Loosing pressure but no air in rads

[vivvov]

Hi..so the Baxi combi at my folks' house has lost pressure twice over a period of a couple of weeks since they started using the central heating this winter....but each time it's happened there's no air build up to be bled from any of the radiators.
Am I right in thinking that the pressure drop is a boiler issue and not a pipework/radiator leak?
Thanks in advance.

 

[cjg]

ok, make sure its at 1 bar now run your heating on full and wait and watch the presure on the boiler, does it go up to or near 3 bar, this may take some time, if it does then it means your expantion vessel needs recharge.