Central Heating Repair London — Same-Day Engineers

No Heat from Radiators

Likely cause: Pump failure, zone valve stuck closed, or programmer fault. If some radiators heat and others don't, the issue is likely a stuck zone valve or an unbalanced system.

→ Engineer tests pump output, checks zone valve actuators, and tests programmer/thermostat signal.

Cold Spots on Radiators

Likely cause: Air trapped in the system (bleed the radiator) or magnetite sludge settled at the bottom of the panel. Bottom-cold radiators almost always indicate sludge.

→ Bleeding resolves air pockets. Bottom-cold radiators need power flushing — often the whole system.

Boiler Pressure Keeps Dropping

Likely cause: Failed expansion vessel (most common), leak in the system, or faulty pressure relief valve. The expansion vessel should be pressurised to 1 bar — they lose charge over time.

→ Re-pressurising the vessel temporarily restores pressure; replacement (~£120–£180 inc. parts) is the permanent fix.

Thermostat Not Responding

Likely cause: Wiring fault, battery failure (wireless stats), or the stat has failed. Smart thermostats (Nest, Hive, Honeywell) sometimes lose connection with the boiler receiver.

→ Engineer tests wiring continuity, checks receiver unit, replaces or reconfigures stat.

Heating On When Thermostat Off

Likely cause: Stuck zone valve (port has jammed open) or a wiring fault at the motorised valve. Very common in systems over 10 years old.

→ Zone valve head replacement or full actuator replacement (£85–£140 inc. parts).

Radiator Leaking at Valve

Likely cause: TRV or lockshield valve body leak — common in valves over 15 years old, especially where limescale has cracked the valve seat.

→ Valve replacement. System must be drained to the affected radiator — usually a 1–2 hour job.