Central heating failures in London tend to happen on the coldest days — often when the system hasn't been used for months. Many causes can be diagnosed and fixed quickly, some without calling a professional at all. Here's a systematic guide to finding out what's wrong.
Check These First (No Plumber Needed)
1. Check the Boiler Display
Your boiler's display panel is your first port of call. An error code narrows down the problem significantly. Look yours up in the manufacturer's manual or search "[brand] error code [number]" online. Common codes relate to low pressure, ignition faults, or overheating.
2. Check the Boiler Pressure
Look at the pressure gauge on the front of your boiler. The needle should be in the green zone — typically between 1.0 and 1.5 bar. If it's below 1, the system needs repressurising. See our guide: No hot water — causes and fixes for step-by-step repressurise instructions.
3. Check the Thermostat Settings
Is the thermostat set above the current room temperature? A room thermostat set to 18°C won't fire the boiler if the room is already 19°C. Also check the timer/programmer — after a power cut, the clock may have reset. Some smart thermostats (Nest, Hive) run on batteries that can die without warning.
4. Try Resetting the Boiler
Press the reset button on your boiler (usually a circular arrow symbol or flame icon). Hold for 3–5 seconds. If the boiler fires up and stays running, it was a temporary lockout. If it locks out again within minutes, there is an underlying fault.
5. Check If the Boiler Is Getting Gas
Check that other gas appliances in the property are working (hob, oven). If none are working, the gas supply may have been interrupted — call your gas supplier. If others work fine, the problem is with the boiler's gas valve.
Radiator Problems — Diagnose and Fix
All Radiators Cold
If no radiators are heating, the issue is almost certainly with the boiler or the system pump. After checking boiler pressure and settings, if the boiler appears to be running but radiators are cold, the pump may have failed. A plumber needs to diagnose and replace it — typically £150–£300 in London.
Some Radiators Hot, Some Cold
This is usually caused by one of three things:
- Air lock: Bleed the cold radiators (see below)
- Closed valve: Check that both the thermostatic valve (TRV, the one you can turn) and the lockshield valve (the capped end) are open
- Sludge build-up: Black sludge from corroding system components blocks individual radiators — a power flush resolves this
Radiator Cold at the Top, Hot at the Bottom
Classic trapped air. Bleed the radiator:
- Turn off the heating and let the system cool slightly
- Find the bleed valve — a small square-headed fitting at the top corner of the radiator
- Use a radiator bleed key (£1–£2 from any hardware shop) to turn the valve anti-clockwise
- Hold a cloth under it — air will hiss out, followed by water
- Once water flows steadily, close the valve
- Check your boiler pressure — it may have dropped; repressurise if below 1 bar
Radiator Cold at the Bottom, Hot at the Top
This is the opposite pattern — sludge has settled at the bottom. Bleeding won't fix it. The radiator needs either a chemical flush or a power flush of the whole system.
Zone Valve or Motorised Valve Problems
Many London homes have zone valves that separately control heating and hot water circuits. If one zone (e.g. upstairs radiators) isn't working but others are, a stuck or failed zone valve is a likely cause. A plumber can test and replace the valve for £100–£250.
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Get Free Quotes Now →Power Flushing — When Is It Needed?
Over time, London heating systems accumulate black sludge (magnetite) and limescale. This causes radiators to heat unevenly, reduces boiler efficiency, and can lead to pump failure. Signs you need a power flush:
- Multiple cold spots in multiple radiators
- Boiler making banging or kettling noises
- System slow to heat up
- Dark, dirty water when you bleed radiators
A power flush for a typical London property costs £300–£600 and takes a full day. It can significantly extend the life of your boiler and radiators.
Central Heating Repair Costs in London (2026)
| Problem | Typical London cost |
|---|---|
| Boiler pressure top-up / reset (DIY) | Free |
| Bleeding radiators (DIY) | Free |
| Thermostat or programmer replacement | £100–£200 |
| Zone / motorised valve replacement | £100–£250 |
| Circulating pump replacement | £150–£300 |
| Radiator replacement (standard) | £150–£300 |
| Power flush (whole system) | £300–£600 |
| Boiler repair (varies by fault) | £100–£400 |
For a full breakdown of boiler costs, see: How much does a boiler repair cost in London?
Frequently Asked Questions
Why does my heating come on but the radiators don't get warm?
If you can hear the boiler firing but the radiators stay cold, the most likely cause is a failed circulating pump. The boiler is heating the water, but it isn't being pushed around the system. This needs a plumber to diagnose and replace the pump.
How often should I service my central heating system?
Annual boiler servicing is recommended — it keeps the manufacturer's warranty valid, catches developing faults, and ensures the system is running efficiently. A London boiler service costs £60–£100.