External pressure vessel

[Maccy]

Fitted an external expansion vessel to closed system central heating system due to failure of internal boiler vessel. Fitted on return below wall mounted boiler. Vessel pre-charge pressure checked prior to fitting, confirmed at 1.3 bar. System purged of air and pressurised at cold to 1 bar. Pressure appears to be unstable and creeps up above 1.5 bar at working temperature. Any ideas?

 

[ShaunCorbs]

That’s normal upto 2 bar standard from a cold pressure around 1.2 bar

 

[Maccy]

That’s normal upto 2 bar standard from a cold pressure around 1.2 bar

Okay, thanks for that. I assumed that the expansion vessel pre-charge air pressure would determine the water pressure. But thinking about it I guess that the diaphram, moving against the air will compress the air and thus increase its pressure allowing the water pressure to rise to meet that increased air pressure?
Hmmmm...physics!

 

[ShaunCorbs]

Exactly when the water is heated it expands creating more pressure