Combination boiler pressure check

Combination boilers or "combi" boilers as they are more commonly known sometimes stop working due to a drop in pressure. In most cases this is easily rectified and will take seconds. Pressure can be lost in a central heating system if for example the radiators are bled, if the combi constantly looses pressure than you have a leak! This could be something as simple as a bleed valve that is not closed properly or a radiator valve that is leaking slightly, check all radiator pipes and valves for evidence of leaking. Leaks are normally very easy to spot as the water in the system is discoloured, if you find a leaking valve try to re-tighten it to stop the leak, otherwise replace the valve!


combination boiler pressure

The picture above shows a regular combi boiler.

The filling loop must not be connected to the boiler permanently as it only used to top up the boiler if pressure has dropped. In reality these are permanently connected as people very rarely disconnect them.

There are two dials one of which is the pressure dial and the other which is the temperature. Below the boiler is a filling loop which comprises of a flexible pipe, a handle and a double check valve. The double check valve prevents water from the central heating being sucked back into the cold water supply for the house. The pressure dial should have a red line on it to signify the minimum pressure required for the boiler to work. This dial will increase in pressure when the combi is heating water so ensure that the central heating part is off and that no hot water is being drawn. If there is no red line then check the manual for the combi as putting too much pressure in the boiler will cause the blow off valve to blow, which will be vented outside of the property.

Most boilers require just over 1 Bar of pressure when switched off. If the pressure is below the minimum required for your boiler then slowly open the valve on the filling loop, this normally has a handle attached but a suitable tool may be needed. You should hear the water moving into the combi! Keep an eye on the pressure dial and when it reaches the correct pressure turn the valve off! You can now use your boiler as normal.


You can watch the video here-



How to top up or re-pressurise a combi boiler