Sealed Heating system: expanison vessel charge/filling

[jackecole]

Can some explain this to me please: with a sealed system (eg a combi boiler) you pressurise the system (on the gauge) to 1 - 1.5 bar depending on what the boiler manual states. Correct? BUT if you under charge the expansion vessel, then it says on heating, there is likely to be a PRV discharge due to nowehere for the water to go/expand in to? Correct?

What I don't understand is this: if you have NO CHARGE then surely the space where the charge should be, can take the 4% volume of water approx. when it is heated??

Or am i missing something here.....

When you fill a sealed CH system via the filling loop/filling key, does the needle on the pressure gauge move straight away or ONLY when the system is full, and the 1.5 bar is the extra PRESSURE you are pressurising the system too??

Sorry for sounding stupid, I am just trying to understand this!....

 

[Agile]

See the FAQ on how to charge the expansion vessel.

Water is incompressible but does neeed a space to expand into.

That space is provided by air which compresses in accordance with Boyles Law.

Tony

 

[jackecole]

Thanks Agile.

I suppose my point I was trying to understand was: when you fill a sealed system up, the gauge moves only when filled, then the 1 bar, is the pressure the system is under?

 

[mogget]

What I don't understand is this: if you have NO CHARGE then surely the space where the charge should be, can take the 4% volume of water approx. when it is heated??

If there's no charge, most of the space in the expansion vessel will be taken up with the fill pressure of 1 - 1.5 bar. This would leave insufficient space for the water expansion due to heat.

In other words, you can use a smaller expansion vessel (with precharge) than would otherwise be required if there was no precharge. This saves money.