Why Is My Boiler Making a Banging Noise?

A noisy boiler is certainly irritating, but it can also indicate that something's not quite right with your unit's components. A boiler making a banging noise is a common complaint with several possible causes.

The problems that cause a boiler to make a banging sound often don't stop it from working altogether — at least, not at first. Therefore, it can be tempting to ignore the banging sound and keep using your boiler as usual to avoid paying for a technician. The bad news is that ignoring a banging boiler can cause it to break down or even pose a safety risk. Understanding why your boiler is noisy can help you figure out how to make it stop banging.

There are several potential reasons your boiler might be making a banging noise, and when the noise occurs is your biggest clue.

A Big Bang

Boilers with pilot lights sometimes make a single loud bang when you fire them up. This issue only occurs in gas boilers, and it happens because of an ignition delay.

When you fire up a gas boiler, the gas valve opens and allows gas inside the combustion chamber. The pilot light then ignites the gas to heat the heat exchanger that makes the water hot. However, sometimes a fault develops that stops the pilot light from igniting the gas right away.

When this happens, larger amounts of gas than usual build up inside the combustion chamber. This causes a loud bang when the gas finally ignites.

There are several possible reasons for delayed ignition inside a gas boiler. Too little gas or a dirty burner can prevent the pilot light from igniting the gas at the correct time. Alternatively, there may be a problem with your pilot light.

Recurring Banging

Constant banging usually comes down to an issue known as kettling. Kettling occurs when the heat exchanger overheats, creating steam bubbles in the water. These bubbles rise and eventually pop, causing a regular banging sound.

One of the most common causes of kettling is too much limescale on the heat exchanger. The limescale layer stops the water from heating up at the usual rate, causing the exchanger to say on for longer until it overheats. Any water between the exchanger and the limescale turns into steam bubbles and causes the boiler to bang.

Alternatively, the heat exchanger can overheat and cause kettling if you set the thermostat too high. A broken thermostat can also cause kettling if the heat exchanger stays on longer than it should.

Another common reason for kettling that doesn't directly involve the heat exchanger is low water pressure. Low water pressure stops the water from moving over the heat exchanger fast enough, causing it to get too hot and form steam.

How Do I Make My Boiler Stop Banging?

If your boiler is kettling constantly, you might be able to stop the banging by lowering the thermostat to prevent the water from boiling so fiercely. If that doesn't work, call a technician to inspect your unit and replace any defective parts.

There's no safe DIY method for fixing a boiler that bangs when you turn it on due to delayed ignition, and it's essential to get it fixed as soon as possible. Allowing a large amount of gas to ignite repeatedly inside the combustion chamber can crack the heat exchanger, which is an expensive part to repair.

Furthermore, a cracked heat exchanger can leak carbon monoxide. Carbon monoxide is odorless and colorless, and inhaling it can cause carbon monoxide poisoning and even death. If you hear a loud bang on startup, call a technician to check the condition of your heat exchanger and fix or replace the parts causing the ignition delay.