No Hot Water But Heating Works? Here's What's Wrong
When the radiators heat up perfectly but the taps run cold, the fault is almost always in the part of the boiler that switches between the two — not the boiler as a whole.
When the radiators heat up perfectly but the taps run cold, the fault is almost always in the part of the boiler that switches between the two — not the boiler as a whole. That's good news: it usually means a component repair rather than a replacement. This page walks through what an engineer actually checks, in the order they check it, so you know what you're paying for before anyone arrives.
The diverter valve is the first suspect
A combi boiler shares one heat exchanger between heating and hot water, and a diverter valve decides where the heat goes. When the valve sticks in the heating position, radiators work and taps don't. Symptoms that point here: hot water that is briefly warm then cold, or water that only heats when the heating is switched off. The valve head can often be replaced without draining the system.
Thermistors, flow sensors and plate heat exchangers
If the valve moves freely, the next checks are the domestic hot water thermistor (the boiler thinks the water is already hot and shuts the burner down), the flow sensor or flow turbine (the boiler never registers that a tap has been opened), and the plate heat exchanger. A scaled plate exchanger is common in harder-water parts of Hampshire and Surrey and shows as hot water that gets progressively weaker over months rather than failing overnight.
Cylinder systems fail differently
On a system or regular boiler with a hot water cylinder, no hot water usually points to the motorised zone valve, the cylinder thermostat, or a failed immersion heater on backup. Timer and programmer settings account for a surprising share of call-outs, so it's worth confirming the hot water schedule hasn't reset after a power cut before booking anyone.
What we do on the visit
We test the water side and heating side independently, read any fault codes off the PCB, check gas working pressure, and confirm the actual flow temperature at the tap rather than trusting the display. You get the diagnosis and a fixed price to fix before we start, and we carry the common valve and sensor parts for Worcester Bosch boilers.
Common questions
Why does my heating work but not my hot water?
On a combi boiler this is nearly always a diverter valve that has stuck in the heating position, or a hot water thermistor or flow sensor that isn't telling the boiler a tap is open. Both are component-level repairs rather than a reason to replace the boiler.
Is a stuck diverter valve worth repairing?
Usually yes. On a boiler under about ten years old, replacing the valve head or cartridge is far cheaper than a new appliance. On an older boiler that has already needed several repairs, we'll give you the repair price and the replacement price side by side and let you decide.
Can low pressure cause no hot water?
Yes. Most modern boilers refuse to fire below roughly 1 bar. Topping up through the filling loop may restore hot water immediately — but if pressure keeps dropping there is a leak or a failed expansion vessel behind it, and that needs investigating.
How quickly can you look at it?
Ring us and we'll tell you honestly when we can be there rather than promising a slot we can't keep. We're Gas Safe registered (197524) and cover Farnborough, Camberley, Farnham, Aldershot, Fleet and the surrounding towns.