Airlocks in hot and cold water pipes: symptoms, checks and when to call a plumber

An airlock is a pocket of air trapped in a water pipe that stops water flowing properly. It usually shows up after the water has been off: after a plumber drained the system, after a repair, or after a cold water tank in the loft ran dry. This guide covers airlocks in the pipes that feed your taps, showers and hot water cylinder.

Air in the heating system is a different problem, with different fixes. If a radiator is cold at the top or gurgles, see our radiator bleeding and balancing guide.

Why airlocks happen

Airlocks are most common on tank-fed (gravity) systems, where a cold water storage cistern in the loft feeds the hot water cylinder and sometimes the bathroom cold taps. The water in these pipes is pushed only by the height of the tank, which is a gentle pressure. A bubble of air sitting at a high point in the pipe, or in a pipe that rises and falls, can be enough to stop it. Our guide to loft cold water tanks explains the tank and its ballvalve.

Typical causes:

• the system was drained down and refilled;
• the loft tank ran dry, for example because the mains was turned off, the float valve stuck, or the mains feed to the tank froze;
• new pipework was added with a high loop in it;
• a pipe was partly opened to the air during a repair.

Mains-pressure systems, such as a combi boiler or an unvented cylinder, are much less prone to lasting airlocks because the mains pressure usually pushes air out through the taps. You may get spitting and spluttering for a short while after the water comes back on, which normally clears.

Typical symptoms

• A hot tap that gives a trickle, splutters or runs dry, while the cold taps are fine.
• A tap that coughs out air and water in bursts.
• A gravity-fed shower or shower pump that runs weakly or not at all after the water has been off.
• Banging or hissing in the pipes when a tap is opened.

Some of these overlap with low pressure from other causes, so see our guide to low water pressure if the problem is not linked to the water having been off.

Careful checks you can make

1. Make sure the water is really back on

• Check the stopcock is fully open. Our guide to stopcocks and water meters shows where to look.
• On a tank-fed system, check the loft tank is filling and has water in it. If it is empty or low, the float valve may be stuck, or the supply to it may be off or frozen. Do not climb into a loft without safe boards and lighting.
• Check any isolation valves on the pipes to the affected taps or shower are fully open.

If the tank is empty, there is no point looking for an airlock until it is full again.

2. Protect the hot water cylinder

If the loft tank ran dry or the hot side was drained, the hot water cylinder may not be full. Switch off the immersion heater and the hot water from the boiler until hot water flows properly from the hot taps again. Heating an empty or part-empty cylinder can damage the immersion element and the cylinder. Our guide to hot water cylinders and immersion heaters explains how the system fits together.

3. Let the system refill slowly

With the tank filling and the heat off:

1. Open the affected hot tap fully and leave it open. Air often works its way out over a few minutes as the tank refills.
2. Open other hot taps around the house too, starting with the lowest and working up. Close each one once it runs steadily without spitting.
3. Run the bath taps, which usually have the biggest pipes and can carry air through more easily.

Be patient. On a gravity system, water moves slowly, and air can take a while to clear.

4. Leave it and come back

Sometimes the air shifts on its own after the system has stood full for a while. If one tap is still blocked, close it, wait, and try again later.

Tricks we do not recommend

You may read about connecting a hose from a mains-fed cold tap to a blocked hot tap and turning both on, to push water back up the pipe. We do not recommend it:

• It connects the mains drinking water directly to stored water and the hot water system, which goes against the backflow principles in the water fittings rules.
• It can push water back into the cylinder or tank faster than it was designed for, and can overflow a tank.
• On an unvented cylinder or a combi system it is the wrong tool entirely, and you should not try it.

Do not use a pump, a compressor or a vacuum on household water pipes either.

When to call a plumber

Call a plumber if:

• a tap or shower is still not running properly after the tank has refilled and you have opened the taps as above;
• the loft tank will not fill, or the float valve keeps sticking;
• airlocks keep coming back, which can mean a problem with how the pipes are laid or a fault drawing air in;
• you have an unvented cylinder or a combi boiler and the water is still spitting or weak after a short while;
• there is any sign of a leak, or water from an overflow pipe outside. See our guide to overflow pipes.

A plumber can find the high point where air is trapped and clear it in the right way for your system, and can correct any pipework that keeps catching air.

If the hot water problem is linked to the boiler itself, for example the boiler will not fire, that is a matter for a Gas Safe registered engineer.

How do I know if I have an airlock?

The usual signs are a tap that splutters, coughs air or gives only a trickle after the water has been off, while other taps work normally.

Will an airlock clear on its own?

Often, yes, especially once the loft tank is full again and the affected tap has been left open for a while. If it does not, ask a plumber.

Is an airlock the same as air in a radiator?

No. An airlock in the water supply affects taps and showers. Air in radiators is in the heating system, and is dealt with by bleeding the radiators.