No Hot Water From Your Boiler? Causes and Fixes

A boiler that fires up but delivers cold (or barely warm) water at the tap is frustrating, especially in the depths of a British winter. The good news is that the fault often isn't catastrophic. Sometimes it's a setting, a flat tyre on the system pressure, or a single worn component. The key is working out which kind of boiler you have, because the likely culprits are quite different.

Combi (combination) boilers heat water on demand straight from the mains, so there's no hot-water cylinder. System and heat-only (regular) boilers heat a stored cylinder of water, usually in an airing cupboard or loft. If you're not sure which you have, a combi has no separate hot-water tank, while system and heat-only setups do.

Gas safety first. Anything involving the gas valve, gas pipework, the flue, the sealed combustion circuit, the pressure-relief valve or removing the boiler casing is strictly a job for a Gas Safe registered engineer. Never attempt it yourself. (The old CORGI scheme was replaced by Gas Safe in 2009.) If you ever smell gas, call the National Gas Emergency line on 0800 111 999.

If you have a combi boiler

Because a combi makes hot water instantly, a hot-water fault usually points to one of these:

Low system pressure. If the pressure gauge reads below about 1 bar when cold, many combis cut out or run poorly. Normal is roughly 1–1.5 bar cold, rising to around 2 bar when hot. This is the most common and most fixable cause.
Faulty diverter valve. The diverter switches the boiler between heating your radiators and heating your taps. When it sticks, you often get plenty of heating but little or no hot water (or only lukewarm water). This is an internal repair.
Domestic hot water (DHW) thermostat fault. If the sensor that controls tap temperature fails, the boiler may not fire for hot water even though the heating works.
Limescale or a blocked plate heat exchanger. In hard-water areas, scale builds up in the small plate heat exchanger that transfers heat to your tap water. Symptoms are weak, fluctuating or slow-to-heat hot water that gets worse over time.
Flow switch / flow sensor. This tells the boiler you've opened a tap so it should start heating. If it fails or sticks, the boiler simply won't fire for hot water.

Of these, only low pressure is a safe DIY check (more on that below). The diverter valve, DHW thermostat, heat exchanger and flow switch all sit inside the boiler and need an engineer.

If you have a system or heat-only boiler

Here the boiler heats a stored cylinder, so the fault is often in the controls or the cylinder rather than the boiler itself:

Programmer or timer set to off. The simplest cause of all. Check the hot-water schedule on your programmer and try an "boost" or "on" override.
Cylinder thermostat fault. The thermostat strapped to the hot-water cylinder may have failed or been knocked off its setting, so the boiler never gets the call to reheat the tank.
Stuck motorised (zone) valve. A two-port or three-port valve directs heated water to the cylinder. If it seizes, the cylinder stops reheating even though radiators may still work.
Immersion heater issue. Many cylinders have an electric immersion heater as backup. If you normally rely on it and the water's cold, the element or its thermostat may have failed, or its switch/fuse may have tripped.
Programmer wiring or power fault. An ageing programmer or a tripped fused spur can stop the hot-water demand reaching the boiler.

Safe checks you can make yourself

Before you book anyone, run through these homeowner-safe steps. None of them involves the gas side of the boiler.

Check the programmer and thermostat. Make sure hot water is scheduled "on", any room or cylinder thermostat is turned up, and you haven't accidentally left the system in a holiday or summer mode.
Check the power and fuses. Confirm the boiler has power and nothing has tripped at the consumer unit or the fused spur near the boiler. For an immersion, check its own switch and fuse.
Check the pressure gauge (combi/sealed systems). If it reads below ~1 bar cold, you can usually top it up via the filling loop following your boiler manual. Aim for around 1–1.5 bar cold, then re-check.
Try one front-panel reset. If your boiler shows a fault code or lockout light, a single press of the reset button can clear a one-off glitch. If it locks out again, stop and call an engineer rather than repeatedly resetting.
Rule out a frozen condensate pipe. In cold snaps, a blocked external condensate pipe can shut the boiler down. Thawing the visible outdoor plastic pipe with warm (not boiling) water is homeowner-safe; the boiler internals are not.

Heating works but no hot water? That pattern is a useful clue in itself, often a diverter valve on a combi or a cylinder thermostat/motorised valve on a stored system. We cover it in detail in No hot water but heating works.

Typical repair costs

The figures below are broad UK indicative ranges for 2026, including parts and labour. Actual prices vary by region, boiler make and engineer call-out rates, so treat them as a guide only.

Job	Indicative cost
Top up system pressure (DIY)	£0
Engineer diagnostic / call-out	£60–£120
Diverter valve replacement (combi)	£150–£350
DHW or cylinder thermostat	£90–£200
Plate heat exchanger replacement	£250–£450
Motorised / zone valve	£150–£300
Immersion heater element	£150–£300
New combi boiler (if uneconomical to repair)	£1,800–£3,500+

Where boiler cover comes in

A repair to a diverter valve or heat exchanger can run into the hundreds, and a string of small breakdowns over a winter adds up. A boiler cover or home-emergency policy spreads that cost into a monthly figure and gives you a number to call when the hot water stops. Whether it's worth it depends on the age and reliability of your boiler. We weigh that up in is boiler cover worth it? and explain the basics in what is boiler cover?.

If you'd rather skip straight to options, our cheap boiler cover and best boiler cover guides compare cover levels, excess amounts and what's actually included.

Why does my combi have heating but no hot water?

On a combi this usually points to a sticking diverter valve, a faulty DHW thermostat or flow switch, all of which sit inside the boiler. Because the heating still works, the boiler is firing, it just isn't switching to or sensing the hot-water demand. It's an engineer job, not a DIY fix.

Could low pressure be the reason I have no hot water?

Yes. If your pressure gauge reads below about 1 bar when cold, many combis cut out or run poorly. Topping up via the filling loop to around 1–1.5 bar cold is a safe homeowner check. If the pressure keeps dropping, there may be a leak that needs investigating.

My water is warm but never gets hot. What's wrong?

Lukewarm or fluctuating hot water on a combi often points to limescale building up in the plate heat exchanger, common in hard-water areas. On a stored system it can be a cylinder thermostat set too low or a failing immersion. Both warrant a closer look from an engineer.

Is it safe to reset my boiler myself?

Pressing the front-panel reset button once to clear a one-off lockout is fine. But if the boiler locks out again straight away, don't keep resetting it, that can mask a genuine fault. Book a Gas Safe registered engineer instead.

Will boiler cover pay for a hot-water repair?

Most boiler-only and boiler-and-controls policies cover breakdowns like a failed diverter valve, thermostat or pump, subject to your excess and any age or service conditions. Always check the policy wording, as exclusions vary between providers.

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Information, not advice. We compare a selected panel of providers, not the whole market, and may earn a commission from links on this page. Costs are indicative UK ranges for 2026 and will vary by boiler, region and provider.

By the Boiler Cover UK editorial team · Last updated: June 2026