Combi system pressure - why

[potterton678]

A domestic combi boiler/system for DHW and central heating.
Why does the system have to be pressurised to 1 bar.
Why not 0.2 to 0.5 bar?
When the pump runs, that will also create pressure in the system.

Thanks

 

[Wayners]

Has an expansion vessel.
Read about that

 

[potterton678]

??
The expansion vessel is only there to accommodate the water expanding as it is heated up.
In gravity fed systems the expanding water went to the tank in the loft.

 

[kidgreen61]

Simply because the boiler "needs to know" it's full of water to stop it dry firing.
Most boilers have a low pressure switch in them for this purpose.