Boiler not working: no heating or hot water? Causes and fixes

A dead boiler in January is not a fair fight. Before you spend money on a call-out, five minutes of checks will tell you whether this is a five-second fix or a real repair. This guide covers all three situations: no heating with hot water, hot water problems with heating fine, and nothing at all. Work out which one you have, then follow the checks for that scenario.

Most sudden boiler failures come down to one of five things: no power, low pressure, a frozen condensate pipe, a lockout, or a failed part like the diverter valve or pump. The first four you can often sort yourself. The last needs a Gas Safe registered engineer.

The 5-minute check before you call anyone

Run through these in order. Any one of them can bring a boiler back with no repair bill.

1. Power. Check the boiler has power and its display is lit. Look at the fuse board for a tripped switch or RCD and reset it. If it trips again straight away, leave it off and book an engineer.
2. Pressure gauge. Cold system should read 1.0 to 1.5 bar. Below 1 bar and many boilers shut the heating down. Below 0.5 bar and most lock out completely.
3. Boiler display. Note any fault code or error light. Photograph it. That code tells the engineer half the story before they arrive.
4. Thermostat and programmer. Turn the room thermostat well up. Set the programmer to “on” or “constant”, not “timed”. Replace thermostat batteries if it is wireless.
5. Gas. Check other gas appliances work. Is the meter credit topped up? Has the emergency control valve been knocked to off?

If all five are fine and the boiler still will not run, it is a fault that needs an engineer. Skip to the diagnostic table below.

Points to take away

• Check power, pressure, the fault code, the thermostat and the gas supply before booking anything.
• Reset the boiler once. Never more than twice on a genuine fault.
• In cold weather, a frozen condensate pipe is the single most common winter breakdown, and you can often thaw it yourself.
• Repairs over about £500 on a boiler older than 10 years are usually the point to price a replacement.
• Smell gas at any point: stop, turn off at the meter, ventilate, leave, call 0800 111 999.

Reading time: about 9 minutes.

First, work out which problem you have

The fix depends entirely on which part of the boiler has stopped. Match your situation to one of these three, then read that section.

A combi boiler makes hot water on demand and has no cylinder, so a fault on one side rarely kills the other. A system or regular boiler heats a hot water cylinder, so “no hot water” can also mean a failed cylinder immersion or a motorised valve stuck shut. If you are not sure which type you have, our guide to combi, system and regular boilers explains how to tell them apart.

No heating, but hot water is fine

The boiler is lighting. Something is stopping the heat reaching the radiators.

Thermostat or programmer. The most boring cause and the most common. A flat thermostat battery, a programmer stuck on “off”, or a holiday setting still active. Turn the stat up past room temperature and listen for the boiler to fire within a minute.

Stuck diverter valve. On a combi, this valve sends hot water either to the taps or to the radiators. When it sticks in the hot water position, you get endless hot showers and stone cold rads. A tell-tale sign is heating that works for a few minutes after a hot tap has run, then dies. This is an engineer job, usually £250 to £400 fitted.

Seized pump or airlock. The circulating pump moves water round the radiators. If it is seized or airlocked, the boiler heats water that then goes nowhere. Some pumps can be freed by an engineer with a screwdriver on the bleed screw. A replacement pump is £250 to £400 fitted.

Motorised valve (system and regular boilers). A small actuator on the pipework near the cylinder. When the motor burns out, it can fail shut and block flow to the radiators. Around £150 to £280 fitted.

Every radiator cold, but pipes near the boiler warm. Points to pump, valve or airlock. Some radiators warm, some cold. Points to trapped air (bleed them) or sludge (needs a power flush).

Hot water problems, heating is fine

The radiators heat up. The taps run cold or only briefly warm.

Diverter valve again, the other way. Stuck in the heating position, so hot water never reaches the taps. Same part, same rough cost as above.

Combi flow rate too low. A combi needs a decent flow of cold mains water to make hot water. A partly closed isolation valve, a blocked filter or a failing plate heat exchanger can starve it. A furred plate heat exchanger is common in hard water areas and costs £250 to £450 fitted.

Cylinder faults (system and regular). No stored hot water can mean a failed immersion heater, a stuck motorised valve on the cylinder circuit, or a cylinder thermostat that has failed. Check the airing cupboard for a tripped immersion switch first.

Lukewarm, never hot. Often a scaled heat exchanger or a diverter valve that is not fully closing. Hot then cold in cycles at the tap. Classic dying diverter valve or a flow sensor fault.

No heating and no hot water at all

The boiler is not firing. Work through these.

Frozen condensate pipe. In cold weather this is the number one breakdown. Modern condensing boilers drain acidic water through a plastic pipe, often run outside, and it freezes. The boiler locks out, sometimes with a specific fault code, and you may hear a gurgle. You can usually fix it: pour warm, not boiling, water over the external pipe, or hold a hot water bottle against it, then reset the boiler. If it keeps freezing, the pipe needs lagging or re-routing.

Ignition lockout. The boiler tried to light, failed a set number of times, and shut down to be safe. Press the reset button once and hold for about 10 seconds. If it fires and stays lit, it was a blip. If it locks out again, stop. Repeated resets on a real fault, such as a failing gas valve or ignition electrode, can make things worse.

Low or no pressure. Covered in the 5-minute check. If topping up from the filling loop brings it back and it holds, you are fine. If pressure keeps dropping, there is a leak somewhere, often a failing pressure relief valve or a corroded radiator.

No gas. Other gas appliances dead too points to a supply problem: meter credit, a wider outage, or the emergency control valve turned off. One appliance dead is a boiler fault.

Printed circuit board (PCB) failure. The boiler’s brain. Symptoms are random: no display, display but no response, intermittent shutdowns, fans that run without ignition. A PCB is £350 to £600 fitted and is often the moment to weigh up a new boiler instead.

Diagnostic table: symptom, cause and rough cost

Use this to gauge whether you are looking at a quick fix or a real bill. Fitted prices are typical UK figures for 2026 and vary by area and boiler.

What you can safely do, and what you cannot

You can, without any risk: reset the boiler once, top up pressure via the filling loop, bleed radiators, thaw an external condensate pipe, replace thermostat batteries, and check the fuse board and gas supply.

You must not: remove the boiler casing, touch the gas valve or any gas connection, work on the flue, or “have a go” at the burner or heat exchanger. Gas work by anyone who is not on the Gas Safe Register is illegal and dangerous. Every registered engineer carries an ID card with a licence number and a list of what they are qualified to work on. Ask to see it.

When repair stops making sense

One repair on a boiler under 10 years old is almost always worth it. The picture changes when: the boiler is 12 years or older, a major part like the PCB or heat exchanger has failed, spares are on back order or discontinued, or you have paid for two or more repairs in the last couple of years.

At that point, get a repair quote and a replacement quote side by side. A £550 PCB on a 14-year-old boiler with a dead pump behind it is money spent on borrowed time. Our guide to what a new boiler costs breaks down the fitted price by job type.

Booking a repair

If the checks above have not fixed it, you need a Gas Safe registered engineer. Have your boiler make, model and the fault code ready, and mention how old it is. That lets the engineer bring the likely part on the first visit.

Get free quotes from Gas Safe registered engineers covering your postcode, or browse the directory to see which heating companies are registered in your area.