Where the gurgling comes from
Gurgling is the sound of water and air moving together. In a sealed heating system that should be full of water only, it means air or gas has got in, the pressure has fallen, or something is stopping water or condensate from draining properly. The most common causes are ones you can check yourself.
| Where you hear it | When | Likely cause | Who fixes it |
|---|---|---|---|
| Radiators and pipes | As the heating starts up | Air trapped in the system | You (bleed radiators) |
| Boiler and pipes, gauge under 1 bar | Whenever the pump runs | Low system pressure | You (filling loop), engineer if it keeps dropping |
| Boiler, then a lockout | Freezing weather | Frozen or blocked condensate pipe | You (thaw it), engineer if blocked inside |
| Inside the boiler near the pump | While the pump runs | Air lock or failing pump | Gas Safe engineer |
| Radiators need bleeding every few weeks | Ongoing | Corrosion producing gas, or a leak drawing in air | Heating engineer |
Air in the heating system
Air gets in when a system is drained or refilled, after work on radiators, or through tiny leaks. It collects at high points, usually the tops of radiators and pipework upstairs, and gurgles as water pushes past it. Radiators that are cold at the top and warm at the bottom are the classic sign. Gurgling that is loudest just after the heating switches on, then fades as the system warms up, also points to air rather than a fault with the boiler itself.
How to bleed a radiator
- Turn the heating off and let the radiators cool, so you are not dealing with hot water.
- Hold a cloth under the bleed valve at the top corner of the radiator.
- Turn the valve anticlockwise with a radiator key (or a flat-head screwdriver on some valves) by about a quarter turn. You will hear air hiss out.
- Close the valve as soon as water comes out in a steady stream.
- Repeat on other radiators that need it, starting downstairs and working up.
- Check the boiler pressure gauge. Bleeding lets water out, so the pressure often drops.
Low boiler pressure
When pressure falls below about 1 bar, the pump can draw in air and the water moving through the boiler starts to gurgle. Many boilers lock out when the pressure drops further. After bleeding radiators, top the system back up to 1 to 1.5 bar with the filling loop. Our step-by-step guide to repressurising a boiler shows how to find and use both external and built-in filling loops.
If you need to top up more than about once a month, water is escaping. That might be a weeping radiator valve, a leaking pipe joint or the pressure relief valve opening. Air can be drawn in at the same weak points. A Gas Safe registered engineer can find and fix the loss.
Frozen condensate pipe
Condensing boilers produce acidic water, called condensate, that drains away through a plastic pipe, often running outside to a drain. In a hard frost, the water in that pipe can freeze. The condensate then backs up into the boiler, which often makes a gurgling or bubbling sound before the boiler locks out with a fault code. The pattern to look for is gurgling when it is below freezing, a fault on the display and a condensate pipe that runs outside, especially over a long or exposed route.
You can thaw it yourself by pouring warm (not boiling) water along the outside section or holding a hot water bottle against it, working back from the open end, where it is most likely to be frozen. Then reset the boiler according to the manual. Our frozen condensate pipe guide covers where to look and how to stop it happening again. If the pipe is clear but the boiler still gurgles and locks out, the blockage may be in the condensate trap inside the boiler, which is an engineer job.
Never pour boiling water on a plastic condensate pipe, as it can crack the pipe or its joints. Do not open the boiler to clear the condensate trap yourself.
Pump problems and air locks
If air collects in the pump itself, the pump runs but moves little water. You may hear gurgling or rattling from inside the boiler, the heating may be slow to warm up and some boilers show a circulation or pressure sensor code, such as Vaillant F75 or Baxi E125. Bleeding the radiators and restoring the pressure sometimes clears it. If the noise continues, the pump may need venting at its own bleed point, or it may be sticking or worn. Those are jobs for a Gas Safe registered engineer, as the pump sits inside the casing on most combis. If the gurgling has turned into knocking, see our guide to a boiler making a banging noise.
Radiators that need bleeding every few weeks are a warning sign too. Corrosion inside the system can produce gas, which collects in radiators and makes them gurgle. An engineer can test the system water, clean the system and add inhibitor to slow the corrosion down.
Safe checks you can do yourself
- Check the pressure gauge with the system cold and top up to 1 to 1.5 bar if needed.
- Bleed radiators that are cold at the top, then recheck the pressure.
- In freezing weather, check and thaw the condensate pipe with warm water or a hot water bottle.
- Check the thermostat and timer are set correctly and the boiler is actually calling for heat.
- Press reset if the boiler has locked out, as described in the manual, but stop after 2 or 3 attempts.
Typical UK repair costs
| Job | Typical UK cost |
|---|---|
| Bleeding radiators and repressurising yourself | Free, apart from a radiator key costing a few pounds |
| Engineer visit to bleed the system and diagnose | Typically £60 to £120 |
| Condensate pipe rerouted, upsized or insulated | Typically £100 to £250 |
| Condensate trap cleared or replaced | Typically £80 to £180 |
| Automatic air vent replaced | Typically £100 to £200 |
| Pump replaced | Typically £200 to £450 |
| Chemical clean or powerflush with inhibitor | Typically £150 to £800 |
Simple visits come in at the lower end, especially outside London. Costs rise in London and the South East, where labour rates are higher, for long or awkward condensate runs that need ladder work, and for large systems where a powerflush takes most of a day (powerflush prices are set largely by the number of radiators). Pump prices depend on whether the engineer can fit a replacement pump head or needs the whole pump, and on the boiler brand.
If the gurgling only starts in freezing weather, a frozen condensate pipe is the usual cause. Our frozen condensate pipe guide shows how to spot it and thaw it safely.