Air locks in plumbing and central heating systems

Air locks occur in central heating systems and in domestic hot and cold water systems. They can be incredibly frustrating and are not always easy to get rid of but we’ll take you through the way we deal with them. Central heating air locks are lower down the page

Air locks in domestic hot and cold water pipes

Mains hot and cold water air locks?

Air locks almost never happen in mains cold water pipes. There is no easy way for air to be entrained in (pulled into) a mains water system and there is normally more than enough pressure to clear any air. Unvented hot water systems also very rarely suffer from air locks. Air may get into them under some circumstances but the high pressure ensures it is easily expelled. Inadequate flow from a hot water tap supplied by a combi boiler is also very unlikely to be caused by an air lock.

Air locks in low pressure hot and cold water

Air locks affecting domestic hot water taps may be in the low pressure cold feed pipe to the hot water cylinder or in the low pressure hot water distribution pipes.
Air locks affecting cold water taps or toilet cisterns are in low pressure distribution pipes.
Low pressure distribution pipe air locks occur most commonly when a loft cold tank runs out of water. They affect both hot and cold low pressure water taps because both are ultimately fed from the same cold water storage tank. Cold pipes, typically to the bath, basin and loo, come directly from the cold tank. Hot water pipes come from the hot water cylinder, which is filled from the cold tank.
If the water is drawn from the loft tank faster than it is replaced, it will eventually run out. Loft tanks in the UK are relatively large, generally 225 litres or 50 gallons, and this acts as a buffer. Filling a bath quickly, lowers the level of water in the tank. However, with maybe 180 litres of buffer available, the tank will not normally run dry.
In the past, if a tank ran dry it was almost always because it was not filling at the normal rate and that was usually a problem with the ball valve. This is still a common reason for loft tanks running dry.
Ball valves can become blocked with debris. This can happen after repairs to the street mains, with grit or small pebbles being washed into the pipeline. If part of your supply pipe is made of iron, it can also happen when flakes of rust break away from the inside of the pipe and lodge in the ball valve.
Modern ball valves have a diaphragm washer which will deform and stiffen over the years. Eventually the washer can deform so much that it largely blocks the inlet port, slowing down the flow into the tank.
If you push the ball float downwards, water should rush out of the ball valve into the tank. If the flow is slow, or very slow, you will need to sort out the ball valve. It is generally better to change it. “Part 1” brass ball valves are no longer allowed for loft cold tanks. You should use a “Part 2” ball valve, also called a diaphragm ball valve. Make sure cold water can enter the tank quickly.
Over the past 10 or 15 years, more and more houses have had shower pumps fitted, either as a separate pump or a packaged shower unit. Shower pumps are a very common cause of air locks. Shower pumps vary but some are capable of delivering a real torrent of water, faster than filling a bath. All this water, hot and cold, ultimately has to come from the loft tank. Powerful showers are a pleasure to use and we tend to stay in them longer, so it can be easy to run the loft tank dry.

In a well designed low pressure water pipe system, air should clear naturally even if you run out of water. As the pipe system fills again, the air should naturally rise to the top and be expelled. Air locks happen when the pipe system is not well designed.
Air bubbles rise to the top because air is much lighter than water. In a correctly installed low pressure pipe there is a natural “fall” towards the taps. Friction between the water and the pipe walls provides a resistance to the flow. The fall in the pipe is needed to overcome the resistance.
If the fall in the pipe is continuous, any trapped air going the other way in the pipe will be following a natural rise. Given a little time it should bubble back into the cold tank or, if it’s a hot water pipe, bubble back towards the cylinder and up the open vent over the cold tank.
Resistance increases with the length of the pipe and long horizontal sections can be a problem because they have no fall. This is particularly true with old iron pipes. Iron pipes are always rusty inside, and very rough, so the resistance to the flow is high. Short horizontal sections of pipe are not normally a problem.
Pipes which rise towards the taps are a big problem. However, we can ignore the last short section of pipe connecting to the tap which nearly always rises vertically. Any air going to the top of that section is simply passed out through the tap. If a low pressure pipe rises on its way towards the taps, any air going the other way would have to go downhill. Because air is lighter than water it gets trapped at the top of the rise in the pipe. This air pocket adds a lot of resistance to the flow of water. If there is not sufficient overall fall along the length of the pipe to overcome that extra resistance, the pipe becomes air locked.
When an air lock forms, the flow of water from the taps may reduce dramatically and can sometimes stop completely. If the air lock is in a cold supply pipe it may also stop the loo filling.

