How to Clear an Airlock in Water Pipes (UK Homes)

A hot tap that splutters, hisses or won't run after the water has been off is usually an airlock. How to tell, how to clear an airlock in hot water, cold water, shower and heating pipes, and when it points to a bigger fault.

If a tap coughs, hisses, spits or produces only a trickle after the water has been off, you almost certainly have an airlock: a bubble of air trapped in a pipe that the water can't push past. It happens most in older London houses with a cold water tank in the loft and a hot water cylinder. The first thing to try is opening every tap, starting from the highest in the house, and letting them run for a few minutes. If a hot tap still won't flow, you can use mains pressure to push the air back through the system with a short hose, as described below.

Signs you have an airlock

A tap splutters, spits air or runs in bursts, especially the first time it is used after a shut-off.
The hot tap gives no water at all or only a trickle while the cold kitchen tap is fine.
Pipes knock, hiss or gurgle when a tap is opened.
A shower runs weakly or not at all, while the bath taps are normal.
Radiators are cold at the top, or the heating pump sounds noisy after the system has been drained.

If nothing comes out of any tap, including the kitchen cold, it is not an airlock. Work through our guide to no water coming out of the taps instead.

What causes an airlock

Air gets into pipes whenever they are partly emptied, and gets stuck where the pipe runs up and then down again, forming a trap the water can't push the air through. The usual triggers are:

The water has been turned off. After a mains shut-off, repair work or a water company outage, air enters pipes as they drain.
The cold water tank has run low. If the loft tank empties faster than it refills, because of a slow or faulty float valve or heavy use, air is drawn into the outlet pipe. Our guide to cold water storage tank problems covers the float valve.
New or altered pipework. Pipes that rise and fall, or long flat runs with little fall, are more likely to trap air.
Draining the heating. Refilling a heating system always leaves some air behind, which is why bleeding afterwards matters.

The type of system matters. A tank-fed hot water system only has the pressure from the height of the loft tank to push water through, so it doesn't take much trapped air to stop it. Combi boilers and unvented cylinders run at mains pressure, which will normally push air out within a minute or two. If you have a combi and a hot tap keeps spluttering, look at the water pressure or the boiler rather than assuming it is an airlock.

How to clear an airlock in water pipes

Method 1: run the taps

Check the loft tank is full

If you have a cold water tank, look in the loft. If it is low or empty, wait for it to refill before doing anything else, or you'll just draw in more air.

Open taps from the top down

Open all the hot and cold taps, starting with the highest in the house and working down. Leave them fully open for a few minutes.

Close them from the bottom up

Once each tap is running steadily with no spluttering, close them in reverse order, lowest first.

Method 2: the hose method for a stubborn hot tap

This uses the higher pressure of the mains cold supply to push the air backwards through the hot pipe and out through the cylinder's vent. It works best between a mixer tap or two taps close together, such as the kitchen sink where the cold is on the mains.

Connect a short, clean hose

Fit it tightly over the cold mains tap and the airlocked hot tap, using hose clips or tape if needed. On a mixer tap you can sometimes hold a cloth over the spout and open both taps instead.

Open the hot tap first

Open the airlocked hot tap fully.

Open the cold tap briefly

Turn on the cold for a few seconds. You may hear the air being pushed back through the pipes. Turn the cold off.

Disconnect and test

Remove the hose and run the hot tap. Repeat once or twice if it still splutters.

Never leave the hose connected. Joining the mains supply to stored hot water creates a cross-connection, and Water Regulations don't allow that as a permanent arrangement because stored water could be drawn back into the drinking water supply. Use it for seconds, not minutes, and don't use this method on an unvented cylinder or combi, where both sides are already at mains pressure.

Airlocks in showers and heating

Showers. Mixer showers fed from a loft tank are prone to airlocks after the water has been off. Take the shower head off, turn the shower to hot and let it run into the tray until the flow steadies. If you have a shower pump, an airlock can make it run noisily or cut out, and running it dry can damage it. Switch the pump off and read our guide to shower pump problems.

Heating. Air in radiators is cleared by bleeding them with the heating off. Air trapped around the pump or boiler can stop water circulating, cause a gurgling noise and make the boiler lock out. Bleed all the radiators, top up the pressure on a sealed system, and if the pump is still noisy or the boiler keeps cutting out, get a heating engineer to check it. After a drain-down, our guide on draining and refilling a central heating system explains how to bleed it properly.

When it keeps coming back

An occasional airlock after the water has been off is normal. Airlocks that come back again and again usually mean something else is wrong:

The loft tank's float valve is slow or partly blocked, so the tank runs low whenever you run a bath.
The tank is too small for the household, or the outlet is too close to the bottom.
Pipework has been altered with a rise and fall that traps air, which may need rerouting.
A leak on a suction pipe is drawing air in, sometimes around a pump.
On an older vented cylinder, the vent pipe may be blocked or badly placed.

These are worth fixing properly, both to stop the nuisance and because a hot water cylinder or immersion heater that is repeatedly starved of water doesn't work well. We can check the tank, float valve and pipe layout, and explain the options. Our labour is £95 an hour, the £60 call-out is waived when the job runs over 2 hours, and we confirm the price before starting. See also our cold water tank and low water pressure services.

Taps still spluttering?

Tell us what type of hot water system you have and which taps are affected. We'll explain the likely cause and confirm the price before any work starts.

📞 Call 020 7870 3200

Or read about our hot water pressure and low water pressure repairs.

Frequently Asked Questions

Will an airlock clear itself?

Small airlocks often clear within a few minutes of running the taps as water pressure pushes the air along. A full airlock in the hot water pipe of a low-pressure tank-fed system may not clear on its own, because the gravity pressure from the loft tank is not enough to shift it. That is when the hose method or a plumber is needed.

How do I know if I have an airlock or no water supply?

Check the cold kitchen tap, which on most homes comes straight from the mains. If that runs normally but a hot tap or bathroom tap splutters or gives nothing, it is likely an airlock or a problem with the stored supply. If the kitchen cold tap is also dry, the problem is the mains supply, the stopcock or the water company.

Is the hose method safe?

It is widely used as a short-term fix, but it creates a temporary connection between drinking water from the mains and stored water, which Water Regulations do not allow as a permanent arrangement because of the risk of backflow. Use a clean hose, keep it connected for no more than a few seconds of flow, and remove it as soon as the tap runs.

Can an airlock damage my boiler or cylinder?

An airlock in the domestic hot water pipes won't usually damage anything, but it can stop a hot water cylinder or immersion heater from getting a proper supply. In a heating system, trapped air can stop circulation, make the pump noisy and cause a boiler to overheat and lock out, so it should be bled out rather than ignored.

Why is only my hot water airlocked?

On a tank-fed system the hot water runs at low gravity pressure from the loft tank, while the kitchen cold tap is at mains pressure. Air trapped in a hot pipe has little force behind it, so hot taps are the ones most likely to airlock. The cold taps in the bathroom may also be tank-fed and airlock in the same way.

Airlock in Water Pipes: How to Clear It and Stop It Coming Back

Key Takeaways