Guide · Boiler repairs
If the taps run hot but the radiators stay cold, the boiler itself is probably working. The fault is usually in the part of the system that sends heat to the radiators.
Hot water proves that the boiler can light and heat water. So the problem is usually the instruction to heat the house, or the route the heat takes to the radiators. That narrows things down a lot.
On a combi boiler, the boiler switches between hot water and heating using a diverter valve. On a system or regular boiler with a cylinder, motorised zone valves open the heating circuit when the thermostat asks for heat. Both can stick.
Work through these in order before you call anyone:
If those checks do not help, the usual suspects are a diverter valve stuck in the hot water position on a combi, a zone valve that will not open on a system boiler, a failed thermostat or wiring centre, or a pump that is not circulating water to the radiators.
A stuck zone valve sometimes has a manual lever on its head. Moving it to manual open may get heat through as a stopgap, but the valve still needs attention. Anything inside the boiler casing must be left to a Gas Safe registered engineer.
It also helps to listen. When the thermostat calls for heat, you should hear the boiler fire and the pump start. If the boiler lights but the radiators never warm, the heat is not being sent their way. If it lights for a hot tap but never for heating, the call for heat is probably not reaching it, which points back to the controls.
If the heating does come on but parts of the house stay cold, that is a circulation or balancing problem rather than a control fault. Our guide to radiators not heating up covers whole rooms or floors staying cold. For broader system work, see our central heating page.
With hot water still running, most homes can manage for a short time with portable heaters, but in winter or with young children or elderly people in the house it should be treated as a priority. If you have neither heating nor hot water, start with our boiler not working guide.
Tell Chris what the boiler display shows, what type of boiler you have and which checks you have tried. Call or WhatsApp 07429 632804, read about our boiler repairs, or use the quote form.
Common questions
Clear answers before you arrange a visit.
The boiler is firing, so the fault is usually the thermostat, timer, a diverter or zone valve, or the pump that moves water to the radiators.
Check the programmer clock first, especially after a power cut, then the thermostat setting and batteries.
On some combi boilers it can. Check the gauge and top it up to the level in your boiler's instructions.
No. It sits inside the boiler, so it needs a Gas Safe registered engineer.
Yes. A power cut can reset the programmer clock or wipe its settings, so the heating stops coming on at the right times while hot water still works on demand. Reset the clock and check the heating schedule.
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