Clearing air locks in domestic hot and cold water pipes

Clearing air locks is not always straightforward. Some can be quite easy to clear and most can be cleared with a little more effort. In really difficult cases you may need to have pipework modified but that’s rare. We’ll start with the easiest things first.
Turn off all the hot taps and low pressure cold taps. Make sure the loft cold tank is full and that the water runs in quickly from the ball valve when the ball float is depressed. The water level will normally be about 5cm (2″) below the overflow pipe when the tank is full.
Push the ball float downwards so that the tank starts to fill beyond its normal level. Watch carefully and release the ball float when the tank level is almost up to the overflow. The extra 5cm of “head” of water can help, as it slightly increases the pressure we need to overcome the extra resistance caused by the air lock.

Clearing cold water air locks

Make sure that the isolating valve in the low pressure cold pipe from the main tank is fully open. Lever valves are easy and the lever should be in line with the pipe. Older systems used gate valves (usually with a red handwheel) and they often become too stiff to turn. If a gate valve is too stiff to turn, assume it is fully open and ignore it for now.
Make sure all hot taps stay closed.
Fully open the basin cold tap and bath cold tap and flush the loo. This ensures that there are 3 pathways open for the cold water flow and it reduces the overall resistance in the cold pipe. Some water is likely to trickle from the taps.
If you are lucky, the taps will start to gurgle and cough and the air will be flushed out of the pipe. If this happens, wait until the taps run freely for about 30 seconds before turning them off.

If the tap flow stays very slow there is probably still an airlock and there is more you can try.
First, turn the cold taps off again.
Open a hot tap for a couple of minutes. Cold water will run from the cold tank into the bottom of the hot water cylinder and push hot water from the cylinder out of the tap. This lowers the level of water in the cold tank back to its normal level (remember, we had deliberately overfilled it before).
The next process is best done with 2 people.
Persistent air locks can be cleared using mains water pressure. You need a garden hose connected to a mains pressure cold tap (most garden taps are mains pressure).
There are two ways to flush the air out.
The first is to put the end of the garden hose into the loft cold tank and feed the end into the outlet from the tank that goes to the cold taps and hold it there. There are typically 2 outlets from the main tank. One goes to the bottom of the hot water cylinder and the other goes to the cold taps. You want the outlet that goes to the cold taps.
One person holds the end of the garden hose tightly entered into the outlet from the cold tank. (Be warned, tank water can be very cold and it will chill your arm!) The other person then turns on one or more cold taps fully (bath cold tap is good) and then fully turns on the mains cold water to the garden hose.
This forces water rapidly along the low pressure distribution pipe and usually forces out the air which has caused the air lock. 30 seconds is usually plenty but you can try longer if necessary, as long as you don’t overfill the loft tank.
The garden tap is then turned off and the hose removed from the tank outlet pipe. Water should now flow normally from the low pressure cold taps.

The other way to flush the air lock out using mains pressure cold water is to go in the other direction.
A hose is run from the garden tap to the outlet of one of the bathroom cold taps and connected to the mouth of the tap. This has to be done carefully and securely and it must be held in place or you may spray mains cold water all over the bathroom.
Next, open the bathroom cold tap you’ve connected to.
The mains pressure hose is then turned on and water is forced backwards up the low pressure cold water pipe, into the cold water tank. This should flush any trapped air back into the cold tank. Again, 30 seconds should be plenty. Make sure you don’t overfill the cold tank!
Turn off the garden hose and the bathroom cold tap.
Disconnect the hose from the bathroom tap and turn on the bathroom cold taps. They should now run freely.

Clearing hot water air locks

Hot water cylinders are not as strong as the pipework and cannot take the same pressures. You must be careful! Cylinders also have an open vent pipe which runs from the top of the cylinder, up and over the top of the cold tank. This vent pipe must never be closed off. The hot water pipe to the taps is teed off from this vent pipe, usually close to the top of the cylinder.
Hot water air locks can be in two places. They can be in the cold feed pipe between the cold tank and the bottom of the cylinder. They can also be in hot water distribution pipes between the hot water cylinder and the hot taps.
Check that the isolating valve in the cold feed pipe between the cold tank and the bottom of the cylinder is fully open. As we said above, if it’s a lever valve the lever should be in line with the pipe. More commonly it will be a gate valve (these usually have a red handwheel). Many gate valves become too stiff to turn if they are not operated occasionally. If a gate valve is too stiff to turn, assume it is fully open and ignore it for now. You should get it sorted out or replaced but that’s another job.

