Low boiler pressure: what it means & how to fix

Low boiler pressure is probably the most common boiler fault there can be, but fortunately, it’s really easy to fix yourself.

Low boiler pressure explained

Heating systems with a combi boiler must be pressurised to at least 1 – 1.5 bar when cold. If the pressure is too low, the boiler simply won’t come on.

Combi boiler heating systems get their pressurised water from a temporary connection to the mains. This connection is known as the filling loop. It consists of a length of metal pipe or flexible braided metal hose with a valve at each end.

Mains water is usually at a pressure of at least 3 bar, so opening the filling loop will restore the pressure to the system. The process takes a matter of seconds.

What causes low boiler pressure?

It is quite common to have to top up the pressure once or twice a year. You also typically have to do it after bleeding the radiators.

However, doing it more often than this is suggestive of a leak in the system, either via your radiators or pipework, or via one of the boiler’s internal components.

How can I check my boiler’s pressure?

It’s really simple. On the front of the boiler or perhaps on the pipes directly underneath it, there will be a pressure gauge. This gauge has units called bars with numbers starting from 0 to 4 or 5.

  • When the boiler is off and the heating system is cold, the needle will show 1 – 1.5 bar.
  • When the boiler is running, the pressure will be anywhere from 1.5 – 2.75.

Some really modern boilers may not have a pressure gauge, and will have a digital display instead. 

How can I top up boiler pressure?

It’s really simple: switch off your heating, find the filling loop and the two valves connected to it, and open them up slowly to allow mains water into the system. Keep an eye on the pressure gauge as you do this, and close both valves once the system is back to 1 – 1.5 bar.

For more information on this topic, click here.