Boiler losing pressure

i can see that it would press the outside of the vessel on most of the sides, but what about where the bag meets the half of the PV which is filled with water? If I leave the drainage valve open when I pump, then the water, as I inflate the bag, is pushed out, so in principle I can keep on filling the bag up with air until it either 1) inflates to the full capacity of the bag or 2) inflates to the full capacity of the PV - I don't know which is bigger. My point is, how do I know - if the PV is normally part full of water and part full of air, and I am pumping air in one end and water out of the other (because the drainage valve is open) - at which point the pressure of the air in the PV is accurately displayed, since effectively there is always an outlet for the water and therefore a lack of resistance against at least one wall of the runner bag? maybe I am over thinking it. I really just want to know whether I did right to
1) open the drainage valve
2) pump air in until the pressure on my gauge and the boiler read 1bar
3) close the drainage valve without letting any more water run out (I had lost a lot less than the 10 litres touted in the FAQ)

Thanks chaps
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As you pump air into the bag, the water is pushed out.

When the bag reaches the steel casing, the pressure increases to 0.9 Bar and then you repressurise the water to 1.5 Bar which compresses the bag a little.

Ignore that bit about 10 litres. It should be deleted!

hi, I hope someone is still watching this post. i pumped my vessel and the system is still losing pressure, which makes me think that the PV is leaking except no water comes through the pumping valve when I press it which makes it seem like the diaphragm is not punctured. Is there any other way it can lose air? if water is leaking from the system rather than air escaping from the pressure vessel, would i not have noticed after this long (four months)? to add some mystery, the pressure is back at zero but the heating and hot mater still work. Does anyone have any idea how this can be please?

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there are lots of places for water to leak from a heating system.

It would be rare for air to leak but you can always measure the air pressure with the system pressure at zero.

The boiler can still sometimes work but you risk damaging it!


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