So I know this is an old thread but I'm sure others will still come across this needing to do this on their Greenstar Heatslave ii 18/25 boiler. Of course MAKE SURE YOU SWITCH BOILER OFF WHILE DOING THIS, and allow pipes to cool down too before attempting or you may scald yourself.
I have today successfully increased the pressure on my boiler using the filling loop which is inconveniently positioned towards the back (of the top left hand side of the internal boiler gubbins).
There are two flexible/braided hoses inside. Ignore the long one closest to the front of the boiler opening which is attached to the red expansion vessel.
The filling loop hose is the smaller one in length further and towards the back of the boiler. On ours it has 2x black plastic on/off valves at either end of the hose. One of these is visible on the right hand side hose end, but the second one is not on the left hand side hose as it is slightly underneath the isolation joint.
To start with I didn't know this as couldn't see the second valve, so turned on the first valve and nothing happened. After routing around a bit at the left side hose I found by feel the second valve and it was quite stiff. Took a bit of jiggle to loosen and then it also allowed to turn on.
My advice would be to turn on the more tricky out of sight (left hose end) valve first to the open position (mine was horizontal for closed and turned downwards to open), then position yourself so you have your hand on the other right side hose valve but at the same time can see the pressure gauge, slowly open the second valve and because you have already opened the first one you will start to hear water squirting through under pressure. I say do it slowly as you want to keep an eye on the pressure gauge as it increases towards around the no 1 mark. So doing this slowly allows you to control the increase easily without over doing it. Then when the needle reaches 1 close this valve. Then remember to close the other valve on the left side too back to the original off/closed position.
Job done
Added some photos to assist too, hope this helps others like this forum thread originally helped me determine this was the right course of action!
