Airlock in Hot Water Pipes? How to Clear It Safely
An airlock in the hot water pipes shows up as a hot tap that splutters, spits air, runs at a trickle or stops altogether while the cold taps are fine. It happens almost entirely on vented systems, where the hot water is fed by gravity from a tank in the loft, because gravity pressure is too low to push a bubble of trapped air out of the pipework. It usually follows a drain down, plumbing work, or the loft tank running low. You can often clear it yourself by pushing mains cold water back through the hot pipe with a short hose, but first check the loft tank is full, and switch the immersion off in case the cylinder is not. On a combi or an unvented cylinder, a true airlock is rare, and a hot tap that has stopped is more likely a fault than trapped air.
What an airlock looks and sounds like
The typical pattern is a hot tap that coughs and spits when opened, runs for a moment and then fades to a dribble, or produces nothing at all while every cold tap in the house works normally. Sometimes one hot tap is affected and the others are fine, usually the highest one in the house, such as an upstairs basin or a shower, because air collects at high points in the pipework.
You may also hear gurgling in the pipes or from the airing cupboard, and a tap that runs hot for a few seconds and then surges and splutters as air moves through. If the water that does come out is hot, the cylinder is heating normally and the problem is just getting it to the tap.
Why airlocks happen, and why mostly on tank fed systems
On a vented system, a cold water tank in the loft feeds the cylinder, and the hot water is pushed out to your taps only by the height of that tank above them. That is a gentle pressure. If air gets into the pipework, the water behind it often does not have enough force to push the bubble through a bend or a high section of pipe, and the bubble stays put, blocking the flow.
Air gets in for a handful of reasons. The system has been drained down for work and refilled. The loft tank has run low or dry, usually because its float valve has stuck or the mains was off, and air was drawn into the cylinder feed. Or pipework has been altered and a new high point created where air can gather. On a mains pressure system, an unvented cylinder or a combi, the water has enough force to carry air straight through, so the air spits out at the taps for a minute and clears on its own.
- After the water has been drained down for plumbing work.
- After the loft tank ran low, often a stuck float valve or a mains supply cut.
- After new pipework created a high point where air can sit.
- On a shower pump or a long pipe run to an upstairs bathroom.
Before you try anything: check the loft tank and the immersion
Go up and look in the cold water storage tank in the loft. If it is low or empty, that is the real problem, and clearing the airlock will not last until the tank is refilling. Push the float arm down gently; if water starts flowing in, the float valve was stuck. If nothing flows in, check the mains is on at the stopcock.
If the tank has been empty, the cylinder may be partly empty too. Switch the immersion heater off at its switch until you are sure the cylinder is full, because an element that is not covered by water burns out very quickly. On most systems it is also sensible to switch the hot water off at the programmer until the flow is back.
If the hot taps give nothing at all rather than spitting, our no water coming out of hot tap guide covers the other causes, including a closed valve on the cold feed and, in winter, a frozen pipe.
If the loft tank is empty and will not refill, switch the immersion off and call an engineer rather than running the hot taps. A cylinder that is emptying with the immersion on can destroy the element.
How to clear an airlock in hot water pipes
The most reliable home method uses mains pressure to push the air back up the pipe. The kitchen cold tap is almost always fed directly from the mains, so it has much more pressure than the hot side of a vented system.
Take a short length of hose and fit one end over the kitchen cold tap and the other over the hot tap that is airlocked, using jubilee clips or holding them firmly with a cloth. Open the airlocked hot tap fully first, then open the cold tap. Mains water will run backwards into the hot pipework and push the air ahead of it up towards the cylinder and out through the vent. Leave it for a minute or so, close the cold tap first, then the hot, remove the hose and try the hot tap on its own. It may need repeating two or three times.
If the kitchen has a mixer tap with mains cold and tank fed hot, you can sometimes do the same thing without a hose: open both sides, then hold a cloth firmly over the spout so the cold water is forced back up the hot side. Be ready for a splash when you let go.
