New Boiler losing pressure ONLY when radiators running.

[Arno]

Over the winter my new valiant boiler was loosing pressure. From memory 2bar down to 0.6 bar when the boiler cut out. This would happen over 2 week period. Over the summer the thermostats are turned off and there is no loss of pressure over 3 months. The boiler installer claims this is a leak in the pipes (I have looked and I don't see anything but not lifted ground floor at this point) am I right in thinking even with the thermostats off the system is pressurised. So if there was a leak the pressure would still fall even if the rads were not running. I have not noticed any overflow/discharge coming from the external wall outlet.
any thoughts/help would be appreciated.
thanks

 

[Last Plumber]

Get a GSR engineer to check it over for you. It sounds like it may be a problem in the boiler

 

[gassafe]

Sounds like the heat is makin the leak worse... I'd say try some leak sealer in the radiators unless you can lift the ground floor & trace it