An airlock in water pipes makes a tap splutter, cough or run weakly and usually clears with the hose trick or by refilling the system. Banging when a tap or valve closes is water hammer, fixed with an arrestor or by securing loose pipes. Air that keeps coming back after you have cleared it is often being drawn in through a leak, and that needs finding.
You turn on the bathroom tap and it coughs, spits, runs for a second and spits again. Or the whole house shudders with a bang every time the washing machine finishes filling. Neither sounds serious, and often neither is. But taps and pipes only misbehave for a reason, and one of those reasons is a leak somewhere you cannot see. This guide sorts the three common causes, gives you the fixes you can do yourself, and explains the pattern that tells us air is a symptom rather than the problem.
Which one have you got?
| What you notice | Most likely cause | What to do |
|---|---|---|
| Tap splutters or spits, flow weak or stops, usually hot or upstairs | Airlock | Hose trick or refill the tank; see below |
| Loud bang or thud when a tap, valve or appliance shuts off | Water hammer | Secure pipes, fit an arrestor, check the pressure |
| Rattling or knocking while water is running | Loose pipe clips or high pressure | Clip the pipe, fit a pressure reducing valve if needed |
| Radiators need bleeding every few weeks, boiler pressure falls | Air entering through a heating leak | Check for a leak before bleeding again |
| Air returns in the cold supply after every fix, meter moves with taps off | Air drawn in through a leak on the supply pipe | Leak detection on the supply |
| Milky water that clears from the bottom up in a glass | Dissolved air, harmless | Nothing; common after mains work |
Airlocks: why they happen and how to shift one
An airlock is a bubble of air trapped at a high point in a pipe, big enough to stop water pushing past it. It happens most in gravity systems, the ones with a cold water tank in the loft feeding a hot water cylinder, because the pressure from the tank is low and cannot force air out of the way. Airlocks appear after the system has been drained for work, after the tank has run dry, or after a ball valve has stuck and let the tank empty. Hot taps upstairs suffer most, since hot water has the lowest pressure in the house.
The hose trick
This uses mains pressure from a cold tap to push the air out of the hot pipe.
- Find a mixer or a pair of taps where a mains-fed cold tap (usually the kitchen) sits next to the hot.
- Connect a short length of hose from the cold tap to the airlocked hot tap, and secure both ends.
- Open the hot tap fully, then open the cold.
- Leave it for thirty seconds to a minute, then close the cold first, then the hot.
- Test the airlocked tap. Repeat once or twice if needed.
If you have a combi boiler or an unvented cylinder, everything is at mains pressure and true airlocks are rare. Spluttering there is more likely to be a failing tap cartridge, a partly closed isolation valve or air in the incoming main after work in the road.
Other ways to clear air
- Refill the loft tank. If the ball valve has stuck, free it and let the tank fill. Then open the lowest hot tap in the house and wait.
- Open every hot tap at once, starting at the bottom of the house. The combined draw can pull the bubble through.
- Bleed the radiators and any air vents on a heating circuit that gurgles, then top the pressure back up to about 1 to 1.5 bar cold.
Water hammer: the bang when something closes
Water moving down a pipe has momentum. When a tap, a float valve or a washing machine solenoid shuts quickly, that moving column stops dead and the shock travels back through the pipework as a bang. Modern quarter-turn taps and appliance valves shut far faster than an old rising-spindle tap, which is why the problem often starts after a new kitchen or bathroom.
Fixes, in order of cost:
- Clip loose pipes. A pipe that can move will knock against joists and walls. Under the bath and in the loft are the usual places.
- Check the mains pressure. In parts of Cornwall the incoming pressure is high, and a pressure reducing valve on the supply calms the whole house.
- Fit a water hammer arrestor. A small sealed cushion of air fitted near the offending appliance or tap absorbs the shock. They are inexpensive and a plumber can fit one in under an hour.
- Replace a worn float valve. A toilet cistern or loft tank valve that chatters as it closes is a common source of repeated banging.
