No water upstairs — cold water tank airlock
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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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