How to clear an airlock in your hot water or heating system

Inspired by a helpful YouTube guide. This walk-through draws on the popular UK video "How to clear airlocks and restore hot water or heating", which shows the hose method and bleeding process clearly and is worth watching before you start.

1. Work out which system has the airlock

There are two places airlocks usually sit: in the hot water pipes (fed from a tank in the loft, or a cylinder) or in the central heating circuit (the loop that runs to your radiators).

If your hot tap splutters, runs cold or barely trickles while the cold tap is fine, it is almost certainly a hot water airlock. If a radiator feels warm at the bottom but cold across the top, that is air trapped inside the radiator itself, and bleeding it is the answer. A radiator that is completely cold when others are warm suggests a heating circuit airlock or a stuck thermostatic valve.

2. Bleed radiators if the heating is the problem

Turn the heating on and let the system get up to temperature. Feel each radiator - any that are cold at the top while warm at the bottom need bleeding. Turn the heating off and let the pump stop before you start, so you are not fighting water pressure.

Hold a cloth under the bleed valve (the small square nipple at the top corner of the radiator). Use a radiator bleed key to turn the valve a quarter-turn anticlockwise. You will hear a hiss as air escapes. Once water appears rather than air, close the valve straight away - a quarter-turn back is all it takes. Check the boiler pressure gauge afterwards; bleeding lets water into the system and pressure can drop, so top it back up via the filling loop if it falls below 1 bar.

3. Use the hose method for a hot water airlock

This is the most effective fix for a hot water pipe airlock in a gravity-fed system (where a cold tank sits in the loft). You need a short length of hose and two jubilee clips, or a piece of hose that fits snugly over both tap spouts.

Connect one end of the hose to the cold tap nearest the affected hot tap - usually both taps are on the same basin or bath. Connect the other end to the faulty hot tap. Turn the cold tap on fully first, then open the hot tap. The cold mains pressure drives air back through the hot pipe towards the tank, clearing the blockage. Leave it running for a minute, then close the hot tap first and the cold tap second. Try the hot tap normally - it should run freely.

4. Try the cold-tap pressure method without a hose

If you do not have a hose to hand, a simpler version works on some airlocks. Turn the hot tap fully on. Now hold your thumb firmly over the spout - a firm seal, not a light press. Turn the cold tap on briefly so cold water pressure pushes back into the hot pipe. Release after a few seconds. Repeat three or four times. It is less reliable than the hose method but worth trying first on a minor airlock.

5. Check the header tank if the airlock keeps coming back

In older gravity-fed systems, the small header tank in the loft refills the hot water cylinder. If the ball valve in that tank is sticking or the water level is too low, air gets drawn into the system every time you run hot water.

Have a look in the loft. The tank should be roughly half-full of cold water. If it is nearly empty with the ball float sitting on the bottom, the ball valve may be stuck or the arm bent. Free the ball valve gently - push the float down and let it rise - and check whether the tank refills to the correct level. If it will not refill at all, you have a failed ball valve that needs replacing.

6. Check thermostatic radiator valves on cold radiators

If one radiator stays cold even after the system has been bled, the thermostatic radiator valve (TRV) pin may have seized. The TRV is the valve with the numbered dial at one end of the radiator. Inside it is a small pin that opens and closes the flow. If a radiator has not been used all summer, the pin can stick in the closed position.

Remove the TRV head by unscrewing the ring nut. You will see a small brass or plastic pin in the valve body. Push it in and out a few times with a finger or a small screwdriver. If it moves freely, refit the head - the radiator should warm up within a few minutes. If the pin will not move at all, the valve body may need replacing.

7. Re-pressurise the system after any work

After bleeding radiators or clearing a heating airlock, always check the system pressure. The filling loop is usually a braided metal hose under or beside the boiler with two isolation valves. Open both valves slowly until the pressure gauge reads between 1 and 1.5 bar, then close both valves. If the pressure drops back below 1 bar within a day or two, there may be a slow leak somewhere in the system.

When to call a handyman or plumber

Call Richard if the airlock keeps coming back after clearing it, if the header tank will not refill, or if bleeding the radiators makes no difference. Persistent airlocks usually point to a failing ball valve, a pump losing power, or a system that needs a proper flush. These are straightforward jobs for someone who knows the system, but they are not worth guessing at yourself if the first fix has not held.