Combi gaining pressure

[dsracing]

Help required please.

I have a Biasi model number M90F 28S, it is gaining pressure I took the filling loop off and released the pressure back down to 1.5 bar. The next day when I check the pressure again only to find that the pressure was back up to 3 bar. I have now turned the the cold water off to the boiler and pressure is now stable.

So the question is what's the most likely thing thats allowing the cold thought into the heating system?

 

[dcawkwell]

If the pressure is increasing when you have the filling loop disconnected
you have a leak in the plate heat exchanger allowing mains pressure into
the central heating system.