A combi boiler that produces no hot water from the taps is one of the most common heating complaints in London homes. Unlike a system boiler with a separate cylinder, a combi boiler heats water on demand directly from the mains supply, which means that a hot water failure has a different set of causes from a heating failure. This step-by-step diagnosis guide helps London homeowners identify whether the problem can be resolved without an engineer or whether a Gas Safe registered engineer needs to attend.
Check the Basics Before Calling an Engineer
Before calling a Gas Safe registered engineer, carry out a series of basic checks that can resolve a no-hot-water fault without a call-out charge. First, check the boiler display for an error code. All modern combi boilers have a digital display that shows a fault or error code when the boiler has locked out or detected a problem. The error code is a number or letter-and-number combination that corresponds to a specific fault type listed in the boiler manual. Common codes on Vaillant boilers in London include F75 (no pressure sensor signal), F22 (low water pressure), and F28 or F29 (ignition failure). Worcester Bosch boilers commonly show EA 338 for a similar ignition fault and E9 for an overheat condition. Identifying the error code before calling an engineer allows a faster and more targeted diagnosis.
Check the boiler pressure gauge, which is visible through the boiler case on most combi boilers and shows system pressure in bar. A combi boiler requires a minimum operating pressure of 0.5 to 1.0 bar to function, with an ideal working pressure at cold of 1.0 to 1.5 bar. If the gauge reads below 0.5 bar, the boiler will not fire and will show a low-pressure error code. Repressurising the system using the filling loop resolves this fault. The filling loop is a short flexible hose with two valves connecting the mains cold water supply to the central heating circuit, and it is usually located beneath the boiler or in the airing cupboard. Open both valves slowly until the pressure rises to 1.0 to 1.2 bar, then close both valves and reset the boiler. If the pressure drops repeatedly within a short period, there is a leak in the system that must be investigated by a plumber.
Hot Water Not Working But Heating Is Working
If the central heating is working normally but there is no hot water from the taps, the fault is likely in the diverter valve, the plate heat exchanger, or the domestic hot water temperature sensor. The diverter valve is a motorised component inside the combi boiler that directs flow either to the central heating circuit or to the plate heat exchanger for domestic hot water production. When a diverter valve fails, it can stick in the heating position and fail to switch over to hot water mode when a tap is opened. A faulty diverter valve requires replacement by a Gas Safe registered engineer and is one of the more common combi boiler repairs in London.
The plate heat exchanger is a compact component that transfers heat from the boiler water to the cold mains water supply for hot water production. In London, where mains water is very hard, limescale accumulates on the plates of the heat exchanger over time and reduces the rate of heat transfer, causing the hot water to emerge at a lower temperature than expected or causing the boiler to cut out on overheat protection. A scaled plate heat exchanger can sometimes be cleared by descaling treatment, but in severe cases the component must be replaced. Regular annual boiler servicing by a Gas Safe registered engineer includes checking the heat exchanger for scale and assessing whether the boiler would benefit from inhibitor treatment or a system filter to slow future build-up.
When No User Action Resolves the Fault
If the pressure is correct, no error code is displayed, the boiler fires and appears to complete a cycle but the hot water is still cold or absent, the fault is internal to the boiler and requires a Gas Safe registered engineer. Internal faults affecting hot water production on a combi boiler include a failed thermistor, a malfunctioning gas valve, a failed pump, a faulty printed circuit board, or a blocked or leaking plate heat exchanger. Prestige Engineers are Gas Safe registered and provide rapid combi boiler diagnosis and repair across all London boroughs, with same-day attendance available for priority hot water faults.