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

 

[EvilDrPorkChop]

I think @John.g has a useful spreadsheet for sizing expansion, that i've found useful in the past.

 

[ShaunCorbs]

1l per rad

 

[John.g]

Unfortunately cannot post "live" calculations, but here is a screenshot of a system with a 8 litre EV + 110 litre system contents. Final pressure still OK at < 2.5 bar.

Add the total radiator contents + say 25 litrs for a oil fired boiler or ~ 5 litres if G.fired + 0.3litres/meter for 22mm piping and 0.14litres/meter for 15mm piping.

 

[Matt0029]

That would be useful

1l per rad

Thanks. Classing doubles as 2 rads?

 

[ShaunCorbs]

Over 1.2 yes won’t harm if you oversize expansion

 

[Matt0029]

That is the system has 34 rads on it. They want 4 single rads swapping for doubles and an extra rad. Adding to the system. The system currently has a 35 litre expansion on it. Wondering whether to recommend up sizing to a 50 litre.

 

[John.g]

Using the same criteria as in the S.sheet above, a 35 litre EV will accomodate 480 litres with a final pressure of 2.46 bar, a 50 litre EV assuming the same 480 litres gives a final pressure of 2.1 bar.
A 50 Litre EV will accomodate 683 litres with a final pressure of 2.46 bar.

The boiler(s) in addition, may have their own internal expansion vessels?.

 

[ShaunCorbs]

Yes upgrade to a 50l

 

[Matt0029]

Thanks it's hard to know the true system volume from the rads listed?