Sizing expansion vessels

[Matt0029]

Has anyone got a good rule of thumb for sizing an expansion vessel for a central heating system. A combi has a 8 litre vessel. And will deal with the expansion of say 8-10 rads. Could one use this as a ball park figure and multiply up from there Obviously pipe work to take in to account 22mm or if any 28mm on the system. Thanks for any input

 

[John.g]

Did/can you confirm that the pressure was 2 bar with boiler/circ pump off? This is important.

That gives a system contents of 720 Litres

 

[John.g]

The 66/55C flow/return will only be valid if the boiler was running constantly, ie a gas boiler, if not, or an oil boiler which always cycles then the system dT and the boiler dT will be completely different.

 

[Matt0029]

Thanks the boiler is pretty much running constantly there is huge heat loss as one wall of the whole building is nearly completely glass. Sorry I would have to check the pressure when the pump is off. This is seen as the true pressure?

 

[John.g]

Yes, it will give the true reading then.
If all the readings are correct, the spreadsheet will be correct, 720 litres seems a huge systems content though,
the exact pre charge pressure also has a huge influence on the results, for example if it was 1.25 bar (or 0.5bar) the system contents will be 540 Litres.
One way or the other though. I think you should be quite confident that a 50 litre EV will have no problem in dealing with the extra rads water content.