How do I clear an airlock in a water pipe?
Why airlocks form in gravity systems and not mains ones, the hose-between-taps method that clears most of them, and how to do it without creating a backflow risk.
- Difficulty
- beginner
- Time
- 25 min
- Read
- 5 min
- Safety
- caution
Short answer
An airlock is a pocket of air trapped at a high point that gravity pressure cannot push past. Clear it by connecting a hose from the affected tap to a mains-pressure cold tap, clamping both ends, opening the affected tap first then the mains tap, and running it for around thirty seconds. Close the mains tap first, then the other, then take the hose off.
An airlock is why one tap in the house splutters, dribbles or gives nothing while everything else is fine, usually after the system has been drained, refilled, or run dry. The physics is simple: water pushed only by the small head of a loft tank does not have enough force to shove an air bubble over a high point in the pipe, so the bubble sits there indefinitely. Mains pressure does have enough force, which is why the standard cure is to borrow some.
Safety
What you'll need
- A short length of hose that fits both tap outlets
- Two jubilee clips, or strong tape
- Towels
Step by step
- Confirm it really is an airlock.The signature is one outlet affected while others on the same system are fine, spluttering and spitting air rather than a steady weak flow, and a recent event — draining down, a burst, tank work, or the supply being off. A steady weak flow at every tap is a supply problem, not an airlock.
- Try the simple things first.Open the affected tap fully and leave it running for several minutes; airlocks sometimes clear themselves. Open all the taps on that circuit at once, then close them one at a time starting from the furthest, which can walk the air out.
- Find a mains-pressure tap.The kitchen cold tap on any system, or any cold tap on a combi or unvented house. That is your pressure source. On a fully tank-fed house with no mains outlet other than the kitchen, the kitchen cold is the one you use.
- Connect the hose and clamp both ends.Push the hose fully onto the affected tap and onto the mains cold tap, and secure each end with a jubilee clip or several turns of strong tape. A hose that blows off under mains pressure soaks the room and can scald if it is a hot outlet.
- Open the affected tap first.Fully. Then open the mains tap, gradually rather than all at once. You should hear and feel the water surging backwards up the airlocked pipe.
- Run it for about thirty seconds.You are pushing the air pocket back to the tank or cistern where it can escape. Longer does no harm, but thirty seconds usually does it. On a stubborn one, turn the mains tap off and on a few times to pulse it.
- Close the mains tap first, then the other.This order matters. Closing the affected tap first traps mains pressure in a tank-fed pipe. Mains off, then the other, then remove the hose straight away and do not leave it connected.
- Test and repeat if needed.Open the affected tap on its own. If it runs cleanly, done. If it still splutters, the air has probably moved to another high point — repeat once or twice. If three attempts change nothing, the fault is not an airlock.
Tips
- Airlocks are a gravity-system problem. If your house is combi or unvented, a spluttering tap is much more likely to be aerated mains water or a failing tap than an airlock.
- Keep a short length of hose and two jubilee clips with the plumbing bits. It is a five-minute job with them and an evening of frustration without.
- If the affected tap is a mixer, the hose method works but the water can cross between hot and cold inside the mixer body. Some mixers cannot be cleared this way and need bleeding at a nearby joint instead.
- Refilling a drained system slowly from the lowest tap upwards prevents most airlocks in the first place.
If it doesn't work
Hot taps airlocked after a drain-down
Cause: The cylinder and its pipework refilled unevenly, trapping air at high points — Fix: The hose method from the kitchen cold to the affected hot tap. Also worth refilling more slowly next time and opening taps from the lowest upwards.
Airlock keeps coming back
Cause: A pipe run with a genuine high point and no vent, or a loft tank running low and drawing air into the outlet — Fix: Check the tank level and the ball valve first — a tank that empties during demand pulls air into the pipework every time. Recurring airlocks in the same pipe usually mean the run needs re-routing.
Tap spits air but flow is otherwise normal
Cause: Aerated mains water rather than an airlock — common after mains work in the street — Fix: Fill a glass and let it stand. If the cloudiness clears from the bottom upwards it is dissolved air and it will settle on its own within a day or two.
Every tap in the house splutters
Cause: The supply has been off and the whole system has air in it, or the loft tank has emptied — Fix: Not a single airlock. Refill slowly and vent from the lowest tap upwards, allowing each to run clear before moving on.
Questions people ask
Why does it only happen to the hot water?
On a vented system the hot side is fed from the loft tank with very little head, and the pipe often runs up over joists. Both conditions favour trapping air. The cold mains has enough pressure to shift a bubble on its own.
Is it safe to connect two taps with a hose?
Briefly and attended, yes — that is the standard method. Left connected it is a backflow risk and a regulations breach. The rule is: connect, clear, disconnect, and never walk away with the hose on.
Can an airlock damage anything?
Not usually, but an airlock in a heating circuit or across a boiler can cause overheating and cut-outs, which is a different and more serious situation than a spluttering tap.