Watch the loft tank while you do this if you can, or have someone else watch it. You are pushing extra water into the system and the tank level will rise; stop if it gets near the overflow. Remove the hose afterwards. A permanent connection between the mains and the hot water side is not allowed under the water regulations, because of the risk of stored water being drawn back into the supply.
If that does not work, or the airlock keeps coming back
An airlock that will not shift, or one that returns every few days, is telling you something about the system rather than being bad luck. The usual causes are a float valve in the loft tank that sticks intermittently, a cold feed to the cylinder that is too small for the draw off, a vent pipe that is pulling air in when the pump runs, or pipework with a high point that has no way to vent. Each of those needs the system looking at rather than another hose.
Air in the heating side can also stop the cylinder heating from the boiler. If the radiators are fine but the cylinder never gets hot from the boiler, trapped air in the coil circuit is one of the causes an engineer will check, along with the motorised valve and the cylinder thermostat, as set out in our page on no hot water but the heating works.
Combi or unvented cylinder: it is probably not an airlock
On a combi or an unvented cylinder, water arrives at mains pressure and air is normally flushed out within a minute of running the taps. If the hot tap spits after the water has been off in the street, run it gently until it settles. If a hot tap on one of these systems has stopped or weakened and stays that way, look for another cause. Milky water that clears in the glass is dissolved air rather than an airlock, and our guide to cloudy hot water explains the difference.
On an unvented cylinder, weak or stopped hot water with good cold usually means a blocked inlet strainer, a failed pressure reducing valve, or an isolation valve left partly closed, all of which are covered in our guide to low hot water pressure. Work on the inlet group of an unvented cylinder is for an engineer holding the unvented qualification. On a combi, no hot water with the cold running normally is usually a boiler fault rather than air, and the guide to no hot water from a combi boiler covers it.
When to call us
Call if the loft tank will not refill, if the hose method has not cleared it after a few attempts, if the airlock keeps returning, if the cylinder is not heating, or if you have an unvented cylinder and the hot water has weakened or stopped. Tell us whether you have loft tanks, when the problem started, and whether any plumbing work or a water supply cut happened just before.
We have 20 engineers across London and hot water systems are our core work, from gravity fed cylinders in period houses to unvented cylinders in new flats, so we can find where the air is getting in rather than just clearing it for another week. Our rate is on the pricing page and agreed before we attend.
Questions we get asked
- How do I know if I have an airlock in my hot water pipes?
- The hot tap splutters, spits air, runs at a trickle or stops, while the cold taps work normally. It usually happens after the water has been drained down, after plumbing work, or after the loft tank has run low, and it is most common on tank fed systems.
- Will an airlock clear itself?
- On a mains pressure system, usually yes, within a minute of running the tap. On a tank fed system it often will not, because gravity pressure is too low to push the air through, and it needs clearing with mains pressure or by an engineer.
- Is it safe to use the hose method on my taps?
- Yes, as a temporary measure, provided you watch the loft tank so it does not overflow and remove the hose afterwards. A permanent connection between the mains and the hot water side is not allowed under the water regulations.
- Can an airlock stop my cylinder heating?
- Yes. Air trapped in the coil circuit between the boiler and the cylinder can stop the boiler heat reaching the stored water, so the radiators work and the cylinder stays cold. An engineer will check that alongside the motorised valve and the cylinder thermostat.
- Can you get an airlock with a combi boiler?
- Rarely. A combi runs at mains pressure, so air is normally pushed out at the taps within a minute. If the hot water from a combi has stopped while the cold works, the fault is almost always in the boiler rather than air in the pipes.
- Why does my airlock keep coming back?
- Usually a loft tank float valve that sticks, a cold feed to the cylinder that is too small, a vent pipe drawing in air when the pump runs, or pipework with a high point that cannot vent. The cause needs finding, or the airlock will keep returning.