If you suddenly have no heating or hot water, and your property has a combi-boiler then the most likely cause is low or no pressure.
When your boiler has no pressure
A loss of water pressure is a good indicator of some common issues that arise with boilers. Low pressure is relatively easy to diagnose, as most boilers have a built-in pressure gauge. Sometimes, it’s possible to correct water pressure yourself
Understanding water pressure
Constant water pressure is essential to the efficient functioning of your boiler system. Pressure in most modern combination boilers is maintained by cold water flowing from the water mains supply through a mechanism known as the ‘filling loop’.
What causes a drop in pressure?
A number of things can cause pressure to drop. It may be that there is a leak somewhere in your boiler system. Or, if you bled your radiators recently, it may be that pressure was lost then.
However, you may not be able to tell that you have a problem until you check the pressure gauge. A water leak, for instance, may be tiny and go unnoticed at first, only to develop into a larger problem down the line.
Checking your pressure gauge
Check the water pressure gauge or indicator, which is typically on the front of your boiler. For most boilers, this should be set around the 1 bar. The position set when the boiler was installed is sometimes shown by a red indicator needle.
If you have noticed repeated drops in your boiler pressure, make sure you check the gauge regularly. A drop in pressure is a good indication that there is a problem with your boiler.
How to tackle lost pressure
Each boiler will come with specific instructions about its pressure system. Check your user manual to see if you can re-pressurise your boiler yourself. There are helpful videos on YouTube, however if the problems persist it is best to arrange for an engineer.
Source: British Gas