No hot water, or the boiler will not fire: what to check first

A fair few call-outs turn out to be a flat battery in a thermostat or a timer that reset itself after a power cut. None of these checks involve opening the boiler, so they are all safe to do yourself.

The five checks

1. 1

   The thermostat

   Turn it well above room temperature and listen for the boiler responding. If it is a wireless one, change the batteries. Flat thermostat batteries are one of the most common reasons a perfectly healthy boiler does nothing.

2. 2

   The timer or programmer

   Check it is on the right mode and the clock is right. After a power cut or a clock change, plenty of programmers come back on the wrong time or default to off.

3. 3

   The pressure gauge

   Most boilers want roughly 1 to 1.5 bar when the system is cold. On or near zero means the system has lost water, and many boilers will refuse to fire to protect themselves.

4. 4

   The condensate pipe

   The white plastic pipe running outside. In a cold snap it freezes, the boiler locks out, and the fix is often a jug of warm, not boiling, water poured along the outside of the pipe until it clears.

5. 5

   The fuse board and any switch

   Check nothing has tripped, and check the switch or fused spur near the boiler is on. Cleaners, decorators and kitchen fitters all turn these off.

If it is a combi and the heating works but the water is cold

That pattern usually points at the diverter valve, or at the plate heat exchanger if the water is lukewarm rather than cold. Both are common repairs on a combi and neither means you need a new boiler.

Write the fault code down

If the display shows a code, note it before you reset. That code tells us what to bring, which often turns two visits into one. Also worth telling us: whether it happens from cold, whether it happens on hot water only, and whether resetting it works for a while or not at all.