First, try the simplest method to shift an air lock.
Turn off all the hot and cold taps and carefully overfill the cold tank as described above.
Now turn on all the hot taps fully at the same time, including the kitchen and utility room hot taps downstairs.
If you are lucky the taps will gurgle and cough the air out of the pipework. If they do, turn them all off then try them one at a time.
If the hot taps now run normally, you are sorted. If not, you will need to try using mains water to flush the air out but you must be careful!

As we said, with hot water air locks there are two places where air can become trapped.
Try to flush any air out of the cold feed pipe from the cold tank to the bottom of the hot water cylinder.
You will again need 2 people and use a garden hose. The hose goes into the cold tank and this time is fed into the outlet pipe which goes to the bottom of the hot water cylinder. There would normally be only one pipe which connects the bottom of the cold tank to the bottom of the hot water cylinder. You are trying to remove any air from that pipe.
Be careful with the next part! There will be a vent pipe which goes from the top of the hot water cylinder, up over the top of the cold tank. The next part could send water up the vent pipe and into the top of the cold tank. That’s not a problem in itself but if the water in the cylinder is hot, you could scald yourself!
Turn on your hot taps. This will reduce the amount of (hot!) water that is likely to be forced up through the vent.
With the hose held securely into the entry to the pipe from the cold tank, turn on the garden tap (mains cold tap). Ten or fifteen seconds of mains pressure water passing through that pipe should be plenty.
Turn off the garden tap and remove the hose from the pipe outlet. If the air lock was in that pipe, the hot taps should now run more freely. Turn them off and try them one at a time.
If one or more hot taps still doesn’t run freely there may be air trapped in the hot water distribution pipe which directly supplies that hot tap (or taps).

Now try to flush any air out of the hot water distribution pipe.
To do this you want to send fast moving water (from a mains pressure cold tap) backwards along that pipe. This should flush any trapped air back into the hot water cylinder, where it will rise to the top and pass safely up the vent pipe. Remember the open vent which passes from the top of the cylinder, up over the top of the cold tank, must never be closed off.
Again, it’s useful to have a second pair of hands. You want to connect a mains cold supply hose to the outlet of a hot water tap. That’s not necessarily easy to do and you will need to have someone hold the connection in place while you try the next part.
Fully turn on the hot tap you have connected to. Then progressively turn on the mains cold through the hose.
Water should pass backwards through the hot tap and be forced back along the hot pipe. Again, 10 to 15 seconds should be plenty.
Turn off the cold mains supply to the hose.
Turn off the hot tap and disconnect the hose from it.
Turn on the hot tap fully again. Hopefully, now it will run freely.
Try the other hot taps too.

If you have a kitchen mixer tap

Most kitchens have a sink mixer tap and you may be able to use this to clear an airlock. Again, the idea is to force mains cold water back along the hot water pipe to the tap. It sounds as if it should be simple, simply by blocking off the mixer tap outlet and turning on first the hot then the cold but it’s not that simple.

Water regulations require mixer taps to be designed to prevent the hot and cold water mixing. Both the hot and cold outlets should terminate right at the end of the spout. If you block this off you will probably block both at the same time and not allow the cold outlet to connect to the hot outlet.

Diffuser from sink mixer tap spout
Kitchen sink mixer tap spout diffuser

Most mixer taps have a diffuser at the end and this can usually be removed. With the diffuser removed, you may be able to block off the end of the spout without individually blocking off the hot and cold outlets in the spout.

With the end of the spout blocked off, you turn on first the hot tap fully, then progressively turn on the cold tap. With luck the cold mains water will feed back along the hot pipe, forcing any air back into the cylinder where it should pass safely up the open vent pipe at the top. Again, 10 or 15 seconds of fast running mains cold water should be plenty to clear the air lock.

Turn off the cold and check that the hot now runs freely. Remember to refit the diffuser and it would be worth cleaning it first!

