Airlocks in heating systems: how trapped air actually stops flow
Air is far more compressible than water, so any pocket of it trapped in a pipe or radiator simply absorbs the pump's push instead of letting water flow past it, stopping circulation dead at that point.
An airlock in the heating system: the short technical version
Radiator airlocks are usually straightforward, cleared with a bleed key at the top corner valve. Airlocks trapped in horizontal pipe runs are harder to shift and sometimes need a combination of raising system pressure briefly and running the pump at higher speed to force the pocket through to a bleed point. Repeated airlocks after bleeding, especially in the same radiator, usually mean a slow leak is drawing air back in through a joint rather than a one-off installation issue.
Things worth knowing about an airlock in the heating system
- Air is compressible, so it stops flow entirely rather than just slowing it
- Radiator bleed points clear most simple airlocks in seconds
- Pipework airlocks may need a pump speed increase to force them out
- Repeated airlocks in the same spot suggest a leak drawing air in
- Newly refilled systems often need bleeding across several radiators at once
Who should undertake the work
- Gas appliance work must be completed by a suitably qualified Gas Safe engineer
- Oil appliance work should be completed by a suitably qualified OFTEC technician
- Jamie Crawford Heating states both Gas Safe and OFTEC registration
Questions we get asked
- Why does my radiator keep needing bleeding every few weeks?
- That pattern points to air being drawn back in somewhere, usually a small leak or a fitting rather than a genuinely trapped one-off pocket.
- Can an airlock damage the pump?
- A pump forced to run against a persistent airlock can run dry at that point and suffer accelerated wear over time.
Where we deal with this
Related help pages
- Cylinder heatingCylinder immersion heater faults and when to use one as backup
- Oil storage regulationsOil tank bunding: the rules behind that inner containment wall
- System contaminationMagnetite sludge: the black gunge behind most radiator faults
- Efficiency controlsWeather compensation: adjusting flow temperature to the outdoor climate
- Winter oil faultOil boiler will not start in cold weather: waxing, water and filters
- Cycling symptomBoiler turning on and off repeatedly through the day
Services that cover this
Book a visit for an airlock in the heating system
Put simply: A single bleed usually clears a simple airlock, but recurring ones are worth investigating rather than treating as routine maintenance.
For gas work, use a Gas Safe registered engineer; for oil work, use an OFTEC-registered technician.
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