Combi Boiler: No Hot Water but Heating Works? Causes and Fixes

If your combi boiler has stopped producing hot water, it’s frustrating — but the cause is often something simple you can check yourself. This guide covers why a combi boiler has no hot water, including the common situation where the heating works but there’s no hot water, and how to fix it.

First, check these quick things

Check the boiler pressure. Look at the gauge — it should read around 1 to 1.5 bar. If it’s below 1 bar, low pressure can stop the boiler heating water. Top it up using the filling loop, or see our guide on boiler pressure.
Check the thermostat and controls. Make sure the hot water setting hasn’t been turned down or a timer isn’t switching it off.
Check other taps. If only one tap has no hot water, the problem is that tap or its cartridge, not the boiler.
Reset the boiler. If there’s a fault code on the display, note it down and try the reset button once.

No hot water but the heating still works?

This is one of the most common combi boiler faults, and it usually points to one thing: a stuck diverter valve. The diverter valve switches the flow of hot water between your radiators and your taps. When it sticks in the heating position, your radiators stay warm but no hot water reaches the taps. A diverter valve is an internal component that needs a Gas Safe registered engineer to repair or replace — it’s not a DIY job.

Other causes of no hot water

Low boiler pressure: the most common and easily fixed cause.
A faulty diverter valve: the usual culprit when heating works but hot water doesn’t.
An airlock: trapped air in the pipework can block the flow of water.
A broken thermostat or PCB: the boiler’s controls can fail and stop it firing for hot water.
A frozen condensate pipe: in very cold weather, a frozen pipe can cause the boiler to shut down.

Is it worth repairing?

Most no-hot-water faults — a diverter valve, thermostat, or pressure issue — are straightforward, affordable repairs and well worth fixing. If your boiler is over ten years old and faults are becoming frequent, an engineer can advise whether a repair or a new boiler makes more financial sense.

When to call a professional

Checking the pressure and controls is safe to do yourself. But a diverter valve, PCB, thermostat, or airlock should be handled by a Gas Safe registered engineer. If you’ve checked the pressure and controls and still have no hot water, it’s time to book a boiler repair. It’s also worth checking your radiators are heating up correctly, as related faults can affect both.

Frequently asked questions

Why does my combi boiler have heating but no hot water?

This is almost always a stuck or faulty diverter valve, which controls whether hot water goes to your radiators or your taps. It needs a Gas Safe engineer to fix.

Can low boiler pressure cause no hot water?

Yes. If pressure drops below 1 bar, the boiler may stop heating water. Top it up to around 1 to 1.5 bar and see if hot water returns.

Should I reset my boiler if there’s no hot water?

You can try the reset button once if there’s a fault code. If it doesn’t restore hot water or the fault returns, call an engineer rather than repeatedly resetting.

Need a boiler engineer in West London?

If you’ve run the checks above and still have no hot water, our engineers can diagnose and fix it fast. Our Gas Safe registered engineers provide fast, reliable boiler repairs and plumbing and heating services across West London, often fixing the problem in a single visit.

Tel: 020 3561 4415