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Why Does My Furnace Make a Loud *BANG/BOOM* Noise When it Turns On?

Here’s a common issue we hear from homeowners with furnaces: “Why does my gas furnace turn on with a loud BANG?”

There are two possible causes of the noise:

  • Delayed gas ignition
  • Expansion and contraction of metal ductwork

We’ll explain these 2 in more detail and what needs to be done to solve them.

Delayed gas ignition: An explosive problem

If the banging noise is located at the heater this is most likely the problem.

Delayed gas ignition is when gas in the furnace builds up rather than igniting immediately. When the gas finally ignites after building up a while, a mini explosion happens—possibly harming your furnace.

Causes of delayed ignition

  • Too much air mixed with the gas, making it hard to ignite
  • Too little gas at the burners caused by low gas supply pressure
  • Dirty, restricted or weak pilot light (if you have an older furnace)
  • Misaligned or dirty burners

You’ll need a professional heating technician to solve these problems.

Expanding and contracting air ducts: Metal under pressure

Do you hear the banging noise several feet away from the furnace?

The issue could be in the rigid metal ductwork that’s expanding and contracting when the heating system’s blower starts.

The ducts expand at the supply side because there’s too much positive pressure, pushing the ductwork out. The ducts contract at the return side because of the negative pressure, allowing the surrounding air to push the ducts in.

Here’s a quick video about static pressure.

Causes of expanding and contracting air ducts

  • Undersized ductwork
  • Dirty air filter
  • Closed air vents or dampers
  • Dirty ductwork

Do this: Change the air filter and make sure your home’s supply vents are open.

If that does not solve the problem, you’ll need a professional to find a solution.

Minneapolis Saint Paul Plumbing, Heating and Air has served the Twin Cities since 1918.