Pressure loss when UFH called for

[ArifP569]

Hello all, hoping someone could help me with this, Boiler pressure is 1.5 bar, but when we turn on UFH pressure drops to 1 bar. then when we turn off UFH, pressure on boiler goes back up to 1.5 bar?? any help much appreciated

 

[John.g]

With a sealed system and if the E.vessel is teed in just before the pump suction then the boiler pressure will/should rise by the pump head each time it starts so a 0.5bar, 5M rise might be normal.
Have you rads on the system as well?, if so watch the pressure if you switch them in and out with no UFH.