Hi, it looks like I have a negative pressure on my collector system causing the brine reservoir to suck in and empty when the compressor runs.  I am being advised to change to a pressurised system by NIBE specialist, has anyone else done this? And if so did it fix the problem and what size of pressure vessel did you use? Many thanks 

should the reservoir have a vent/ hole in the filler cap?

This is the reservoir, it has no fill point and will contract further with no brine left in it on occasion before refillling when system is not circulating

^ what's the fitting on top of the bottle and where does the copper pipe go?

I’m back up next week so will confirm the connections and their routes.

 

Copper pipe goes to overflow bottle. Have now subsequently been advised against a pressurised system, as the deflation was initially caused by impact on the bottle (one of my tradesmen knocked it ) causing a brine leakage.  Following resealing and cleaning up it doesn’t look good but is actually working as it should.  Was advised the system would operate the same if converted to pressurised, adding extra maintenance and cost for no benefit.  Happy to close off thread unless anyone has strong views on pressurised v not ? 

This is like an unsealed expansion bottle in a car then, the only other way for it to deflate is a blockage in that pipe to the "overflow" bottle.