If the radiators heat up but the taps run cold, the boiler is working. The fault is in the part of the system that only handles hot water. On a combi boiler that is nearly always the diverter valve. On a system boiler with a cylinder, it is usually the cylinder thermostat, the motorised valve, or the immersion.
If you have no hot water cylinder anywhere in the house, and hot water comes instantly when you open a tap, you have a combi. If there is a cylinder in an airing cupboard, you have a system or regular boiler.
The likely fault is different for each, so this is the first thing to establish.
A combi does two jobs with one heat exchanger. When you open a hot tap, a valve inside the boiler diverts the hot water away from the radiators and towards the tap. That is the diverter valve.
When it sticks in the heating position, the radiators keep working perfectly and the taps run cold. Which is exactly the symptom you have.
It is a common failure and a well-known one. Diverter valves are a wearing part, and on a boiler over about eight years old it is the first thing we would look at.
If you have a cylinder, the boiler heats water and sends it there. Several things can break that chain.
The cylinder thermostat can fail, so the system never realises the water is cold and never calls for heat. The motorised valve, usually a two or three port valve near the cylinder, can stick or its actuator can burn out. And the programmer may simply be set to heating only, which is worth checking before you ring anyone.
Is the programmer actually set to give you hot water? People change the clocks and forget.
Has the cylinder thermostat been knocked down? They sit on the side of the cylinder and get bumped.
If you have an immersion heater as a backup, switching it on will give you hot water while you wait. It costs a lot to run, so it is a stopgap and not a solution.
It is not the boiler being broken, because the boiler is visibly working and heating your radiators.
That is genuinely good news. It narrows the problem considerably and it usually means a component rather than a replacement.
Because the fault is in the part of the system that only serves hot water. On a combi that is usually a stuck diverter valve. On a system boiler with a cylinder it is usually the cylinder thermostat, a motorised valve, or the programmer settings.
A valve inside a combi boiler that switches the flow of hot water between the radiators and the taps. When it sticks in the heating position, the radiators work but the taps run cold. It is one of the most common combi faults.
Yes. If the radiators are working, the boiler is safe to run. You just will not have hot water until the fault is fixed.
Not usually, but no hot water in a house with a family in it is something we try to prioritise. Ring us rather than emailing if you want it looked at quickly.
We diagnose the actual fault before quoting to fix it. No jargon, no scare tactics.
