Pressure Gauge -Sealed System -General question

HUMBUG

Any Plumbers and Scientists out there?
Just a little confused about why there is a temporary drop in pressure in a sealed system as soon as one switches on the Central Heating ? I've noticed the pressure gauge falls about 0.5mb almost instantaneously and then creeps back up. Does the sudden flow of water in a sealed system cause pressure to drop (trying to recollect my physics lessons at school)?

Further, the engineer who just installed my new boiler/expansion vessel, etc etc said that air was being dragged into the system by my pump. But how can that be? If its a sealed system, the pump would have to be pretty powerful to overcome the internal water pressure to suck in air from outside. Also , wouldn't the internal pressure just get higher and higher until some equilibrium is reached between the internal pressure and the outside atmospheric pressure? And if there is enough space for air to get in why doesn't water leak out of the pump when its switched off?

Help as I can't get my head around this!!!

Andy_WSM

Without seeing the system design I'd guess that the pressure gauge is on the suction side of the pump. Thus when the pump starts pressure at the gauge drops. When the water heats the pressure in the system will rise.

If the gauge was on the output of the pump you might see a slight rise when the pump is running.

Just a guess having not seen the system.