We’ve all been there. You’ve started running a bath, killed 15 minutes watching TV in delightful anticipation of a relaxing candle-lit hour, stepped into the tub and let out a shivery yelp – it’s stone cold. As well as being a bit of a downer, there’s clearly something wrong with your boiler. So put some clothes on and we’ll see if we can get to the bottom of it.

Is the central heating working?

If the heating is on (or supposed to be on), feel a radiator and see if it’s warm. Perhaps you can switch on the central heating in your usual way (you might have to turn the thermostat up) and see if the boiler fires up. If it doesn’t, it seems there’s something wrong with the boiler. Call an engineer.

If the heating is on, then there’s a good chance the fault lies with a valve that redirects the cycled water between the central heating and the hot water as it’s needed. It’s called a divertor valve, and it might have got stuck. You might be able to loosen it by turning the hot taps on and off a few times while the central heating is on. But it could need replacing or fixing.

Was any water delivered at all?

In the intro we described cold water coming through the hot taps. But if there was no water delivered at all, there could be a problem with your water supply, for example roadworks or frozen pipes. If you have a combi boiler or system boiler, you won’t get any water as there will be no pressure. If it’s a heat only boiler, you’ll get a cylinder’s worth of hot water before it runs out.

Does the boiler have power?

Your boiler requires electricity to work, so quickly check if it’s on. Check for lights, backlights on displays or LCD displays.

If everything’s off, the problem could be with your power supply. Check lights and plugged-in appliances. If they have no power, there’s either a general power cut or your consumer unit (fuse box) has tripped. If it’s just your boiler that’s off, check the boiler’s switch on the consumer unit to see if that has tripped. It might have its own circuit-breaker or it could share one with sockets on that floor.

Has the timer been reset?

If power has been off and has come back on again, it’s possible that your timer has reset itself to default, which could mean you don’t have hot water when you thought you would (for system or heat only boilers especially).

Another possibility is that British Summer Time has just started or ended – that can throw some types of timer out of sync. It shouldn’t affect combi boilers, however.

What temperature is the water set at?

On many boilers, you can set the temperature of the hot water and the central heating separately. It’s unlikely that the temperature has accidentally been set to tepid, but have a quick check to make sure. If it’s set below 30 °C, it will feel quite cold, especially for a bath. The hot water temperature control could be part of a digital readout or a knob on the front of the boiler unit.

What kind of boiler do you have?

If you have a combi boiler, your hot water is fed by mains pressure, so if you’ve been getting cold water through the hot taps, you can rule out a water blockage or supply cut-off.

If it’s a system boiler, it will also be fed by mains pressure, but the hot water will be coming from a cylinder. If it’s not hot now, it could mean that it was never heated up. Check your timers to ensure it was due to heat up. Sometimes people set different times for weekends, for example.

If you have a heat only boiler, the water is fed by gravity from a tank and cylinder. You would be able to fill the bath even if your water had been cut off, but the cylinder should still be hot. If it’s cold when it should be hot, it’s either a boiler problem or a timer problem.

Are there any error messages on the boiler?

Modern boilers have displays that show error messages. If the sensors in the boiler detect any fault within it, it will switch off the boiler. Consult your handbook to find out what any error message means. 

Has anyone else in the house just had a bath?

If you have a system boiler or heat only boiler, your hot water is stored in a cylinder, and once it’s been used up, you’ll have to wait until it re-heats before that bath. Give it an hour.

If you have a combi boiler, it will make no difference if someone has just had a bath – it heats water on demand when you turn on the tap.

Do you have a pre-payment meter?

If you pay for your gas in advance with a prepayment meter or smart pay as you go meter, check you have enough credit – you might need to top it up.

What does the pressure look like?

Water pressure inside the boiler and central heating system should be in the range of 1–1.5 bar. If it has dropped below or risen above this range, it might switch off for safety reasons. If it’s too low, put some more water in the system with the filling loop. If it’s too high, you need to reduce the pressure. The easiest way is to bleed a radiator.

Do you have a pilot light?

Older boilers have a pilot light, which is a small flame that’s on 24/7 and is used to ignite the burner. Sometimes they get extinguished, so you won’t get any heat. Consult your boiler’s documentation to find out how to light it.

Modern boilers (roughly post-2004) generally don’t have pilot lights. Restarting the boiler will put it through a sequence that primes the functions, including the burner function. Give that a try.

Time to call an engineer

If none of the checks and fixes above solves the problem, or if you discover that there is something wrong (for example the divertor valve), you need to call a Gas Safe engineer to check over your boiler to find and fix any issues.

Remember to have your boiler serviced once a year, too – this keeps it in tip top condition so you should never again need to step into a cold bath.