No water upstairs — cold water tank airlock
If you've suddenly lost cold water to your upstairs taps despite the loft tank being visibly full, you've almost certainly got an airlock. This is one of the most common issues in traditional gravity-fed systems, particularly in older properties or after pipework has been disturbed. Airlocks occur when pockets of air become trapped in the distribution pipes, blocking water flow completely — even though there's plenty of water sitting above ready to come down. The good news is that clearing an airlock is straightforward and something most homeowners can tackle themselves in under ten minutes using nothing more than a garden hose. This guide walks you through confirming the problem and clearing it using mains pressure from your kitchen tap. However, if the tank itself is empty or won't refill, that points to a different fault with your ball valve, which may need professional attention.
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Most likely cause & what to check
In a traditional UK gravity-fed system, the cold water storage tank in the loft supplies cold water to the bathroom (and sometimes the hot water cylinder). An airlock can prevent flow even when the tank is full.
Go into the loft and confirm the cold water tank has water in it. If the tank is empty, check the ball valve — push the arm up by hand to confirm water enters when the valve opens.
To clear an airlock in the distribution pipe: find the cold tap nearest the tank (usually the bath cold tap). Connect a length of hose between this tap and the kitchen cold tap (which is on the mains supply). Turn on the kitchen tap first, then the bath tap — mains pressure will blow back through the pipe and clear the airlock.
After clearing: turn off both taps, remove the hose and test the supply. This should immediately restore flow.
If the tank was empty: investigate why the ball valve failed to keep it full before using the system again.
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Frequently asked questions
Why would an airlock happen if my tank is full of water?
Air can become trapped in the pipes during installation, after maintenance work, or if the system has drained and refilled unevenly. Because the tank relies on gravity alone to push water down, even a small air pocket can completely block the flow, whereas mains pressure systems would overcome it. Once trapped, that air pocket sits stubbornly in place until you physically shift it out.
Will the airlock clear itself eventually?
Unfortunately, no — an airlock in a gravity-fed system won't shift on its own because there's no pressure driving it out. The only way to clear it is to apply force from outside the system, which is why the mains pressure method works so effectively.
Is it safe to use the mains pressure method on an old system?
Yes, this method is safe and recognised across the industry for clearing airlocks in gravity-fed pipework. The backflow is momentary and gentle, and modern systems have checks in place to prevent mains water entering the tank. If you're uncomfortable doing it yourself, any qualified plumber can clear an airlock in minutes.
What if clearing the airlock doesn't work?
If flow still doesn't return after following these steps, the problem may lie elsewhere — perhaps a blockage, a kinked pipe, or a failed isolation valve. At this point, it's worth calling a plumber to inspect the pipework and rule out other faults before investigating further.
How can I prevent airlocks happening again?
There's no foolproof prevention, but minimising disturbance to the system helps — for example, draining the tank carefully if you need to do maintenance work, and refilling it slowly. If your system is prone to airlocks, a plumber may recommend fitting a small isolation valve or air release valve on the distribution pipe.
Do I need to bleed all the taps after clearing the airlock?
No, once you've cleared the blockage, water should flow normally straight away. You may hear some spluttering at the tap for a few seconds as any remaining air clears, but there's no need to bleed multiple taps or let water run to waste.