Hot water runs cold after a few minutes
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Most likely cause & what to check
Check if it only happens when someone else opens another tap — combi boilers have a limited flow rate and can't supply two outlets well at once. Try one at a time.
Check the boiler pressure is between 1 and 1.5 bar. Low pressure causes inconsistent hot water.
If those don't explain it, this is most likely an internal boiler fault (diverter valve or heat exchanger). Call a Gas Safe engineer and tell them: "Hot water starts hot then goes cold after a few minutes."
🛠 Tools & materials you may need
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