Why is my boiler making noise when it’s not in use?

3 June 2024

Hearing unexpected noises from your boiler can be disconcerting, especially when it’s not in use. We’ve already written about the noises your boiler might make when it’s running, so if that’s what you’re worried about, please read the article and get in touch with us if something looks like it needs our attention.

Understanding why your boiler could be making noises even when it is seemingly inactive helps in unearthing potential issues and maintaining the system’s health and safety. This article covers both the normal noises and the potentially problematic noises a boiler might make when it’s not running.

Sounds that are normal in a boiler that is not running

These are sounds that should be nothing to worry about – they’re all part of a healthy boiler that’s operating as expected. Of course, if your boiler is making these sounds and you’re not convinced that these are the reasons, it’s always worth calling out a boiler specialist. But hopefully this list will put your mind at ease.

Expansion and contraction

When heated, metal expands and similarly contracts when cooled. Your boiler, which is largely made of metal components, can emit clicking or popping sounds as it cools down after heating. You’ve probably heard a car making exactly the same noises if you stop in a quiet place. This is usually normal and indicates the metal returning to its resting state.

Pressure releases

Some boiler systems are equipped with pressure relief valves which naturally emit a small amount of steam or water. If those vents operate infrequently, they might release slight noises as they manage internal pressure. This is critical for the safe operation of the boiler, and it’s quite normal to hear a slight hissing sound that lasts a few seconds every now and again.

Condensate pipe drips

If your boiler includes a condensate removal system (common in condensing boilers), you may hear occasional dripping sounds. This simply means that the condensate produced from the burning gases is being drained away. This will continue after the boiler has stopped operating, and does not necessarily indicate a leak. Leaks tend to produce visible drips or puddles underneath the boiler, but normal condensate removal will be designed into the boiler.

Quiet buzzing

Your boiler has a host of circuitry and electrical components inside it. From transformers and rectifiers to the display unit and warning lights, many components can create an almost imperceptible buzzing on their own. In combination, however, the buzzing or vibrating sound can become more audible when the boiler is switched off. Remember – many of these electronics need to be running 24/7. It’s probably nothing to worry about, unless it’s noticeably louder than usual, as that could indicate a short-circuit or loose connections somewhere.

It’s worth noting that your boiler is never really turned off – it just isn’t heating water all the time. The sounds listed above are the boiler doing what it needs to do so it’s ready for action when you turn on the hot tap or put the central heating on.

However, while these sounds can be part of regular boiler operations, persistently loud noises or dramatic changes in the type or volume of these noises might suggest issues that require professional inspection. 

Noises that could indicate a fault in a boiler that’s not running

Now, we’ll look at (or, should we say, listen to), some of the sounds that could require some further investigation in a boiler that isn’t heating water. In short, your boiler should make a small amount of noise when it’s off, but anything other than the sounds covered above could be a cause for concern.

Gurgling or bubbling

Persistent gurgling noises from your boiler when it’s not running could signify air trapped within the system or low water pressure, both of which can affect the efficiency of your heating system and could lead to more serious problems.

Solution: Bleeding your radiators to release trapped air may resolve this issue. If the problem persists, check the pressure gauge and consider re-pressurising the system. Failing this, please seek professional help.

Excessive humming or vibrating sounds

A boiler that makes loud buzzing or humming sounds while off might be suffering from an electrical issue, such as a failing pump, loose wiring or malfunctioning thermostat causing the pump or fans to run unnecessarily.

Solution: It’s a good idea to call a certified professional to investigate electrical components safely. Anything that involves removing the boiler’s cover is not a job for regular householders.

Whistling sounds (kettling)

This sound, similar to a kettle boiling, often indicates that there is scale or sludge build-up on the boiler’s heat exchanger. This can cause hot water to become trapped in the heat exchanger, overheating and boiling the water. In some cases the sound can continue for a few minutes after the boiler is switched off.

Solution: A thorough cleaning or descaling by a professional can typically resolve this issue.

Bangs or rattling

Loud banging or rattling suggests mechanical issues or potentially serious malfunctions. This could be due to anything from loose internal components, external piping becoming unsecured, or more seriously an imminent failure. If this happens when the boiler is not running, something has gone seriously wrong, and the boiler should definitely not be turned on again.

Solution: Loud noises that sound like something has broken usually mean just that – seek professional advice straight away and if you know how, turn off the water, electricity and gas supply to the boiler.

If you smell gas, open windows and doors, evacuate the property and call 0800 111 999.

Familiarise yourself with the normal boiler sounds

Some noises from an inactive boiler are normal and result from routine processes. Others, however, demand further investigation to avoid potential safety risks or damage. Always err on the side of caution. If you’re unable to eliminate normal causes for mysterious boiler noises, or the noises are particularly loud and persistent, it’s wise to seek professional advice and service for your boiler. Regular maintenance checks by qualified professionals are key in ensuring the safety of your boiler system.