Central Heating Not Working: Diagnosis Guide

Central heating failure always seems to happen on the coldest day of the year. Before calling a plumber, run through this systematic diagnosis — many common heating failures have simple fixes that don’t require a professional.

Step 1: Check the basics

It sounds obvious, but a surprising number of heating call-outs turn out to be programmer settings or thermostat issues rather than plumbing faults.

Check the programmer/timer. Has it been reset following a power cut? Many programmers lose their settings when power is interrupted. Check that the heating is set to come on (not just the hot water), that the time is correct, and that the programme days are set correctly (particularly if you have a separate weekday/weekend schedule).

Check the room thermostat. Is it set high enough? If the room temperature is already above the thermostat setting, the heating will legitimately not come on. Try turning it up to 25°C to test.

Check TRVs (thermostatic radiator valves). Individual radiator valves with temperature settings can cause individual radiators to stay cold. Check they’re not set to minimum (the snowflake setting) and that the pin inside the valve isn’t stuck — stuck pins are common after summer when the heating hasn’t been used.

Step 2: Is the boiler firing?

Look at the boiler. On most modern boilers, there’s an indicator light, display or sound to show whether the burner has fired.

If the boiler isn’t firing at all:

• Check it has power (boiler isolator switch)
• Check for an error code on the display — consult your boiler manual for what it means
• Check the gas supply (try a gas ring on the cooker — if there’s no gas, contact your supplier)
• Check the boiler pressure gauge — if it reads below 1 bar, repressurise via the filling loop

If the boiler is firing but radiators aren’t heating: The problem is likely in the system rather than the boiler itself — see Steps 4 and 5 below.

Step 3: Boiler pressure and the filling loop

Combi boilers and system boilers need to be pressurised to circulate water. The normal operating pressure when cold is 1.0–1.5 bar (check your boiler manual). If it’s below 1 bar:

1. Locate the filling loop — a flexible braided hose connecting two valves, usually under the boiler
2. Open both valves slowly
3. Watch the pressure gauge rise
4. Stop when it reaches 1.2–1.5 bar and close both valves

If your boiler loses pressure repeatedly (you need to repressurise every few weeks), there’s a leak somewhere in the system — call a plumber to find and fix it.

Step 4: Are any radiators heating?

Feel each radiator individually when the system is running.

All radiators cold: Either the boiler isn’t circulating water (pump fault, diverter valve fault), or there’s no heat getting to the system (gas supply, boiler fault). Check the pump by feeling whether it’s vibrating when the heating is running — a completely cold and silent pump has failed.

Some radiators hot, some cold: The system needs balancing, or some zones have valve issues. Note which radiators are cold and which are hot — the pattern tells you a lot. Radiators on one floor cold and others hot suggests a zone valve fault. Radiators furthest from the boiler cold suggests poor circulation or pressure balance issues.

Radiators warm at top, cold at bottom: Sludge. Black iron oxide sludge settles at the bottom of radiators and blocks the internal passages. A powerflush is the appropriate treatment.

Radiators cold at top, warm at bottom: Air. Bleed the radiators — see our bleeding guide.

Step 5: The pump and the diverter valve

The circulating pump moves water around the heating circuit. On most system boilers, it’s inside or adjacent to the boiler. On older gravity-fed systems, it may be in the airing cupboard. Signs of pump failure: all radiators cold despite boiler firing, pump hot but not vibrating (seized impeller), loud grinding noise from the pump.

The diverter valve (on combi boilers) switches between heating mode and hot water mode. A stuck diverter valve is a common fault: the boiler produces hot water but the valve doesn’t divert to the heating circuit, so radiators stay cold. You may notice the hot water works fine but heating doesn’t. This requires a plumber to diagnose and replace.

Step 6: Check the condensate pipe

Modern condensing boilers have a plastic condensate pipe that drains acidic condensate from the boiler to an outside drain. In freezing weather, this pipe can freeze solid, causing the boiler to lock out (stop firing) as a safety measure.

The condensate pipe is usually a 22mm or 32mm plastic pipe that exits through an external wall and drains to an outside gully or soakaway. In cold weather, pour warm (not boiling) water along the outside section to thaw it. Once thawed, reset the boiler.

To prevent recurrence, insulate the external section of the condensate pipe with foam pipe lagging.

When you need a plumber

Call a plumber or heating engineer if:

• The boiler is displaying a fault code you can’t resolve by resetting
• The boiler pressure keeps dropping (indicating a leak)
• You suspect a pump failure
• You suspect a diverter valve fault (hot water works, heating doesn’t)
• There’s a gas smell near the boiler — call the Gas Emergency Service on 0800 111 999 immediately and do not attempt any work yourself
• Radiators are cold at the bottom throughout the system (powerflush required)
• You’ve worked through everything above and the system still won’t heat

Gas safety note

Any work on the gas connections to the boiler must be carried out by a Gas Safe registered engineer. This includes gas pipe repairs, burner servicing, gas valve replacement and heat exchanger work. Verify your engineer’s Gas Safe registration at gassaferegister.co.uk before any gas work is undertaken.