I'm at my wits end with my 7-year-old Baxi 105HE combi. It's losing pressure quite rapidly, i.e. from 1 bar to nearly zero overnight. Here's a summary of how things are: 1) New expansion vessel fitted 6 months ago. 2) No problems over the summer (but no heating used). 3) Since using heating again, pressure dropping constantly - I think slowly when heating not on, more quickly when heating on. 4) Pressure rises at first about half a bar, from 1 to 1.5, when central heating starts - seems right - but then gradually drops. 5) No obvious leaks from pressure relief valve. 6) No obvious leaks from boiler. 7) No obvious leaks in radiator system - not a drop of air to bleed anywhere. 8) Have used leak sealant just in case - problem seemed to improve for a few days, now back to just as bad as before. 9) Now here's a weird thing I've noticed (well, may just be normal or may be a clue) - if I try to top up the pressure when the heating is actually on, the pressure won't increase - it rises a little when it's filling but drops straight back down again. I can hear the water going into the boiler but it's just 'disappearing' somewhere. Where can that be going? Any bright ideas welcome. Many thanks.