Baxi Combi 105HE Loss of pressure on central heating

[Fireman]

My boiler is just over a year old and has been great until this week. I turned on two radiators which haven't been used for a few months so that all the radiators in the house were working a total of nine. the following day the hot water was not working but when I looked at the boiler the pressure was down to zero. Topped up the system with the filler loop and seemed ok but the pressure started to drop again. It takes approx 20 mins to drop from 1.75 bar to zero.
If I close the flow and return valves off on the underside of the boiler for the central heating I can have hot water and the pressure does not drop.
But then I have no warm radiators.
I've lifted boards, looked at rad valves, radiators etc but can't find any leaks.
HELP!

 

[Baxi-Remeha]

with boiler and system cold, top up to 1.5 bar, and close isolation valves on flow and return. leave boiler off for about 2 hours.

if presure remains normal, open the isolater's.......if presure then drops, you have a leak from your system, not the boiler.

 

[Fireman]

Thanks
Any good ideas how to locate where I am losing the pressure. So far I've looked at all rad valves and radiators and all seem ok. Looked at most of the pipe work that I can get at, and everything appears dry.

 

[Baxi-Remeha]

the 'law of sod' will make sure its from a peice of pipe where you cant get at, probably under the floor downstairs somwhere, as if it was upstairs you would have a damp patch on the cieling.......

 

[gazthepottertonengineer]

check your pressure relief pipe outside and see if water is coming out of that

 

[Fireman]

Already checked pressure relief on the outside ity was dry.