Why Is My Boiler Making Loud Noises?

If your boiler is making loud noises, you’re right to be concerned. While your boiler may make some mild noises as it runs, louder noises could be a sign of something more problematic, especially if it’s a sound you’ve never heard before.

When homeowners call in about loud boiler noises, most of the time their system is making a:

• Banging noise
• Humming or vibrating noise
• Gurgling noise

Below, we’ll explain all 3 of these noises, what causes them and how to stop them.

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If your boiler makes a single BANG when it fires up (and only when it fires up), it’s most likely due to delayed ignition.

However, if your boiler makes recurring banging and popping noises as it operates, it’s most likely due to “kettling”.

Let’s look at both of these issues below.

If your boiler is making a single BANG when it fires up…

...the problem is most likely delayed ignition.

Delayed ignition is a common problem in gas boilers which use pilot lights (as opposed to electronic ignition systems). 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. That gas is then ignited by the pilot light.

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

Typically, delayed ignition is caused by:

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

What to do:

Have a professional examine your boiler 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, so it’s important to have a professional take a look at your boiler ASAP.

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

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

Kettling means that your boiler’s heat exchanger (the part that actually heats the water) is too hot. When this happens, water that comes in direct contact with the heat exchangers causes 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.

Kettling can be caused by:

• Limescale deposits on the heat exchanger. When 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, which in turn causes steam bubbles to erupt when they reach cooler water.
• Bad thermostat. If the boiler’s thermostat is faulty, it will allow the burners to stay on longer than they 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 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

If you hear a loud humming or vibrating noise coming from your boiler, it’s likely because of:

• An issue with the circulating pump
• Loose components

Circulating pump issue:

The circulating pump moves water from your boiler to your home’s radiators (and vice versa). If the circulating pump is not operating normally or set to high of a speed, it can cause a sort of humming or vibrating noise.

What to do:

Check to see if the circulator pump is abnormally hot to the touch, or if it is leaking. Your pump should be located on a pipe near your boiler. You can also check the speed settings and adjust them if necessary. Just keep in mind, not every circulator pump has adjustable speeds. If it does, you will see a dial on the pump itself. Turn the dial down to a lower speed and see if the humming noise stops.

If your boiler continues to work properly (boiler isn't cycling on and off too fast), there shouldn’t be an issue with setting your circulating pump to a lower speed. However, if your boiler starts to struggle to heat your home, or if your energy bills increase, you’ll want to reach out to a professional to check your settings.

If you aren’t sure how to change the speed of your circulating pump, or if you do change the speed and the humming continues, you’ll want to reach out to a professional.

Loose components:

If there are any loose components of your boiler, like a bracket or a panel, they can make a humming or vibrating noise when your boiler is on.

If your boiler was recently maintained or repaired, this could be your issue.

What to do:

Our suggestion would be to take a look at your boiler when it’s making noise...is the entire boiler vibrating or just one part? You can tighten any obvious loose parts or brackets, but if your entire boiler is vibrating, you’ll need to reach out to a professional to secure your boiler.

If your boiler is making a gurgling noise, it's most likely due to:

• Trapped air
• Low water pressure

Trapped air:

If there is air trapped inside your boiler system, your boiler can make a gurgling noise as the air and water move through the system. While some air is normal, too much can cause a gurgling sound.

What to do:

To get rid of any unwanted air in your boiler system, you will likely need to bleed your radiators. If you're familiar with bleeding your radiators and have done this in the past, we'd suggest you do so to get rid of the air in your system.

If you'd like to attempt bleeding your radiators yourself, you can follow the below steps:

Step 1: Turn your heat on and wait until your radiators have reached max heat. If you run your hand near the top of your radiators and feel any cool or cold spots, that's a sign that your radiators need to be bled.

Step 2: Turn the heat off at the boiler and wait until the radiators cool off. You can easily be burned by hot radiators, so make sure to let your system cool down completely.

Step 3: Using a bleed key or screwdriver, turn the key in the bleed valve radiator counter clockwise until a little bit of water comes out. Make sure not to open the valve too far as you’ll release water quickly. Once a few drops of water have come out, close the valve.

Step 4: Repeat this process for every radiator. Make sure you close each valve after the radiator is bled.

If you are unfamiliar with bleeding your radiators, don’t worry. A profession can have your boiler back to normal in no time.

Low water pressure:

Another reason your boiler may be making gurgling noises is because of low water pressure.

When the pressure in your boiler is low, it can allow air in, which, as we just discussed, causes a gurgling noise. Typically, the pressure if your boiler should stay between 1 to 2 bars. If the water pressure is below this, that's likely why you're hearing a gurgling noise.

What to do:

Check your boiler’s water pressure.

Typically, the pressure gauge is located on the control panel or near the pipes below the boiler. The pressure should be between 15-20 psi or 1- 2 bars. If the water pressure of your boiler is below this, you’ll want to reach out to a professional for help.

If you notice that your water pressure is declining steadily, you'll want to reach out for help as soon as possible as this is usually an indication that there is a leak in the system.

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.