Hot water goes cold after a short time then hot again
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Most likely cause & what to check
This symptom — hot water running cold briefly then returning to hot — has a specific name: "cold water sandwich." It is caused by the sequence of water through the pipes rather than a fault.
It happens when you've recently used hot water, the system cooled slightly, and you've started using it again. You first get residual hot water from the pipe, then the cold water that filled the pipe when you stopped, then genuinely hot water from the boiler.
For combi boilers: this is most pronounced if the boiler takes a few seconds to fire up and the pipe run to the tap is long. It's a feature of instant hot water systems, not a fault.
The only real fix is to fit a secondary return circuit — a loop pipe that continuously circulates hot water from the boiler back to the taps, so hot water is always immediately available. This is expensive to retrofit but standard in new large homes.
If the water goes cold and stays cold: your combi boiler may be cutting out (check for a fault code), the diverter valve may be stuck in the heating position, or the hot water temperature is set too low.
For a vented cylinder system: hot water going cold after several minutes suggests the cylinder is running out — the cylinder thermostat is set too low, the heating programme isn't on long enough, or the cylinder is undersized for demand.
If the water alternates hot-cold-hot continuously at the tap, it's likely a thermostat or flow sensor fault inside the boiler. An engineer can diagnose this with the boiler running.
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