Draining and re-pressurising a combi boiler heating system is a relatively straightforward task which can be carried out in a few simple steps. Learn how to re-pressurise your combi boiler with our quick guide…

Some models of combi boiler have a water pressure display at the front so it’s easy to check the pressure level. Normally, if the boiler is set at 1 or below, it will switch itself off or not operate as efficiently.

Boilers can either have an internal or external filling loop, whichever the case, you must turn off the power at the boiler before you attempt to reset it.

Your boiler has an internal filling loop:


Once your boiler is off, follow these easy steps:

• If your boiler has an internal filling loop, you must remove the tray at the bottom of the boiler, once you have done that you will find a filling key attached to the tray.

• Take the key and look for a pipe fitting with several lateral outlets, or manifold, as you will need to insert the key here. This manifold links your cold-water mains to the boiler through your central heating return.

• Fit the key into the manifold with the arrow symbol on the key facing the open padlock position.

• You should now be able to see a plastic nut positioned next to where the key was inserted. You must turn this nut anticlockwise, slowly, until you start to hear the boiler filling with water.

• As you are filling the boiler, check the pressure gage; you should notice that it has started to fill up. Keep filling the boiler until the pressure gage needle has moved into the green area which starts at 1.5 bar.

• Once you are in the green water pressure zone, you can start to tighten the nut again, until you hear the water stop.

• Turn the key back to the open padlock position before removing the key. A small amount of water may leak out of the manifold, this is normal. However, if water continues to spill out, do not panic, just tighten the nut a bit more.

• If you have slightly overfilled the boiler, and the pressure gage is displaying a 2 bar, don’t worry, all you have to do is go around your house and bleed a few radiators until the water pressure gage is showing at 1.5.

• You can now switch the boiler back on.

Your boiler has an external filling loop:

If your boiler has an external filling loop or hose, follow the instructions below, not forgetting to firstly switch off the power of the boiler.

• Check the location of the filling hose, normally it is close to the boiler and, in some cases, under the kitchen sink. Check that the hose is attached to the mains cold water to boiler pipe securely at both ends. Check for a valve at either or both ends.

• Open this valve or both if there are two, and allow your boiler to fill with water and continue to fill until the water pressure gage needle moves into the green zone at 1.5 bar.

• Isolate the valves so you do not overfill the boiler and the pressure stops rising.

• Release the loop. A small amount of water will escape from the loop, again this is normal.

• As before, if the water pressure needle is positioned a bit too high, just bleed some radiators until you reach the 1.5 mark.

• You can now switch the boiler back on.

We hope this guide helped you to get your combi boiler back up and running! Let us know if you have any problems down below.