expensive, it’s also extremely dangerous. In fact, tiny cracks in your heat exchanger allow toxic carbon monoxide to escape into your breathing air, potentially leading to carbon monoxide poisoning.

Possible cause #2: Popping ductwork

Why is my air duct popping?

Problems with your ductwork can make ducts “pop” when the pressure inside changes. Think of how a paper bag expands when you blow into it and contracts when you suck the air out of it. Your air ducts work the same way.

You see, when your furnace turns on, the blower is both sucking in air via the return air ducts and blowing out air via the supply ducts. So when the furnace starts, those metal ducts expand and pop out on the supply side and cave in on the return side. Which means you’ll also hear the ductwork pop back into place when the furnace turns off.

But ductwork should be designed to withstand a certain amount of pressure (and not pop), so let’s take a look at some of the problems that can cause your air ducts to pop.

What causes it?

The most common issues that lead to popping ductwork include:

  • Ductwork that is too small for your HVAC system

  • Closed vents

  • A clogged air filter

What can I do to fix it?

Step 1: Check your air filter and replace it if it looks like the one to the right.

Dirty Vs Clean Fiberglass Air Filter1

Step 3: If you still hear the popping/banging noises, have a professional inspect your ductwork if you still hear the popping/banging noises. They’ll be able to determine the cause behind your noisy air ducts and offer the best solutions.

Need a professional to diagnose your booming/banging furnace?

If you still have questions about the noises your furnace is making, we can help.

Just contact us and we can send a tech out to inspect, diagnose and repair your furnace.

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