I have a combi boiler which looses pressure, the relief valve is ok, I fixed a balloon over the end to prove it the expansion tank has the correct pressure and the system is correctly pressurised I have access to all the pipework no leaks there and no apperent leaks on the boiler any ideas please.

dave c, January 2010

Is it a condensing combi? Either way i would be looking at the primary heat exchanger. Older vaillants (242, 282) would do this quite regularly.

Henry, January 2010

have you checked the auto air vent inside the case? sometimes they leak but it mostly evaporates due to the heat but can depressurise the system, it will be a bit crusty if this has been happening. i assume all of your pipework is above ground, so any leaks would show and no gland nuts are dripping on the rad valves

john, January 2010