Combi pressure....Why?

[scudo12]

Just a question.....
In a combi central heating system why does it need to be pressurised as it has a pump to circulate the water? Would this not circulate the water without having pressure?

 

[Mottie]

Not sure but maybe the system needs to know there is pressure (and therefore water) in it otherwise it could damage the boiler running it without water maybe? Unlike conventional systems, water is not automatically added when low. Just a guess, I’m sure I’ll be corrected though.

 

[bernardgreen]

One reason for having some pressure in the heating system is so that the boiler can use a pressure sensor to determine if there is enough water in the system for the boiler to fire up safely.

 

[Johntheo5]

A CH circulating pump is just that, a circulator in a closed loop, the static pressure or head must all ways be = to or slightly higher than the highest rad in the system which normally is ~ 5M above the boiler, the header tank is then mounted above that in the attic so ~ 0.6 bar pressure or 6M head, if a drain is opened to drain down the water to below the level of the upper floor rads to say 2.5m (0.25 bar) then the pump will only circulate water through the rads below this level. A pressurized system works the same way, generally the minimum pressure/head is 0.75bar/7.5M when cold.
Quite a lot of boilers (and nearly all oil fired boilers) do not have a pressure sensor but have a extra thermostat wiured in series with the control stat for safety reasons, if the water level keeps falling in the boiler then the control stat (set to say 65C to 75C) will cut out the burner and if that has failed then the hi limit stat set to ~ 113C on oil fired boilers will trip the burner and lock it out.

 

[scudo12]

Thanks all for the replies. I now understand the logic behind it.

Thanks.