Combi boiler producing no hot water
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A combi boiler losing hot water is one of the most common issues we see, and the good news is that it's often fixable without calling an engineer. Modern combination boilers are brilliant when they work properly — they heat your water on demand without needing a separate tank — but when hot water suddenly stops, it can feel like a complete breakdown. The problem usually comes down to low pressure, a misplaced control setting, or an internal fault the boiler is flagging up. This guide walks through the three quick checks any homeowner can do in minutes, using nothing more than a look at your boiler's display and a quick check of your controls. If these steps don't restore your hot water, you'll need to ring a Gas Safe registered engineer, but at least you'll know you've ruled out the simple fixes.
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Most likely cause & what to check
Check the boiler pressure gauge on the front of the boiler — it should read between 1 and 1.5 bar. If it's below 1, the boiler won't heat your water. Use the filling loop underneath to top it up, then press the reset button.
Check your programmer or thermostat hasn't accidentally been set to heating-only. Make sure hot water is switched on.
Look at the boiler display for any fault codes — take a photo and use our Boiler Fault Code tool to find out what it means.
If those checks don't fix it, call a Gas Safe engineer. Tell them: "My combi boiler isn't producing hot water — heating may still be working. I've checked the pressure and programmer."
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Frequently asked questions
Why does my combi boiler have hot water one day but not the next?
The most common culprit is low boiler pressure, which can drop gradually over time or suddenly if there's a minor leak you haven't noticed. Combi boilers are very sensitive to pressure — even dropping below 1 bar stops hot water production. Check your pressure gauge first; if it's low, use the filling loop to top it up and you'll likely restore hot water immediately.
Can low pressure damage my boiler?
Running a boiler below 1 bar for extended periods can damage internal components and force the boiler into lockout mode to protect itself. However, a one-off dip in pressure won't cause permanent harm — just top it up and keep an eye on it. If pressure keeps dropping, you may have a leak and should contact a Gas Safe engineer.
Is it safe to use the filling loop myself?
Yes, filling loops are designed for homeowner use and are very straightforward — you'll find it beneath the boiler with a simple lever or valve. Turn it gently until you hear water flowing and watch the pressure gauge rise to between 1 and 1.5 bar, then turn it off. Never force it; if it won't budge, leave it for a Gas Safe engineer.
What should I do if my boiler shows a fault code but still has pressure?
Take a photo of the fault code on your boiler's display — it's usually a number or sequence like F28 or E119 — and search it using your boiler's manual or an online fault code tool. Different codes mean different things, from sensor faults to gas supply issues, and knowing the specific code helps an engineer diagnose the problem much faster when you call them.