Central Heating problem

I have a 2 year old Baxi 105e Instant. About 4 months ago I noticed the system was losing pressure, alittle at first,then alot so I kept topping it up(the pressure went down to 0, then I refilled it to 1 bar (when cold)). There was no obvious signs of a leak (no damp patches on the ground floor ceiling and I accessed the sub-floor void several times - no leaks (its a two storey house with no pipework in the roof)), so was advised it may be a faulty overflow relief valve. I have been looking into the problem for a while, and today changed the valve, but the same thing happened - rads got hot, pressure went up accordingly, ORV let water out (when pressure got to above 3 bar) then stopped letting out water. However, I lost all pressure in the boiler again - but only when the rads were on - when the boiler is set to taps only the system does not lose pressure. Where can I be losing water from? Is it the boiler or the system? All rad's are getting hot with no cold spots.

Any ideas would be most welcome.

Many thanks, Tom Sendall

do a search on expansion vessels

Check that your filling loop is fully shutting off first, then if it is do as corgi says for expansion vessel faults.

Thanks for the response, expansion vessel it is then....

Cheers for your help

Highest regards, Tom