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John correct me if I'm wrong but if the vessel pre charge is 2.1 bar then before it will accumulate any expansion the pressure of the water at that part of the system must be greater?
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That's correct, the E.vessel diaphragm will be up against the water end so the system pressure will have to rise and exceed 2.1 bar before the e.vessel can accomodate any expanded water, if the pre charge pressure was 3 bar then the boiler PRV would lift almost immediately on boiler start up.
Saw your update, thanks.
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Hi Everyone, I’m new to this site so not quite sure how it all works or if this is even in the right place but here goes.

I live in a large property that used to be one of the training centers for Lucas Engineering. However it has since been converted back to a residential dwelling.
The house has a single pipe system with very large pipe work and 42, 4 column cast iron ideal radiators. We used to have an old ideal floor mount boiler (I think 70kw). We have since upgraded it to a vailliant ecotec 64kw and made it a pressurized system along with a 80l expansion vessel.
We are having problems maintaining the right pressure.
When the heating is off the boiler reads 1bar, the second it turns on and the big external pump kicks in it jumps to 1.5bar and stays there. When left at this the majority of the radiators work fine but around 8 or 10 don’t come on until the pressure is increased to around 1.8bar at the boiler. And we seem to have to top this up most days. The plumbers who installed the new boiler just don’t want to know and won’t come back, we’ve had a couple other people out but no one seems to know much about this type of system.
There seems to be no sign of leaks as most of the pipe work is surface mounted.
Could the expansion vessel pressure need to be increased?
Any advice would be greatly appreciated.
Thank you.
A lot of these old single pipe systems worked on gravity, basically hot water rises & cold water falls & naturally tended to run in one direction. Is your pump pushing it the same way?, Is the single pipe under the cold radiators hot? If it is it's got a blockage in the radiator or valves.

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