BoilerFaultCodesHot water fine, radiators stone cold? Your boiler is probably working.
This one sounds worse than it is. If the taps still run hot, the boiler is lighting, burning and heating water correctly. What has failed is the part that decides where that heat goes. That is a much smaller repair than a new boiler, and on some systems it is not a repair at all, just a setting.
Do the free checks first. A surprising number of these callouts turn out to be a timer between heating periods or a flat battery in the room thermostat. Five minutes below could save you a callout fee.
Rule out the free fixes
The programmer is set to hot water only
Most programmers run heating and hot water on separate schedules. If heating is off or sitting between timed periods, the radiators stay cold while the water stays hot. Put it on constant for ten minutes and see what happens.
The room thermostat is set too low
If it is set below the current room temperature, the heating has no reason to come on. Turn it up well past room temperature and listen for a click.
Flat batteries in a wireless thermostat
A wireless thermostat with dying batteries stops telling the boiler to fire. The screen may still look alive. Change them anyway. This is one of the most common faults engineers find in October.
Every radiator valve turned down over summer
If the thermostatic valves were all wound to zero or frost setting in spring, the water has nowhere to go. Open a few fully and check again.
If the free checks did nothing
Now it depends on what kind of boiler you have. The part that has failed is different, and so is the price.
Combi boiler: the diverter valve
£250 to £450A combi has one heat exchanger doing two jobs. The diverter valve swings between sending heat to the taps and sending it to the radiators. When it sticks, it usually sticks on the hot water side, because that is where it sits every time you run a tap. The result is exactly what you are seeing: endless hot water, no heating.
Sometimes the valve itself is fine and only the small motor or actuator on top has failed, which is cheaper. An engineer will tell you which after testing.
System or regular boiler: the motorised valve
£150 to £300If you have a hot water cylinder in an airing cupboard, look for a small box clamped to the pipework nearby, usually branded Honeywell or Drayton. That is the motorised valve, and it does the same job as a combi diverter. When the motor burns out the valve stays in the hot water position. Many have a small manual lever on the side. If moving it to the middle position brings the radiators to life, you have found your fault.
Any system: a seized pump
£200 to £350The pump pushes hot water round the radiator circuit. On a combi, hot water at the tap does not need the pump in the same way, so a seized pump can leave you with working taps and cold radiators. A pump that is hot to the touch but silent is a clue.
One more thing worth checking
Look at the pressure gauge on the boiler while you are there. Cold, it wants to be somewhere around 1 to 1.5 bar. Ideal puts the figure for a Logic Heat at "between 1 & 1.5bar on the system pressure gauge". Low pressure tends to stop everything rather than just the heating, but it costs nothing to look and it is the one thing on this page you can fix yourself in five minutes.
How to repressurise a boiler →Where the DIY stops
Checking a timer, a thermostat, a radiator valve or a manual lever on a motorised valve is fine. Replacing a diverter valve, a motorised valve or a pump means draining the system and working inside the boiler or on sealed components, so it is a job for a Gas Safe registered engineer. Smell gas at any point? Leave the house and call 0800 111 999.
Avoid the next call-out bill
Monthly cover means no unexpected bills when your boiler breaks down.
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