Is it the boiler or the controls?
Before assuming a fault, it is worth ruling out the simple things, because a surprising number of “broken” boilers are a setting that has changed. A power cut can reset a timer, a new battery can be needed in a wireless thermostat, and a hot water dial can be knocked down.
What are the common causes of no hot water?
On a combi boiler:
- Diverter valve sticking. The valve that sends heat to the taps or the radiators can seize, leaving you with heating but no hot water. It is one of the most common combi repairs.
- Flow sensor not detecting the tap. The boiler only fires for hot water when it senses water flowing. A failed sensor, or low mains flow, means it never starts.
- Low system pressure. Many combis will not run at all below around 1 bar.
- Ignition problem. If the boiler tries and fails to light it will lock out, usually with a code.
On a system or conventional boiler with a hot water cylinder:
- Timer or programmer set so the hot water channel is off.
- Cylinder thermostat turned down or faulty.
- Motorised valve stuck, so heat never reaches the cylinder.
- Immersion heater not switched on as a backup, if you rely on one.
What can you safely check yourself?
- Check the pressure gauge. If it is below 1 bar, see our guide to a boiler losing pressure.
- Check the display for a fault code and note it down.
- Check the controls. Make sure the hot water is switched on, the time and date are right, and the hot water temperature is not turned right down.
- Run a hot tap fully open. A trickle may not be enough for a combi to fire.
- Check other gas appliances. If the hob will not light either, the problem may be the gas supply or a prepayment meter out of credit.
- Reset the boiler once using its reset button. If it locks out again, stop there.
When do you need an engineer?
If the pressure is fine, the controls are set correctly, a single reset does not help, and the boiler still will not give hot water, there is a component fault. Diverter valves, flow sensors, pumps and motorised valves are routine repairs, and in many cases they can be finished in a single visit when the part is a common one. The engineer will confirm the cause and give you a price before starting.
If the boiler is older and has needed several repairs recently, it is worth asking whether repair or replacement makes more sense. Our cost guide covers typical prices for the parts involved.