When your heater stops working on a cold evening, it feels like an emergency. But before you call out an engineer — and pay a call-out fee — there are several safe checks you can do yourself. Many heater problems turn out to be simple: a tripped switch, air trapped in a radiator, or a thermostat set wrong after a power cut.

This guide covers how to fix a heater safely, whether you have central heating radiators, electric heaters, storage heaters or a gas boiler. It also explains clearly which jobs you must leave to professionals under UK law.

Safety First: What You Must Not Touch

In the UK, it is illegal for anyone who is not Gas Safe registered to work on gas appliances, including boilers, gas fires and gas pipework. Doing so risks gas leaks, carbon monoxide poisoning and invalidating your home insurance. If you smell gas, call the National Gas Emergency Service immediately on 0800 111 999.

Similarly, do not open up electric heaters or storage heaters to poke at wiring unless you are competent — always unplug them first, and call an electrician for anything beyond basic checks.

Step 1: Check the Basics

Run through this checklist before anything else — it solves a surprising number of “broken” heaters:

• Power supply: Is the heater actually plugged in and switched on at the wall? Check the fuse in the plug (most portable heaters use a 13A fuse).
• Circuit breaker: Look at your consumer unit (fuse box). A tripped switch for the heating circuit is a common cause after a power surge.
• Thermostat: Check the room thermostat is set above the current room temperature, and that any timer or programmer is calling for heat. After a power cut, programmers often reset to factory defaults.
• Boiler pressure: For combi boilers, check the pressure gauge — it should read between 1 and 1.5 bar when cold. Low pressure is the single most common reason a boiler refuses to fire up.
• Pilot light / error codes: Modern boilers display error codes. Note the code and check the manual — many have a simple reset procedure.

How to Fix a Radiator That Is Cold at the Top

A radiator that is hot at the bottom but cold at the top has trapped air inside. The fix — bleeding the radiator — takes five minutes:

1. Turn the heating on and let the radiators warm up fully, then switch the heating off.
2. Find the bleed valve: a small square-headed screw at the top corner of the radiator.
3. Hold an old cloth or small jug under the valve and turn it anticlockwise a quarter-turn with a radiator key (about £2 from any hardware shop).
4. You will hear hissing as air escapes. When water starts to dribble out steadily, close the valve.
5. Check your boiler pressure afterwards and top it up via the filling loop if it has dropped below 1 bar.

Bleed all your radiators once a year, ideally before winter. If you find yourself bleeding the same radiator repeatedly, there may be a leak or corrosion issue worth getting checked.

How to Fix a Radiator That Is Cold at the Bottom

Cold at the bottom but hot at the top usually means sludge — a build-up of rust and debris in the system. This is more involved:

• Try turning all other radiators off and running the heating with just the affected one on full — sometimes this flushes minor blockages through.
• A magnetic system filter fitted to the pipework catches sludge going forward.
• For badly sludged systems, a professional powerflush (£300–£500 in the UK) cleans the whole system.

Fixing Electric Heaters

Portable fan and convection heaters

• Check the plug fuse and try a different socket to rule out a dead outlet.
• Many have a thermal cut-out that trips if the heater overheats (often because it was covered). Unplug it, let it cool for 30 minutes, and look for a small reset button — usually on the underside or back.
• Vacuum dust from the air intake grilles; clogged heaters overheat and cut out.

Storage heaters

• Check the dedicated switches — storage heaters usually have two: one for the overnight charge and one for the daytime boost. Both are often on a separate consumer-unit circuit.
• Make sure the input and output dials are not both set to minimum.
• If the heater charges overnight but gives no heat by day, the thermostat or thermal fuse may have failed — this needs an electrician.

Oil-filled radiators

These are sealed units. If one stops heating, check the power and thermostat as above. Never attempt to open an oil-filled radiator — the oil and internal wiring are not user-serviceable. Fortunately, they rarely fail; a quality oil-filled radiator lasts 15–20 years.

Fixing a Boiler That Will Not Fire Up

For combi and system boilers, safe DIY checks include:

1. Check the pressure gauge — below 1 bar usually means the boiler locks out. Top up via the filling loop (two taps under the boiler) until the gauge reads 1–1.5 bar, then close both taps firmly.
2. Reset the boiler — hold the reset button for 3–5 seconds. If it fires and stays on, the problem may have been a one-off glitch.
3. Check the condensate pipe — in freezing weather, the white plastic condensate pipe outside can freeze solid, shutting the boiler down. Pour warm (not boiling) water over the exposed pipe to thaw it, then insulate it with foam pipe lagging.
4. Check the thermostat batteries — wireless thermostats with dead batteries cannot call for heat.

If the boiler shows the same error code after a reset, or you hear unusual banging or gurgling, stop and call a Gas Safe engineer. Describe the error code when you book — it helps them bring the right parts.

How Much Does Heater Repair Cost in the UK?

• Boiler service: £80–£120 annually (recommended by manufacturers and often required to keep warranties valid).
• Boiler repair call-out: £60–£100 call-out plus parts; common fixes like a new pump or diverter valve run £150–£350 all-in.
• Powerflush: £300–£500 for an average 3-bedroom house.
• New thermostatic radiator valves (TRVs): £15–£30 each fitted, and they can cut heating bills noticeably.
• Electrician call-out: £50–£80 for the first hour.

Compare quotes on Checkatrade or Rated People, and always confirm gas engineers are on the Gas Safe Register before they start work.

Preventing Heater Breakdowns

• Service your boiler every year — ideally in summer when engineers are less busy and cheaper.
• Bleed radiators each autumn before you first switch the heating on.
• Keep the area around boilers and heaters clear for ventilation.
• Fit a carbon monoxide alarm near any gas appliance and test it monthly — NHS guidance on carbon monoxide poisoning explains the symptoms to watch for.
• Lag exposed condensate pipes before the first frost.

Frequently Asked Questions

Why is my heater blowing cold air?

For electric fan heaters, cold air usually means the heating element has failed or the thermal cut-out has tripped — try the reset procedure above. For central heating, cold radiators point to air, sludge, low boiler pressure or a pump problem.

Can I fix my boiler myself?

You can do basic checks: pressure top-up, reset, thermostat batteries and thawing a frozen condensate pipe. Anything involving opening the boiler casing, touching gas components or altering pipework must be done by a Gas Safe registered engineer — it is the law.

How often should radiators be bled?

Once a year before winter is enough for most homes. If a radiator needs bleeding every few weeks, get the system checked for leaks or corrosion.

Why does my boiler keep losing pressure?

Frequent pressure loss usually indicates a leak — check radiator valves and visible pipework for drips. It can also mean a faulty pressure relief valve or expansion vessel, both of which need an engineer.

Is it cheaper to repair or replace an old boiler?

As a rule of thumb, if repair costs exceed a third of replacement cost (new combi boilers run £1,500–£3,000 fitted) or the boiler is over 12–15 years old, replacement usually makes more financial sense — modern condensing boilers are far more efficient.

Most heater problems yield to the simple checks in this guide — power, thermostat, pressure, air. Work through them methodically before spending money on a call-out, but never hesitate to call a professional for gas or complex electrical faults. A warm, safe home is worth the price of an engineer. For more home maintenance help, see our guides on fixing a sagging door and drilling into concrete safely.