Boiler Gurgling

What's happening?

A boiler gurgles when water moves through the system past trapped air, or when a waste pipe is not draining freely. The sound is more like a drain than a kettle, and that separates it from the banging or rumbling of limescale. Three places produce it. Air in the radiators and pipework gurgles as the pump pushes water round it, most noticeably when the heating first comes on. Low system pressure lets air in and makes the same noise. And the condensate pipe, which carries the acidic waste water from a condensing boiler to a drain, gurgles when it is partly blocked or frozen, because water and air are fighting through it. That last one is the one to watch in winter, because it goes on to shut the boiler down.

What usually causes it?

• Air trapped in radiators, pipes or the pump after draining, a leak or topping up
• System pressure below about 1 bar, which lets air in
• A condensate pipe that is partly blocked, frozen or running uphill
• Sludge restricting flow so the water has to force past it
• An air-locked pump or a failing auto air vent
• A dripping or leaking joint drawing air in

Is it dangerous?

No for the sound itself. It matters in winter, because a condensate pipe that gurgles in a hard frost will freeze solid and lock the boiler out.

Can I fix it myself?

Bleed the radiators one at a time, starting with the lowest floor, with the boiler off and cool. Then check the pressure gauge and top up to about 1.2 bar. In freezing weather, find the plastic condensate pipe outside, usually a white pipe running to a drain, and pour warm, not boiling, water over it if it is frozen. Do not open the boiler.

When should you call a heating engineer?

Call a heating engineer if the gurgling continues after bleeding and repressurising, if the pressure keeps falling, or if the condensate pipe freezes more than once. Call a Gas Safe registered engineer if the boiler is also locking out, since that points to the flue or condensate trap.

What will the heating engineer actually do?

• Bleed the system and the pump and find where air is getting in
• Check pressure, the expansion vessel and the auto air vent
• Clear and test the condensate trap and re-fall or insulate the pipe
• Flush the system and fit a magnetic filter where sludge is present
• Check flow through the boiler and the pump speed setting

Who fixes this

• Boiler repair
• Heating not working

Questions people ask

Sources & References

1. Vaillant: why is my boiler making a noise?
2. Vaillant: getting ready for winter, your condensate pipe
3. Worcester Bosch: frozen condensate pipe
4. Vaillant: how to repressurise a boiler