What size Epansion vessel?

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Happy new year all,
Can anyone advise me on what size of expansion vessel I need to fit?
I have a Vokera 80-20 RS Turbo combi boiler and I have concluded the sealed unit expansion vessel has gone as the cold to hot system pressure ranges from 0.5bar to 2.7bar. If I top the system up then the pressure exceeds the 3 bar safety valve and dumps water. There are also lots of creaking noises when the boiler cycles.
I have added a couple of rads and a towel rail to the system since it was installed, I have 7 doubles, 4 singles plus towel rail of varying sizes.
How do I work out the volume in the system to size a separate expansion vessel correctly, or do I just get one more than big enough to cope, what size would this be?
Should I fit it onto the heating flow or return side of the boiler?

Any help appreciated!
 
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How do I work out the volume in the system to size a separate expansion vessel correctly, or do I just get one more than big enough to cope, what size would this be?

Get manufacturers data sheets (mostly downloadable) Calculate pipe volume, then get a vessel at least 10% of the total. Anything bigger is ok.
 
:confused: Thanks Oilman, I guess what I was asking was if anyone knew a volume figure for a CH system in a typical or average 4 bedroom house. Alternatively what typical size of CH expansion vessel is seen in your average 4 bed house?
To calculate the figure by adding up all the radiators and guessing the pipework is not only a headache but is more than likely to have a large error!
Anyone got a view?
 
use the same volume as fitted to the boiler or one size up you cant oversize an ex vessel :)

alternitive is to check the pressure in the vessel might just need re charging :idea:
 
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the pressure in the vessel might just need re charging
exactly :eek: - what's the reason for thinking you need a new vessel?

Answer would otherwise be 12L or so min - most combis are 10L.

(I'm agreeing - I thought I'd better point that out..)
 
I cannot see any valve on the expansion vessel which is tucked away at the very back of the boiler and is very innaccessible, the manual descibes it as a sealed unit expansion vessel, indicating it cannot be repressurised, this is why I have concluded I must fit a new additional one, thanks for the advice, 12L seems the way to go. Cheers all!
 

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