Why Is There No Hot Water but the Heating Still Works?
It can seem strange when your radiators are warming up perfectly normally but the hot tap runs cold, but this combination of symptoms is actually a useful diagnostic clue. It tells an engineer that the boiler itself is firing and working, and that the fault most likely lies specifically within the hot water side of the system.
This guide explains the most likely causes depending on whether you have a combi boiler or a system with a separate hot-water cylinder, and what to expect from a repair visit.
Combi Boilers: Diverter Valve Faults
In a combi boiler, a component called the diverter valve switches the flow of heated water between the central heating circuit and the domestic hot water circuit depending on demand. If this valve becomes stuck or fails, the boiler may continue heating radiators normally but fail to switch over properly when a hot tap is opened.
This is one of the most common causes of this exact symptom combination in combi systems, and it is a job for a qualified engineer to diagnose and replace, as it involves working inside the boiler casing.
Combi Boilers: Plate Heat Exchanger Issues
Another common cause in combi boilers is a partially blocked plate heat exchanger, which is responsible for transferring heat to the water as it passes through on its way to the tap. Limescale build-up, particularly in hard water areas, can restrict flow through this component over time, sometimes causing reduced or absent hot water while heating continues to function.
System Boilers and Cylinders: Cylinder Thermostat or Valve Faults
If your home has a separate hot-water cylinder, a fault with the cylinder thermostat, the motorised zone valve controlling flow to the cylinder, or the immersion heater (where fitted as a backup) can result in radiators heating normally while the cylinder fails to heat the stored water.
A stuck zone valve is a particularly common culprit, as it can mechanically fail in a position that allows flow to the heating circuit but not to the cylinder circuit.
Programmer and Wiring Faults
Occasionally the fault lies not in the boiler itself but in the programmer or wiring centre that controls which circuits receive heat and when. A wiring fault or a failed programmer setting can effectively disable the hot water schedule while leaving heating unaffected.
What a Repair Visit Involves
An engineer will typically check the boiler's operation on both heating and hot water demand, test relevant valves and the diverter mechanism, and check the programmer settings before concluding what needs replacing. Repairs are charged at our standard hourly rates starting from £99 + VAT on weekdays, with a one-hour minimum and 30-minute increments thereafter, plus materials at a 20% mark-up where parts are required.
We attend properties across Wimbledon, Wandsworth, Merton and neighbouring Surrey areas for this type of fault, and same-day appointments may be available depending on demand.
Need a plumber or heating engineer?
We arrange appointments across Wimbledon and south west London at clear hourly rates. Call 020 3727 2578 or request an appointment.