If you have hot and cold washing machine valves

If you have hot and cold washing machine valves you may be able to use these to clear a hot water air lock. Washing machine isolating valves are usually cheap and may not work well but they are necessary, so if they are not working you should replace them anyway.
Turn off both valves (hot and cold) where they connect to the washing machine hoses. The valves are almost always ¾” threaded, matching the washing machine hose ends.
Connect a standard washing machine hose directly between the hot and cold valves.
Open first the hot valve then the cold valve. Allow the cold mains water to run for 10 or 15 seconds and then turn both valves off.
Disconnect the hose and reconnect your washing machine, remembering to turn the valves back on.
The mains water should have flushed any air trapped in the hot pipe back up into the hot water cylinder where it should safely pass up and out of the vent pipe at the top.

Preventing hot and cold water air locks recurring

Air locks in hot and cold water pipes are almost always caused by the loft cold tank emptying too far. This happens either because the flow into the tank is too slow or because water is drawn out of the cold tank too quickly. It may be both.
Make sure that cold water flows quickly from the loft tank ball valve when the ball float is depressed. If it is slow, it may be the ball valve.
If you’ve checked or replaced the ball valve, and the flow into the tank is still slow, you’ll need to investigate the mains cold pipe pressure and flow.
Make sure any mains cold stopcock in the pipe to the ball valve is fully open.
Shower pumps can draw water from the cold tank far more quickly than it can be replaced. Remember, both hot and cold water for the shower come ultimately from the main cold tank. Even though the shower pump may lower the water in the cold tank there is usually enough of a buffer to prevent it emptying completely.
If the shower empties the cold tank completely, and the ball valve is filling at a normal rate, you either need to take shorter showers or slow down the shower pump (if this is possible). Setting the shower head to a mode which passes less water will achieve this. Allowing time for the tank to refill between showers will help too.
Even if a cold tank empties, the system should not air lock when it refills. If it does, you may choose to be more radical and sort out the problems with the pipe design but this is not always possible.

Air locks in central heating pipes

Flow and return pipes for the central heating radiators often run vertically on walls, or in walls or cupboards. Depending on the system design, the flow of water in a pipe might be upwards or downwards (but it will always stay the same). If air enters a vertical pipe it tends to go to the top. If the normal direction of flow is upwards, the air will usually be flushed out of the vertical pipe.
If, however, the normal direction of flow is downwards, the air is not so easily flushed out of the pipe. If there’s a lot of air and not a lot of pressure from the pump, an airlock forms which can stop the flow of water in the pipe. This can prevent one or more radiators from getting hot, depending on how many radiators the pipe serves.

To clear a central heating airlock

An airlock in a vertical pipe can often be cleared by increasing the pump pressure in the pipe:
First, note the setting on the boiler thermostat and then set the boiler thermostat to minimum. If it is an older boiler, and the boiler thermostat can be switched right off without it shutting off the pump, switch the boiler stat off.
Second, if you haven’t already done so, turn the pump speed to maximum.
Third, if your boiler heats the radiators and a cylinder, turn the hot water program off. This should stop the cylinder receiving flow from the pump and should increase the flow to the radiators. This increases the pump pressure in the radiator pipes.
Fourth, go round the house and close off all the hot radiators except one. (We leave one hot radiator open to minimise the risk of the boiler overheating. If you have an older boiler which allows the boiler thermostat to be switched off while the pump keeps running, you can close all the hot radiators.) Leave open the radiators that don’t get hot.
To close a radiator off you only need to close the valve on one end. If the valve you are closing was not fully open, note the number it was set to if it was a TRV. If it was a wheelhead or lockshield valve, note how many turns and part turns it took to close it. If you successfully clear an airlock you will want to re-open the valves you had closed, setting them to their original positions.

Boiler and pipes banging?

If you get a loud banging in the boiler and pipes, the boiler is probably overheating. It should shut itself down automatically but you should immediately re-open some of the radiators you had closed. The boiler may lock itself out and need resetting. If so, follow the User Instructions for your boiler.


If your cold radiators suddenly start to get hot (and this may be accompanied by a gurgle of air as an air lock clears) that’s great! This method often clears an air lock in vertical pipes. Now go back to the other radiators and set them back as they originally were (valves open to the previous positions).
Turn the boiler stat back to its previous setting and switch the hot water (cylinder) program back on if required. Turn the pump back to its original speed.
If the radiators are still not balanced you will have to carry on with the balancing process. It takes time but it is not technically difficult.

