Hot water runs cold after a few minutes
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →There's nothing quite as frustrating as stepping into a hot shower only to have it turn cold within minutes. This is a common problem in UK homes, particularly those with combi boilers, and it's usually telling you that something isn't quite right with your heating system. The good news is that in many cases, it's straightforward to diagnose and fix without calling an engineer. This guide walks you through the most likely culprits — from simple flow rate issues to boiler pressure problems — so you can work out what's happening and know whether it's a DIY fix or time to contact a Gas Safe registered engineer. If you've already tried the basics and nothing's changed, don't ignore it; a fault inside the boiler itself won't heal on its own.
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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."
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Frequently asked questions
Why does my hot water start hot then suddenly go cold?
Your boiler is struggling to maintain consistent heat delivery, usually due to low water pressure, insufficient flow rate when multiple taps are open, or an internal fault with the diverter valve. Start by checking your boiler pressure gauge — it should read between 1 and 1.5 bar — and test whether the problem occurs when only one tap is running.
Is this problem dangerous?
Temperature fluctuations alone aren't a safety hazard, but they're a sign something's wrong with your boiler. If you suspect an internal fault like a faulty diverter valve or heat exchanger, you should arrange an inspection by a Gas Safe engineer within a week or two rather than leaving it unattended.
Will my boiler need replacing if this keeps happening?
Not necessarily. If it's a pressure issue, a simple re-pressurisation can fix it. If it's a diverter valve or heat exchanger fault, a Gas Safe engineer can often repair or replace just that component without replacing the entire boiler, depending on age and cost.
Why does it happen when someone else uses the kitchen tap?
Combi boilers have a maximum flow rate they can deliver. When two outlets demand hot water simultaneously, the boiler can't supply enough heat to both, so your shower temperature drops. This is normal operation, not a fault — it's simply a limitation of how combi boilers work.
Can low boiler pressure cause other problems?
Yes, low pressure affects your entire heating system. You may notice weak radiator performance, slow hot water delivery, or the boiler shutting down and needing a reset. Checking and re-pressurising your boiler is a basic maintenance task that often resolves multiple issues at once.