Radiators Keep Filling With Air

What's happening?

Bleeding a radiator once after a repair is normal. Bleeding the same radiators every few weeks is not, because air does not appear from nowhere. It is either coming in, through a leak or an air vent on the return pipe that sucks air as the pump runs, or it is being made inside the system, by corrosion producing gas, or coming out of the water as it heats after a top-up. The boiler's pressure gauge is the most useful clue. If the pressure drops between bleeds, water is leaving and air is entering in its place. If it holds steady and the radiators still fill, the likelier causes are corrosion in a system without inhibitor, or a vent in the wrong place. On an older open-vented system a low or failing loft tank can also pull air in.

What usually causes it?

• A slow leak at a valve, joint or pump, letting air in as water leaves
• An automatic air vent on the return pipe or at the pump that draws air as it runs
• Corrosion inside an uninhibited system, producing gas that collects in radiators
• A pump fitted on the return side or set too fast, lowering pressure at the top
• A failing expansion vessel causing pressure swings that pull in air
• A low or leaking feed and expansion tank on an open-vented system
• Air released from fresh water after repeated topping up

Is it dangerous?

No. The harm is slow: air and corrosion cause cold spots, noise and sludge, and the boiler may lock out on low pressure.

Can I fix it myself?

Note the pressure gauge reading each morning for a week after bleeding and topping up to about 1.2 bar. Falling means a leak, so look under radiator valves and along pipes. Steady pressure with returning air is something an engineer can test. Ask an engineer whether the system has inhibitor in it, since they can add it.

When should you call a heating engineer?

Call a heating engineer if you need to bleed the same radiators more than once a month, if the pressure falls, or if radiators are noisy and cold at the top again within days. They find the source, fit an air vent in the right place and treat the system.

What will the heating engineer actually do?

• Pressure test the system to find a leak that shows as falling pressure
• Check the position of the pump and any automatic air vents
• Test the expansion vessel charge and the loft tank level and float valve
• Test the water and system for corrosion and add inhibitor
• Flush the system and fit a magnetic filter and air separator where needed

Who fixes this

• Heating not working
• Power flush

Questions people ask

Sources & References

1. Vaillant: how to repressurise a boiler
2. Vaillant: why is my boiler losing pressure?
3. Vaillant: what is a magnetic boiler filter?
4. Vaillant: what is an expansion vessel?