Central Heating Not Working? How to Diagnose the Problem

When the central heating stops working, the cause is usually one of a small handful of common faults — and most of them you can identify (and sometimes fix) yourself in under ten minutes. This guide walks through the diagnosis in the order an engineer would do it, so you can either resolve the issue or pass a clear description to whoever you call out.

Step 1: Check the obvious things first

Before assuming the boiler is broken, rule out:

• Thermostat batteries. Wireless thermostats run on AA batteries that usually last 12–18 months. A dead thermostat will not call for heat.
• Programmer / timer schedule. Daylight saving changes or a power cut can reset the schedule. Confirm the heating is programmed to be on at the time you expect it.
• Boiler power. Check the fused spur next to the boiler is switched on, and the boiler display is lit. A blank display means no power.
• Room thermostat setting. If the thermostat is set lower than the current room temperature, the boiler won't fire.

These four account for roughly 30% of "broken boiler" call-outs we attend that turn out to be no-fault. Always check them first.

Step 2: Check the boiler display

If the boiler display is lit, look for:

• A fault code — write it down before doing anything else. Common codes: F22/E22 (low pressure), F75 (pressure sensor), F28/E133 (ignition failure), F29 (flame loss), F1 or F4 (flow temperature sensor).
• A flashing indicator on units without a digital display
• Pressure gauge reading — should be between 1.0 and 1.5 bar when cold

The user manual for your specific boiler lists every fault code with its cause. Most are also searchable online by model and code number.

Step 3: Pressure issues are the most common

If the pressure gauge reads below 1 bar, the boiler will refuse to fire as a safety measure. Repressurising via the filling loop (a braided hose under the boiler) takes 2 minutes and usually restores heating immediately. See our step-by-step pressure guide.

If pressure drops again within days or weeks, you have a leak somewhere — radiator valve, pipe joint, or the boiler itself. Don't keep repressurising indefinitely — call a plumber to find the leak.

Step 4: Frozen condensate pipe (winter only)

In cold weather, the condensate pipe (the white plastic pipe exiting the boiler through an external wall) can freeze and block the boiler's ability to drain. The boiler then locks out, often showing a specific fault code (F1, F4, or F22 depending on brand).

To thaw: pour warm (not boiling) water along the external section of the condensate pipe. Repeat for 10–15 minutes until water starts flowing freely. Reset the boiler. To prevent future freezing, lag the external pipe with foam pipe insulation — a £5 fix that saves a winter call-out.

Step 5: The heating fires but radiators stay cold

If the boiler is running but no radiators are warming, the issue is in the heating circuit:

• Stuck diverter valve (combi boilers) — the boiler is producing heat but sending it to the hot water side only. Symptom: hot water works fine, heating doesn't.
• Failed pump — the boiler heats water but can't circulate it round the radiators. Listen near the boiler — a healthy pump hums quietly when the heating is on.
• Stuck zone valve (in multi-zone systems) — heating circuit isolated from the boiler.
• Air-locked system — bleed all radiators starting from the one closest to the boiler.

Step 6: Heating works but is taking ages to warm the house

Different problem — the boiler is running and radiators are heating, but the house never reaches temperature. Causes include:

• Radiators sludged up — only the tops are hot, bottoms cold. Power flush needed.
• System undersized — boiler too small for the property, especially after an extension
• Poor insulation — heat is escaping faster than the boiler can produce it
• TRVs set too low — check each radiator's TRV is on a useful number

What to tell the plumber when you call

To get the right engineer, with the right parts, in the right time, have ready:

• Boiler make, model, and year if known
• Any fault code showing on the display
• What you've already checked (thermostat, pressure, programmer)
• Whether hot water is also affected, or only the heating
• How long the symptom has been going on

For a typical fault in a Peterborough home, we attend within 24 hours and most repairs are completed in a single visit. Common parts (pumps, diverter valves, pressure sensors, fans) are carried on every van. See our central heating services for coverage and pricing.

If the boiler is more than 12 years old and the repair is significant (£500+), check our guide on repair vs replacement before committing — sometimes the right answer is a new install rather than a third or fourth repair.