What size Epansion vessel?

[rgate]

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!

 

[oilman]

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.

 

[rgate]

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?

 

[kevplumb]

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

 

[ChrisR]

the pressure in the vessel might just need re charging

exactly - 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..)

 

[kevplumb]

 

[rgate]

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!