MY IDEAL BOILER KEEPS LOSING PRESSURE?

I SET THE PRESSURE GAUGE TO 1.5 BAR AND WHEN THE HOT WATER IS USED IT IS FINE, BUT AFTER THE CENTRAL HEATING SYSTEM IS USED THE PRESSURE GAUGE LOSES PRESSURE AND GOES DOWN TO NOTHING. I THEN HAVE TO REFILL THE PRESSURE TO 1.5 BAR BEFORE I CAN USE IT. WHAT CAUSES IT TO LOSE PRESSURE WHEN THE HEATING SYSTEM IS USED.
IDEAL BOILER 120