Why Is My Boiler Humming or Buzzing?

A boiler that hums or buzzes is usually telling you about the pump or about vibration. A steady low hum that starts when the heating comes on is most often the circulating pump, either running faster than the system needs or starting to wear. A buzz or rattle that seems to come from the wall or floor is often normal vibration being amplified by a loose casing panel, mounting bracket or pipe clip. Air in the system and, less often, an electrical component such as a relay or the gas valve can also hum. Most causes are not dangerous, but a noise that is new, getting louder or comes with a burning smell should be checked.

Is It Normal for a Boiler to Hum?

A quiet hum while the boiler is running is normal. The fan and the pump both make some noise, and you will usually hear them when the heating or hot water first starts.

What matters is change. If the boiler has always made a gentle hum, that is just how it sounds. If it has become louder, harsher, started buzzing or rattling, or now carries on when the heating is off, something has shifted. Note when it happens: only on heating, only on hot water, all the time, or just when the boiler first fires. That one detail narrows the cause down more than anything else.

A Hum That Starts With the Heating: The Pump

If the humming starts when the heating comes on and stops when it goes off, the pump is the most likely source. It is the part that pushes water around the radiators, and it runs whenever the heating is calling.

There are three common pump-related causes:

• Speed set too high. Many pumps have speed settings. Set higher than the system needs, the pump drives water faster than the pipes can take quietly, and you get a hum or a rushing noise through the pipework.
• Worn bearings. As a pump ages, the bearings wear and the steady hum turns into a grinding or buzzing sound. It may also run hot.
• Air in the pump. Trapped air makes the pump rattle or buzz, and it cannot move water properly, so some radiators may stay cool.

On a system or regular boiler, the pump is often a separate unit near the hot water cylinder, so the noise may seem to come from the airing cupboard rather than the boiler. On a combi it sits inside the casing. Either way, adjusting, bleeding or replacing a pump is a job for an engineer. Our central heating pump repair service covers testing and replacement.

Vibration Through the Casing, Brackets or Pipes

If the noise is a buzz or rattle that you can feel as well as hear, the boiler's normal vibration is probably being amplified by something loose. It is one of the most common causes and one of the cheapest to put right.

Typical culprits are a front panel that is not fully secured, a mounting bracket that has worked loose, or copper pipes that touch each other, a joist or the wall without a clip. Boilers fitted to thin stud walls or inside kitchen cupboards can also sound louder than the same boiler on a solid wall.

You can look at any pipework you can see outside the boiler, under it or in cupboards, and note where it touches something. Do not remove the boiler casing. The panels on a gas boiler form part of its sealed combustion chamber, so anything under them is for a Gas Safe registered engineer.

Air in the System and Low Pressure

Air in the heating system can cause buzzing and gurgling as it passes through the boiler and pump. It often comes with radiators that are cold at the top.

Check the pressure gauge on the boiler. If it reads low, top it up to about 1 to 1.5 bar, following our guide to repressurising a boiler safely. Bleeding radiators with the heating off and cold can also release trapped air, but it lowers the pressure, so check the gauge again afterwards. If air keeps coming back, the system may be drawing it in somewhere or producing gas from corrosion, and an engineer should look at it.

Electrical Buzzing From the Boiler or Controls

A sharp electrical buzz is different from a mechanical hum. It usually comes from a relay, transformer or wiring centre, and occasionally from the gas valve inside the boiler.

If the buzz comes from a programmer, thermostat or wiring box rather than the boiler itself, the component may be failing or a connection may be loose. If it comes from inside the boiler and changes when the burner lights or shuts off, the gas valve or a control component could be the source. Neither is something to investigate yourself. If you notice a burning or hot plastic smell, scorch marks or the boiler display flickering, switch the boiler off at the fused spur and call an engineer.

Squeaking, Screeching and Other Sounds

A squeak or screech is not quite the same as a hum. It usually comes from a worn bearing in the pump or fan, or from pipes creaking as they expand against floorboards.

Pipe creaks and ticks as the system warms up and cools down are common and harmless. A high-pitched squeal from inside the boiler that gets worse over time is more likely a bearing on its way out and should be looked at before it fails. If the sound is more of a whistle or kettle-like rumble, that points to a different set of causes, which we cover in our guide to what different boiler noises mean.

When to Call an Engineer

Call an engineer if the hum or buzz is new and getting louder, if it comes with a fault code or lockout, or if radiators have started heating unevenly at the same time. The same applies if you have topped up the pressure and bled the radiators and the noise has not changed.

Before you ring, note when the noise happens, where it seems to come from and what the pressure gauge reads. That helps the engineer tell a pump problem from a vibration or electrical one on the first visit. We work across London, and you can book a visit to have the noise traced and fixed.