Where does the air in central heating systems come from?

First, the “air” in a central heating system may not be air. It may be gases formed as part of the corrosion process if corrosion inhibitor chemicals have not been added to the central heating system water. It may be hydrogen so I’d be careful not to get a flame anywhere near if I was bleeding radiators. It’s always worth adding a good quality, long-life corrosion inhibitor liquid to the system water.

Dissolved air

When a heating system is filled with water, the water contains dissolved air. Apparently, the water can hold dissolved air which would have a volume of up to 17% of the water volume. That’s not to say that, when the air is released, the water would only be 83% or the original volume. It means that, if all the air was released, the air would have a volume of 17% of the water volume.
In newly filled central heating systems some air may be released from solution in the first few days. This would need to be bled out of the radiators.

Air entrained via the open vent

With open-vented central heating systems (most older central heating systems) air can be pulled down the open vent if the pipework has not been well designed. Most of this air would be passed into the radiators, from where it could be bled out, but some could be trapped in pipework, causing air locks.
Traditionally there was a vent pipe from the system pipework going up over the top of the water in the central heating feed and expansion tank (header tank). There was also a separate cold feed pipe down into the central heating system pipework. Both were normally connected “behind” the pump in the negative pressure zone, with the feed pipe connected closer to the negative side of the pump.
This design prevented air being pulled down the open vent and system water being pushed back up the feed pipe. However, after a few years and without corrosion inhibitors added, sludge deposits can start to block pipework connections. This can alter the pressures in the feed and vent pipes when the pump is running, sometimes allowing the vent to start pulling in air. This is most likely to happen each time the pump starts to run.
The air that is dragged into the heating system tends to end up in the radiators. As it builds up it makes matters worse. When the pump starts it can compress the now larger volume of air in the radiators and this allows a sudden rapid movement of water through the pump and a rapid drop in pressure behind the pump. This makes it easier to pull air down the open vent.
If a system is pulling air down the open vent, bleeding all excess air from all radiators will make this less likely to happen. Slowing the pump speed down will help too, provided the pump is still running fast enough to allow the radiators to stay balanced and get evenly hot.
The best way to prevent air being pulled down the open vent is to modify the pipework design. Capping off the old feed pipe and connecting the feed directly into the vent pipe combines the feed and vent. They then only share one common connection with the central heating pipework so they are both connected to the same pressure zone. This radically reduces the risk of air being pulled into the system via the open vent.

Micro leaks

A less obvious cause of air entrainment are micro leaks. These are leaks which are so small that water does not easily pass through them. They are typically found at badly made compression joints or at valve glands. Though they may not let water out, air is much less viscous (thick and sticky) than water and may be pulled in.
If micro leaks are located in the negative pressure zone “behind” the circulating pump, a continuous stream of microscopic bubbles can be pulled into the heating system water. Over time these build up. If the micro leak is close to the pump, the build up of air can be rapid.
One common place for a micro leak is the pump isolating valve on the negative side of the pump. This can be from the compression nut connecting the valve to the pipe or from the isolating valve gland nut.
Micro leaks entraining air can be anywhere in the negative pressure zone of the system pipework. The closer they are to the negative side of the pump, the more air they can entrain. Micro leaks can be very difficult to find but in a well installed system they are much less likely.

Can sealed central heating systems pull in air?

Sealed systems do build up gases in the radiators under some circumstances. Since most sealed systems are pressurised to between 1 and 1½bar (above atmospheric pressure) it seems illogical that air could be pulled in but I’m not sure. I’m not a physicist but it might be possible that the required localised drop in pressure needed to entrain air in a sealed central heating system could be caused by a venturi effect. If anyone has a definitive answer to this I’d be glad to hear it.
My best guess for “air” build-up in a sealed central heating system is that it will be hydrogen being released from sludge deposited over years in the central heating system. Hydrogen could continue to be produced if corrosion is continuing but I would expect it to stop once some of the corrosive reagents were used up. If it is persistent, the central heating system may need to be thoroughly powerflushed before a long-life corrosion inhibitor liquid is added on completion.

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