Water hammer is not a leak, but it can cause one. Years of shock loading work compression joints loose and split old solder, so a house that has banged for a long time is worth checking. Our guide to the sound of running water in the walls covers what a leak sounds like once it has started.
When air in the system means a leak
This is the part that matters. Water in a sealed pipe cannot make air out of nothing. If you clear an airlock and it comes back, or radiators need bleeding again and again, the air is getting in from somewhere, and the same fault that lets air in usually lets water out.
On the heating circuit
A sealed heating system that keeps gathering air almost always has a leak. When the boiler is off and the system cools, the water contracts and pressure drops. If there is a small hole, air is drawn in at that moment. When the system heats, water is pushed out of the same hole. The homeowner sees a pressure gauge that falls, radiators that go cold at the top, and a filling loop they use more than they should. Our guide to central heating losing water with no visible leak explains the pattern in full. Constant bleeding and refilling brings in fresh oxygen, which corrodes radiators and produces sludge, so the leak costs more the longer it runs.
On the cold supply
A split or a failed joint on the supply pipe between the boundary and the house can pull air in when the pressure drops overnight or when a neighbour draws heavily. The tap splutters first thing in the morning, then runs fine. On the long supply pipes common to Cornish farms and cottages this is a regular finding. The simple check is the meter: with every tap closed, watch it for ten minutes. Movement means a leak. Our article on a water meter moving when nothing is on explains how to read it.
Finding the entry point
A leak that admits air is usually small and hidden, which is why it has not been found already. Water leak detection uses acoustic listening for pressurised pipes, thermal imaging for heating circuits under floors, and tracer gas for buried or encased pipe where nothing else will reach. We do this across Cornwall, and the report tells your plumber where to open up and your insurer what was found.
A short diagnosis routine
Before booking anyone, spend fifteen minutes on this.
- Note which taps misbehave. Hot only and upstairs points to an airlock. Cold and every tap points to the supply.
- Note when. First thing in the morning suggests air drawn in overnight. After an appliance fills suggests hammer.
- Check the loft tank if you have one. Is it full, is the float valve free, is it overflowing?
- Check the boiler gauge cold. Below 1 bar with a history of topping up is a heating leak until proved otherwise.
- Watch the water meter with everything off. Any movement is the clincher.
If the answers land on airlock or hammer, the fixes above will sort it. If they land on a leak, do not keep bleeding and refilling. Find it.
Frequently asked questions
Can an airlock clear itself?
Sometimes, if there is enough pressure behind it to push the bubble through to a tap. On a low-pressure gravity hot water system it usually will not, and the hose trick or refilling from the loft tank is needed. An airlock that keeps returning after clearing points to air entering through a leak.
Why do my taps splutter only in the morning?
Overnight the pressure in the supply drops and a small split or failed joint can draw air in, which is pushed out through the first tap used. Check the water meter with every tap closed. If it moves, the supply pipe has a leak and the spluttering is a symptom.
Is water hammer dangerous?
Not immediately, but repeated shock loading loosens compression joints and stresses old solder, so a house that bangs for years often ends up with a leak. Clip loose pipes, fit an arrestor near the appliance that triggers it and have the mains pressure checked.
Why do my radiators keep needing bleeding?
A sealed heating system cannot make air on its own. If radiators need bleeding every few weeks and the boiler pressure falls, air is being drawn in through a leak when the system cools. Keep a note of the gauge and book a heating leak survey rather than continuing to refill.
Does spluttering mean the water supply is about to fail?
Rarely. Air in the mains after work in the road clears within a day. Ongoing spluttering with a moving meter or a boiler losing pressure is a leak that will get worse rather than a supply problem, so it is worth finding while the damage is small.
Air that keeps coming back?
If you have cleared the airlock twice and it returns, or the radiators never stay bled, let us find where the air is getting in. One survey, one small opening, and a report your insurer will accept.
