No Hot Water London — Emergency Repair

If your central heating radiators are warm but you have no hot water, the most likely cause is a faulty diverter valve — the component that switches the boiler between heating the radiators and heating the domestic hot water. On combi boilers, a stuck diverter valve is the number one cause of this exact fault. The fix typically costs £150–£350 including parts and takes 1.5–2 hours. If you have a system or regular boiler with a hot water cylinder, the fault could be the motorised valve, the cylinder thermostat, or a failed immersion heater element.

Lukewarm hot water on a combi boiler is usually caused by limescale build-up in the plate heat exchanger — London's hard water (300mg/l+) deposits limescale rapidly. Other causes include a failing thermistor (temperature sensor), low gas pressure, a partially closed service valve, or a blocked heat exchanger. On a cylinder system, a partially failed immersion heater element or a cylinder thermostat set too low are common causes. We carry descaling agents and can often restore hot water on the first visit.

The cost depends on the fault. A diverter valve replacement costs £150–£350. A plate heat exchanger replacement costs £200–£450. A cylinder thermostat replacement costs £90–£180. An immersion heater element replacement costs £100–£200. A thermistor/sensor replacement costs £80–£150. We diagnose the fault first, give you a fixed quote, and start work only when you agree. There are no hidden labour supplements for evening or weekend callouts on agreed jobs.

This is the classic sign of a scaling plate heat exchanger on a combi boiler. The boiler heats the water initially but the heat exchanger's flow capacity is reduced by limescale, so it quickly overheats and the safety thermostat shuts it down. The fix is either a professional descale (£150–£250, suitable for moderate scaling) or a plate heat exchanger replacement (£200–£450 for severe scaling). In London this is extremely common given the water hardness of 280–320mg/l.

A pilot light that won't relight or stays lit for a few seconds then goes out is usually caused by a faulty thermocouple (the safety device that detects the pilot flame). Thermocouples cost £20–£40 and the repair takes around an hour. Do NOT repeatedly attempt to relight a boiler that keeps going out — call a Gas Safe engineer who can check whether the issue is the thermocouple, the gas valve, gas supply pressure, or a more serious fault. Never attempt to repair gas components yourself.

Yes. We dispatch Gas Safe engineers across all 33 London boroughs 24 hours a day, 7 days a week including bank holidays. For morning calls before 2pm, same-day attendance is available in most areas. Evening and weekend emergency callouts are dispatched within 60 minutes. We carry the most common replacement parts (diverter valves, thermistors, plate heat exchangers, thermocouples) on our vans, so the majority of no hot water faults are resolved on the first visit.