Your boiler should run smoothly and relatively quietly whether it’s heating or not. People who have their boilers hidden away in a compartment or in a garage might never hear them at all, but if yours is exposed, there are some sounds that are normal, and some that will require expert investigation.

In this article, we’ll look at both kinds, so you can judge whether to call an engineer. However, if you’re in any doubt, we’d always recommend making that call just in case. Fixing a faulty-sounding boiler can be cheap and effective – ignoring it could be expensive or even dangerous.

Normal sounds your boiler should make

Just like a car or even a laptop, your boiler contains moving parts, and inevitably, that’s going to create noise. Boilers are doing some pretty hard work when they’re on – heating water and pumping it around is it quite intense, so you shouldn’t be surprised that sounds come from within it.

If you get a new boiler, the engineer will test it, so listen to the sounds it makes and ask any questions about where they’re coming from. That way, you’ll better identify anything abnormal.

Here’s what you should hear:

Quiet whirring sound

If the heating is on, it’s normal to hear a quiet whirring. This is the pump circulating water around the boiler’s burner and heat exchanger, and possibly pumping your hot water to the radiators.

Deep rumbling sound

When you turn on your gas heater, the flames ignite and start vigorously boiling the water passing over them. Although it should be quiet, it might still be audible, just as you can hear your gas burner on your cooker hob.

Occasional quiet clicking

Modern boilers have information coming from thermostats and the control panel, and this triggers switches and valves inside the boiler as components are switched on and off and water is redirected. Although they are usually silent, you might hear them from time to time. 

Boiler noises that require investigation

Once you’ve got used to the sounds your boiler makes in normal operation, you’ll notice straight away if it’s behaving unusually. These are the most common sounds.

Banging noises

More often than not, a banging noise from your boiler is traced back to a process called “kettling”. This is usually when limescale builds up on the heat exchanger and restricts the flow of water to the point where it literally boils. (Contrary to the name, boilers shouldn’t actually boil water to 100 °C – it’s usually in the 65–70 °C range.) The banging sound you hear is the water turning to steam and expanding, just like the sound you get from a kettle.

In the absence of limescale, it’s possible that you have a faulty boiler thermostat (not the room thermostat) or that it’s set up incorrectly so it’s overheating the water.

Whistling sounds from the boiler

While kettling can cause a whistling sound (think of the old whistling kettles), it’s more likely that whistling is caused by trapped air in the system. It’s a relatively quick process for an engineer to release the air, but a good engineer will want to know how air is getting into the system, as that’s not normal.

Mechanical devices can’t be ruled out, either. Pumps that need oiling are prone to a squeaking or whistling sound. Again, leave it to your engineer to fix.

Gurgling noises

If you’ve ever gurgled mouthwash or emptied a bath, you’ll recognise this sound – a rapid, low, liquid sound caused by water being agitated and having air passed through it. So it will come as no surprise that the gurgling from your boiler is probably cause by trapped air. This air can end up finding its way into radiators too, and most people with central heating will have experienced that sound when they’ve switched theirs on. You can bleed your radiators as a first step. If this doesn’t work, it’s time to call in a professional – you might need to drain your whole system.

It’s also possible that your condensation pipe is frozen, which can happen in cold weather when it’s outside. This causes a blockage which stops proper circulation through the condenser. Again, you might be able to sort this out yourself by warming up the condenser pipe. Hot water bottles will do the trick. To prevent freezing in the future, consider insulating the pipe.

Grinding

A loud grinding noise will almost certainly be caused by a faulty pump. It’s vital that you switch off your system and call in an engineer, as a boiler without a pump is no use – and could even overheat.

What to do if your boiler is making a noise

In all the cases above, we’ve offered a few typical causes and some possible remedies, but any action by you should be a preliminary one just to rule out other causes. If you are in any doubt whatsoever about the cause of your boiler’s noises, please get in touch with a Gas Safe engineer.

Many of the issues that cause noises can be prevented by ensuring your annual services are carried out. Your engineer will identify any worn-out parts and replace the ones that have a limited lifespan, as well as checking for potential wear and tear.

Your boiler is too important to go without, but keeping it working is a matter of safety as well as comfort. Keep it checked, use it as the manual instructs and get help if you hear any unusual noises coming from it.

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