How to Clear an Airlock in Your Water Pipes

Updated July 2026 · Emergency Plumber Chelsea

An airlock is simply air trapped in a pipe where water should be. Water cannot compress but air can, so a bubble sitting at a high point in the pipework acts like a spring and blocks or chokes the flow. It usually shows up on the hot side, because hot water in a traditional Chelsea system is fed from a cold water storage tank in the loft at low gravity pressure, and that gentle pressure is easily beaten by an air pocket. The cold mains, by contrast, arrives at far higher pressure and rarely airlocks, which is exactly why the mains is your tool for clearing one.

The classic signs are a tap that splutters and spits air, a flow that starts then stops, or no water at all from one outlet while others run fine. You may hear banging or hissing as air moves, and the problem is almost always on the hot tap or a specific mixer rather than across the whole house. Airlocks typically appear right after something disturbed the system: the water was drained down for a repair, the loft tank ran low or was refilled, a new tap or bathroom was fitted, or a stopcock was turned off and back on. If instead every tap in the property is weak, the cause is more likely low mains pressure, not an airlock.

Before you start, confirm it is actually an airlock. Turn on every tap in turn and note which are affected. A single hot outlet spluttering while cold runs strong points to trapped air. Check the loft cold water storage tank if you have one and can reach it safely, because a tank that has run dry or has a stuck ball valve will starve the hot side and mimic an airlock. Make sure your stopcock is fully open too. It also helps to note whether the flow was fine until a recent event, since a sudden onset after works or a tank refill is the signature of an airlock rather than a gradual pressure problem. Once you are satisfied it is trapped air, work through the methods below in order, easiest first.

Method one, the hose method, is the most reliable fix. You need a short length of hose that fits both a hot and a cold tap, ideally with a jubilee clip or push-on connector at each end. Attach one end to the airlocked hot tap and the other to a cold tap fed directly from the mains, such as the kitchen cold tap. Open the hot tap first, then open the cold tap. Mains pressure now flows backwards up the hose into the hot pipework and drives the trapped air back towards the tank or vent, where it escapes. Hold it for 30 seconds to a minute, then turn the cold off first, then the hot. Test the hot tap on its own. If it still splutters, repeat once or twice, because a large pocket sometimes needs more than one push. Use a hose in good condition and clip the ends firmly, since the cold mains can build real pressure and a loose connector will pop off and soak the room.

Method two works on a kitchen mixer or a monobloc tap without a hose. Hold a cloth firmly over the spout to seal it, then open both the hot and cold sides fully. The high-pressure cold water is forced across into the hot line and pushes the air out. Keep the spout sealed for around 30 seconds, release, and run the hot tap alone to check. If your taps have separate hot and cold spouts rather than a single mixed outlet, this method will not work and you should use the hose instead.

Method three tackles airlocks lower in the system, such as after a boiler, cylinder or pump has been drained. Start by turning off the loft tank supply if you have one, then open the affected hot taps and let them run to draw water and air through. For an indirect hot water cylinder, bleeding at the highest hot outlet while briefly using mains pressure through a hose usually shifts stubborn pockets. Radiators are a related but separate job: air trapped in a radiator is bled with a radiator key at the valve, not by the hose method, and if your heating feels cold at the top that is a bleeding task rather than a supply airlock.

For airlocks that keep coming back or will not shift, work methodically rather than repeating the same step. Turn off the cold water storage tank, open all hot taps and drain the hot system fully, then close the taps and let the tank refill slowly so water pushes air ahead of it rather than trapping it. Reopen taps one at a time starting with the lowest in the house and working up. This full flush-through often clears air that a single-tap method leaves behind, and it is the sensible approach in a multi-storey Chelsea townhouse where pipework rises through several floors.

Know when to stop and when an airlock points to a bigger issue. If air returns every few days, if you can never fully clear it, or if the flow was never right after recent works, the problem is usually a design or fault issue rather than a one-off bubble. Undersized or badly routed pipework with high points that trap air, a failing loft tank ball valve, a sludged-up cylinder, or a supply pipe partly blocked with scale or debris all produce airlock-like symptoms that DIY cannot cure. Persistent air on the hot side can also signal a pump or cylinder fault in a pressurised system. It is worth noting when the trouble started and what happened just before, because a fault that dates from a recent bathroom fit or a drained-down repair usually means the pipework was left with a permanent air trap that needs re-routing rather than re-clearing. Repeatedly bleeding the same airlock only masks that underlying defect.

There is one situation to treat with real caution. If air is coming from a combi boiler or an unvented (pressurised) hot water cylinder rather than a traditional tank-fed system, do not improvise. These systems are under mains pressure and unvented cylinders are safety-critical, so trapped air or repeated air discharge should be looked at by a qualified engineer, and any gas or unvented work needs the right registration. If you smell gas at any point, leave the property and call the National Gas Emergency Service on 0800 111 999 first.

Airlocks are especially common in Chelsea's period and mansion-block homes because so many still run on gravity-fed systems with a loft tank and long, old pipe runs that were extended or altered over decades. A loft tank that sits low, a length of pipe that dips and rises across a converted floor, or a system that was drained for listed-building or bathroom works all make air more likely to lodge. If the hose and flush methods do not give you a steady, full-pressure hot flow, or the fault keeps returning, call Emergency Plumber Chelsea on 020 3002 6667. We trace the real cause across SW3, SW10 and nearby postcodes, clear stubborn air, and put right the tank, cylinder or pipework fault behind it, with clear pricing before major work starts.

Quick tips

• Identify the airlock first: a single hot tap spluttering while cold runs strong is trapped air; every tap weak usually means low mains pressure instead.
• Keep a short hose with tap connectors handy; the hose-and-tap method clears most household airlocks in under a minute.
• Always turn the cold tap off before the hot when using mains pressure, so you do not leave the hot line pressurised.
• Refill a drained system slowly from the lowest tap upward so water pushes air ahead of it rather than trapping new pockets.
• If air returns repeatedly, do not keep retrying the same step; a low loft tank, sludged cylinder or badly routed pipe is the likely cause and needs inspection.

FAQs