On the central heating boiler there is this air pressure switch at the chimney fan (Vaillant ThermoCompact), what is the reason to have two air sensing hoses connected to this pressure switch. Would have thought it only needed one ?

It is designed to use both for sensing purposes as in differential.

Thanks hvoc1000 -- have you any idea what the water gauge pressure and vacuum levels are approximately ?

I have no real info on Vaillant units. They are not that popular in the USA as of yet.

But I remember working on a Vaillant water heater 30 years ago for a church. It heated the baptism tank and the parts for that had to come from Germany.