No hot water in London?

Quick self-checks before calling an engineer

Before picking up the phone, there are a handful of checks that take under five minutes and may save you a callout fee. First, check your boiler pressure gauge — on most combi and system boilers the needle should sit between 1 and 1.5 bar when the system is cold. A reading below 0.8 bar means the system has lost pressure, and the boiler will lock out rather than fire. Repressurising via the filling loop (the braided flexible hose under the boiler) often restores hot water immediately.

Next, check the pilot light or boiler display for a fault code. Modern boilers show alphanumeric codes — look yours up in the manual or search the model number plus the code online. Common codes relate to ignition failure (the boiler tried to light and gave up), low pressure, or a frozen condensate pipe. In cold snaps the condensate pipe — the white plastic pipe that exits through an external wall — can freeze solid, causing the boiler to lock out. Pouring warm (not boiling) water over the external section of pipe usually thaws it within minutes.

Also check your room thermostat and programmer. It sounds obvious, but a thermostat set below the current room temperature will prevent the heating circuit from calling for heat, and on some older systems this affects the hot water circuit too. If you have a separate hot water cylinder with an immersion heater, check the immersion heater switch — typically a red double-pole switch on a fused spur near the airing cupboard. If the immersion element has failed or the overheat thermostat has tripped, you can reset it manually by pressing the small button in the centre of the element.

Most common causes of no hot water in London

The single most common cause of no hot water on a combi boiler is a faulty diverter valve. The diverter valve is a motorised component that switches the boiler output between the central heating circuit and the domestic hot water (DHW) circuit. When it sticks in the heating position — usually because of scale or a worn actuator — the boiler fires perfectly for radiators but produces no hot water at the taps. You may also notice hot water running briefly then cutting out, or the hot tap running warm rather than properly hot. Diverter valve replacement costs between £120 and £250 on most London callouts depending on the boiler model.

A frozen condensate pipe is a winter-specific fault that has become more common as London experiences colder snaps. Condensing boilers produce acidic condensate water as a by-product of combustion efficiency — this drains through a plastic pipe to an outside drain or soakaway. When temperatures drop below freezing the condensate can freeze at the elbow or terminal, blocking the pipe and causing the boiler to lock out. Other causes include low system pressure (pressure drop due to a leak or radiator bleed), a faulty motorised valve on a system or heat-only boiler (the valve that opens to allow hot water to flow to the cylinder), and a broken immersion heater element on properties with a hot water storage cylinder.

Combi boiler vs system boiler: why the fault type differs

Understanding your boiler type matters when diagnosing a no hot water fault. Combi boilers heat water on demand — there is no storage cylinder. When you open a hot tap, the diverter valve switches the boiler to DHW mode and the plate heat exchanger heats cold mains water directly. A diverter valve fault or plate heat exchanger blockage means you get heating but no hot water. System boilers and heat-only (regular) boilers work with a separate hot water cylinder. If you lose hot water but retain heating on a system, the fault is typically in the cylinder circuit — check the immersion heater, the cylinder thermostat, and the motorised valve that controls hot water flow to the cylinder. Our Gas Safe registered engineers carry parts for both system types and can diagnose the fault accurately in a single visit across all 33 London boroughs.