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.

 

[jj4091]

Is 1 bar not just a general figure suitable for the varying instalations?. Our combi boiler will continue to work until the pressure is almost down to zero. Although it is difficult to tell the exact pressure it falls over at. Or so I noticed after a recent leak. Never noticed any change in efficiency as the pressure was dropping due to the leak..

 

[Johntheo5]

Is 1 bar not just a general figure suitable for the varying instalations?. Our combi boiler will continue to work until the pressure is almost down to zero. Although it is difficult to tell the exact pressure it falls over at. Or so I noticed after a recent leak. Never noticed any change in efficiency as the pressure was dropping due to the leak..

Where is the combi (with presumably its integral EV) located in relation to the highest rad?, if the rad is higher than the boiler pressure then there will be no circulation through this/these rads, if the rad elevation is, say, 3M, 0.3bar and if the boiler pressure is , say 0.1/0.2bar, then there should be no circulation through those rads.

 

[jj4091]

Where is the combi (with presumably its integral EV) located in relation to the highest rad?, if the rad is higher than the boiler pressure then there will be no circulation through this/these rads, if the rad elevation is, say, 3M, 0.3bar and if the boiler pressure is , say 0.1/0.2bar, then there should be no circulation through those rads.

Yes it has an internal EV. I think that is why ours works ok at low pressure. The height between the top of the boiler and the upstairs pipework is less than 1 metre.