Why Does My Boiler Make A Banging Noise When Firing Up?

November 16, 2017


When a very large boiler makes loud banging noises, you’re right to be concerned. So what’s causing that noise and is it dangerous?

Well, it depends:
  • If your boiler makes a single BANG only when it fires up, it’s most likely due to delayed ignition.
  • If your boiler makes recurring banging, popping noises as it operates, it’s most likely due to “kettling”.

We’ll explain both of these problems, what causes them and how to stop the banging noises.

Need a professional to inspect and repair your boiler? Just contact us and we’ll send one right over.

If your boiler makes a single BANG when it fires up…

...the problem is most likely delayed ignition.

Delayed ignition is a problem that’s common in gas boilers that use pilot lights (as opposed to electronic ignition systems). 

You see, during normal operation, when a gas boiler receives the call for heat, the gas valve opens and releases a small amount of gas into a sealed combustion chamber via small “jets”. That gas is then ignited by the pilot light and the jet flames heat up a “heat exchanger”. That heat exchanger then is used to heat water. 

Need a more in-depth look at the boiler process? Just check out our blog, “How a Boiler Works”.

But “delayed ignition” is when the gas that’s sent into the combustion chamber isn’t ignited immediately. And when that happens, gas builds and builds inside the combustion chamber until BANG—the pilot light finally ignites all that gas, creating a mini-explosion. 

Problems that cause delayed ignition:

  • Not enough gas
  • Not enough surrounding air
  • Dirty burners
  • A bad gas pilot light

What to do:

Have a professional examine your furnace as soon as possible. If ignored, the intensity of the flames caused by the gas buildup can eventually cause your boiler’s heat exchanger to crack (easily a $1,200+ repair).

Beyond being expensive, a cracked heat exchanger can also lead to carbon monoxide poisoning. You see, the heat exchanger contains the combustion chamber, which is where dangerous combustion gases are produced. And tiny cracks in the heat exchanger allow toxic carbon monoxide to escape into your breathing air, potentially leading to carbon monoxide poisoning.

If your boiler makes recurring banging, popping noises as it operates…

...the problem is most likely “kettling”.

Kettling can happen in any kind of boiler. It essentially means that your boiler’s heat exchanger (the part that actually heats the water) is too hot. When this happens, water that comes into direct contact with the heat exchangers gives rise to tiny steam bubbles. As soon as those bubbles drift away from the heat exchanger into cooler water, they burst—creating that persistent banging/popping noise you hear.

Problems that cause kettling:

  • Limescale deposits on the heat exchanger. Limescale is common in areas with “hard water” (water with a high amount of dissolved minerals). As the layer of limescale builds on the heat exchanger, it acts as a barrier between the heat exchanger and the water. Because the water is much slower to heat due to this barrier, the burners stay on longer, causing the heat exchanger to overheat. And when this happens, small pockets of water trapped between the heat exchanger and the limescale heat up to the point of steam bubbles (which burst as they escape).
  • Water pressure/flow is too low. When the water pressure/flow is too low, water runs over the heat exchanger much slower than it should. This slower pace causes the water to overheat, giving rise to steam bubbles which erupt forcefully when they reach cooler water.
  • Bad thermostat. If the boiler’s thermostat is faulty, it will allow the burners to stay on longer than it should, causing the heat exchanger to overheat. And when water comes into contact with the super hot heat exchanger, steam bubbles are created, which then erupt noisily.
  • Water temperature on boiler is set too high. Setting your boiler to a very high temperature can overheat the water, causing steam bubbles.

What to do:

Have a professional inspect your boiler to determine what’s causing your boiler kettling. They’ll fix the problem to get your boiler back to normal, quiet operation.

Need help from a NY tech?

If you’re still struggling with a boiler that’s making loud banging noises, we can help.

Just contact us and we’ll send over a tech that can inspect and repair your boiler.

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