Why do I need to top up the pressure in the CH each time I turn it off?

Why do I have to top up the water pressure in the CH boiler every time I turn it off. The pressure falls to below 1 bar so the boiler does not kick in automatically and I have to add water to the system to get it to go. My service engineer says it is because of low mains water pressure but I believe the system is a sealed unit and this should not affect it.