Combi boiler not holding pressure

[Nozza]

Hello,
My daughter has just bought her first house which has a combi boiler. The house was a repossesion and the heating system had been drained down for a while, possibly 4-5 months before they got the house. We have filled the system back up and it runs fine. However we keep loosing pressure over a period of about 1-2 hours. We have checked for leaks at all valves and fitting under the boards but can't see any signs of a leak. We have found all routes for the pipe-work to all rads and from the boiler, but no signs of any water. We have checked the Pressure release valve pipe-work and this is also dry. Has anybody got any ideas?
Thanks
Darren

[plumbbob]

Have you checked the condensate drain?

[Trixsales]

I had a similar problem with a Baxi 105E - after a great deal of time looking for leaks without success I eventually found that there was no pressure in the Expansion vessel - this turned out to be a leaking valve used to pressurise the vessel - the valve is similar to ones used on a car tyre. I used a car foot pump to take the pressure back up to 1 Bar. Since then no problems.

Rgds

[Nozza]

Thanks for you help, it was a leak after all. It was coming from the flow pipe at the connection to the boiler, I think the leak was so small the water was evaporating when the pipe-work was hot.