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.

 

[fixitflav]

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.

Also because the boiler is usually on the ground floor, and need to ensure the highest rads are under positive pressure.

 

[Johntheo5]

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

If the EV (expansion vessel) is installed on the ground floor then ~0.5bar is the minimum required to reach the highest point (usually a rad) in the system, if the EV is positioned in say the hot press then ~ 0.25bar would be sufficient, you should then increase the filling pressure by 0.25/0.5bar to give a few litres of water reserve to make up for any very small leaks, generally, but not allways, the EV is pre charged (air end) to 1.0bar and then charged to 1.5bar, With a 10L EV,this gives a 2L reserve or a 2.27L reserve with precharge/filling pressures of 0.7/1.2bar.
The pump only produces a differential pressure (head), if you watch the pressure gauge on a boiler then you should see the pressure increasing by ~ 0.3bar when the pump starts.