Boiler Humming Or Vibrating

What's happening?

Every boiler hums a little because it has a pump and, in modern ones, a fan. What you are listening for is a change: a hum that is louder than before, one that has a rattle or a whine in it, or one you can feel in the walls and pipes. Where the sound comes from tells you most. A hum from the boiler body that appears when the burner starts is usually the fan, whose bearings wear. A hum that is constant while the heating runs points to the pump, which can be partly air-locked or on its way out. A hum felt in the pipes when hot water runs, in a house with high mains pressure, comes from valves vibrating. And a hum with a rattle can be something simply loose, a pipe clip, the casing or the flue.

What usually causes it?

• A worn fan or fan bearings, usually louder at start-up and as the boiler modulates
• A pump that is air-locked, running on the wrong speed setting or wearing out
• High incoming water pressure making valves hum in the boiler or house pipes
• Limescale or sludge restricting the heat exchanger so the pump has to push harder
• A loose pipe clip, pipe against a joist or a rattling boiler casing
• A restricted flue or air supply making the fan work harder

Is it dangerous?

Not usually, but treat a new and worsening hum as a warning. A fan on its way out can seize and lock the boiler out, and a restricted flue is a combustion issue.

Can I fix it myself?

Bleed the radiators and check the pressure is around 1.2 bar. Then work out when it hums: only when the boiler fires, only with the heating, or only when a tap runs. Look for a loose pipe clip you can see in the airing cupboard or loft and tighten what is obviously loose. Do not remove the boiler casing.

When should you call a heating engineer?

Call a Gas Safe registered engineer if the hum is loud, has changed recently, comes with a fault code or rattle, or the boiler shakes. Ask about the fan and pump first. Call a plumber if it only happens when a tap runs, which is a pressure or valve matter outside the boiler.

What will the heating engineer actually do?

• Locate the sound with the casing off, running the boiler and the pump separately
• Check the fan, its bearings and the air pressure switch
• Bleed and test the pump and set its speed for the system
• Check incoming pressure and fit a pressure reducing valve if it is too high
• Descale or flush the heat exchanger and system where flow is restricted
• Secure any loose pipework, bracket or casing panel

Who fixes this

• Boiler repair

Questions people ask

Sources & References

1. Vaillant: why is my boiler making a noise?
2. Vaillant: what is a heat exchanger?
3. Vaillant: what is a magnetic boiler filter?
4. Vaillant: most common boiler problems