Airlock in water pipes: signs, causes and how to clear it

Signs you have an airlock

Air in a pipe behaves differently from a blockage. Water may surge and stop, the tap may cough and hiss, and you may hear knocking or gurgling in the pipes as the air moves. Often only one outlet is affected, or only the hot taps, while cold taps fed straight from the mains run normally.

Timing is the biggest clue. Airlocks nearly always appear after something has let air into the pipes: the water supply being turned off for a repair, a street main being repaired, a cold tank running dry, or a system being drained for work such as a new radiator or cylinder.

• Hot tap spluttering, spitting or delivering only a trickle.
• No water at one tap or the shower while others run normally.
• Knocking, gurgling or hissing in the pipes when a tap opens.
• A shower or bath mixer that runs cold or surges between hot and cold.
• Problems that started straight after the water was turned off or restored.

Why tank-fed hot water is most affected

In many older London houses, the hot water comes from a cylinder that is filled from a cold tank in the loft. That supply relies on gravity, so the pressure depends on how high the tank sits above the tap. It is enough to run a bath, but it is often not enough to force a stubborn pocket of air through a long run of pipe, particularly where the pipe rises and falls on its way to an outlet.

Cold taps in the kitchen, and all taps in homes with a combi boiler or an unvented cylinder, are fed at mains pressure. Mains pressure usually pushes air through and out of the tap within seconds. So if a mains-fed tap is still spluttering or weak after a supply interruption, the cause is more likely to be debris, a partly closed valve or a problem on the supply, and our guide on water not restored after mains repair is a better place to start.

How to clear an airlock safely

Start with the gentle method. It clears most airlocks in tank-fed systems and cannot do any harm. Make sure the cold tank in the loft is full before you start; if it is empty or low, the float valve may have stuck and that needs sorting first.

• Check the loft tank has water in it and that the stop tap and any isolation valves are fully open.
• Open every hot and cold tap in the house, starting with the lowest one and working upwards.
• Leave them running for several minutes until each one flows steadily without spitting.
• Close them in reverse order, starting with the highest tap.
• If one outlet is still affected, run just that tap fully open for a few minutes more.

You will often see advice to join the hot and cold taps with a hose so mains pressure pushes the air out of the hot pipe. That briefly connects drinking water to stored water, which the Water Supply (Water Fittings) Regulations are designed to prevent because of the backflow risk. We would not recommend it; if the gentle method fails, get the pipework checked instead.

Airlocks in central heating

Air also collects in heating systems, where it shows up as radiators that are cold at the top, gurgling in the pipes or a pump that runs noisily. Bleeding the affected radiators with a radiator key usually sorts it. On a sealed system, check the boiler pressure afterwards because bleeding lets water out, and top it up through the filling loop if it drops below about 1 bar.

Air in the pump itself can stop water circulating altogether, so the boiler fires but the radiators stay cold. If bleeding the pump does not help, see our guide to heating pump repair or replacement. If radiators need bleeding every week or two, air is getting in or gas is forming inside the system, often from corrosion. Our guide to a radiator cold at the top explains what that pattern usually means.

When an airlock keeps coming back

A one-off airlock after the water has been off is nothing to worry about. Airlocks that return without any obvious reason point to a fault that is letting air in, or to pipework that traps air every time it is used heavily.

The usual culprits are a cold tank that is not refilling fast enough because the float valve is failing, a tank that is too small for the house, pipework with high points where air collects, or a shower pump drawing air through a poor connection. In older properties, pipework altered several times over the years is often to blame. An engineer can trace the runs, check the tank and its valve, and reroute or vent the section that is trapping air.

When to call a plumber

Call if running the taps has not cleared the problem, if you have no hot water at all, if the loft tank is empty or overflowing, or if you are hearing heavy banging in the pipes. Also call if the airlock appeared after someone else's work and they are not coming back to finish it. Sometimes what looks like an airlock is actually a frozen pipe, a closed valve or a leak, and those need finding quickly. If there is no water coming out of the taps at all, start with that guide instead, because an airlock rarely stops every outlet at once.

Loyal Plumbers has 10 vans covering London and everywhere inside the M25, 24 hours a day. When you get in touch, tell us which taps are affected, whether you have a tank in the loft, a combi or an unvented cylinder, and what happened just before the problem started. That is usually enough for us to know whether it is an airlock or something else before we arrive.