Central Heating Not Working in London: Diagnosis Guide for 2026

The Right Order to Check When Central Heating Is Not Working

Central heating not working in London is one of the most common call-outs we attend — and in a significant number of cases, the fix takes under five minutes because something simple was missed first. Before assuming your boiler has failed, work through this sequence. It follows the same diagnostic ladder a Gas Safe engineer uses on arrival, and it will either resolve the problem or give you clear information to pass on when you call for help.

Step 1 — Thermostat: Confirm the room thermostat is set at least 2°C above the current room temperature. Smart thermostats (Nest, Hive, tado°) can lose their schedule after a power cut; check the app as well as the physical unit. Wired programmable stats sometimes have their batteries replaced incorrectly, which resets all settings.

Step 2 — Boiler pressure: Look at the pressure gauge on your boiler. The needle should sit between 1 and 1.5 bar when the system is cold. Below 1 bar and most modern boilers will lock out entirely. Repressurise using the filling loop (usually a silver or grey braided hose beneath the boiler) until the gauge reads 1.2 bar, then close the valve and reset the boiler. If you are unsure about this step, see our detailed guide on why your boiler keeps losing pressure in London.

Step 3 — Programmer or timer: Confirm the heating programme is active for the current time slot. British Summer Time changes and power outages are common culprits, especially after London's occasional grid blips. Many older timers use a 24-hour dial — make sure the pins or tabs are correctly set for AM vs PM.

Step 4 — Boiler error code: If the boiler display is showing a fault code (e.g. EA on a Vaillant, E1 on a Baxi, F.22 on a Worcester Bosch), look that code up in our London boiler error codes guide or your manual. Codes are specific and will tell you whether the fault is ignition, overheat, sensor, or pressure related.

Step 5 — Zone valve and pump: If the boiler fires normally but heat does not reach the radiators, the circulation system is the likely culprit. More on this in the section below.

Common Causes of Central Heating Failure in London Homes

London's housing stock skews older than the national average. A large proportion of inner-city properties — particularly across Islington, Hackney, Lambeth, Lewisham, and Southwark — are Victorian or Edwardian terraced houses with original pipework that has never been power-flushed. This creates a specific set of failure patterns that differ from newer builds in outer boroughs.

The table below shows the most common faults we encounter, approximate repair costs in London (as of 2026), and whether it is a DIY or engineer job.

London's water hardness also plays a role. Thames Water supplies water with a hardness rating of approximately 293–360 mg/l across most of Greater London — among the hardest in the UK. Over time, limescale deposits form inside heat exchangers and pipework, reducing efficiency and eventually causing blockages. If your boiler is over eight years old and has never had a descale or inhibitor top-up, this is worth discussing with an engineer.

If you are also losing hot water at the taps while the heating works, that points to a different fault path — read our guide on no hot water but heating works for a targeted walkthrough.

Boiler Fires But No Heat Reaches the Radiators

This is one of the more confusing faults homeowners encounter: the boiler lights, the display looks normal, there are no error codes — but the radiators stay stone cold. The heating circuit is operating in isolation from the distribution system, which almost always means the problem lies in the zone valve or the circulation pump.

Most London homes built or refitted from the 1980s onwards use a mid-position or two-port zone valve to direct water between the heating circuit and the hot water cylinder. When this valve sticks in the closed position — a common failure mode, especially on Honeywell V4043 and Drayton models — the pump runs but water cannot flow to the radiators. You may hear the boiler fire and even feel warmth around the boiler itself, but no heat travels to the rest of the system.

The circulation pump can also seize, particularly in properties where the system has been drained and refilled without inhibitor. A seized pump will sometimes make a faint humming noise but will not move water. An engineer can confirm this quickly with a clamp meter and by feeling for vibration on the pump body.

For radiator-specific faults — where some radiators heat but others do not — the problem is more likely to be a balancing issue, a stuck thermostatic radiator valve (TRV), or localised sludge. Our guide on radiators not heating up in London covers those scenarios in detail.

If the boiler itself is not firing at all — no ignition, no flame, no display — the fault path is different again. See our boiler not working in London guide for a step-by-step breakdown of complete boiler failure.

When to Call a Gas Safe Engineer in London

The DIY checks above — thermostat, pressure, programmer, error codes — are safe for any homeowner to carry out. Everything beyond that point requires a Gas Safe registered engineer. In London, this is not just best practice; working on a gas appliance without registration is a criminal offence under the Gas Safety (Installation and Use) Regulations 1998, and will also void your home insurance if a claim arises from the work.

Call an engineer immediately if you notice any of the following alongside your central heating not working: a smell of gas anywhere in the property, a yellow or orange boiler flame (should always be blue), black sooty marks around the boiler casing, or carbon monoxide alarm activation. These are not heating faults — they are safety emergencies. Open windows, leave the property, and call the National Gas Emergency line on 0800 111 999 before calling a plumber.

For non-emergency faults — zone valve, pump, pressure loss, PCB, sludge — our boiler repair London team can typically attend the same day across all Greater London boroughs. We carry the most common zone valves, pump cartridges, and pressure relief valves on the van, which means the majority of jobs are resolved in a single visit.

London engineers are in high demand during the first cold snap of autumn — typically late October in most years — and again after the Christmas period when properties have been unoccupied. If you are calling during a cold weather surge, booking in advance for the following morning is often faster than waiting for a same-day slot to open.

Frequently Asked Questions