Cold tap runs warm — hot and cold water mixing
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Most likely cause & what to check
If cold water from a tap is warm, hot water is getting into the cold supply — known as cross-flow contamination. This is a plumbing fault and should be rectified promptly, as it can affect Legionella control.
The most common cause on a monobloc mixer tap is a failed ceramic cartridge that no longer fully separates hot and cold water. Replace the cartridge (see our cartridge replacement guide).
Another common cause is a check valve missing or failed on the hot or cold supply under the sink. Both hot and cold should have check (non-return) valves — especially if there is no back pressure from a tank.
On combi boiler systems, mains pressure on cold and hot should be balanced. If the cold supply pressure is much higher, water can push back through the mixer and into the hot — a check valve resolves this.
If you cannot identify the cause, a plumber can diagnose and fit check valves or replace the tap cartridge.
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