Alpha CD50 - Pressure Loss

[BloggLam05]

I recently remove a rad in order to properly paint the wall behind it, and when the rad was reinstalled, refilled with water and making sure the pressure on the boiler is within the green section on the dial, it appears to lose pressure overnight after the heating has been on in the evening. I have to refill the CH system to the correct pressure virtually every day, but it has since stopped losing pressure. this is not the first it has happened.

So i wonder if there may be an intermittent problem that may be it needs to look at. i need help as to what the problem may be, although the boiler is working fine and have had no loss of pressure since.

thanks

BL