Expansion Vessel Size

[Shower Engineer]

Hi
I have a Grant 26-36 S Pro boiler fitted with underfloor heating and DHW. The system was fitted about 3 years ago (not by me) and has worked well since then, only issue is topping the system up every now and then. Pressure has never fallen sufficiently to stop the system working I have just noticed the pressure has dropped and topped it up.
Anyhow I have been looking to flush with F3 and add F1 to the system as I don't think this was ever carried out and was doing some calcs to find out the system volume and came up with 300L of system water (boiler, DHW, 3 x towel rads and 400 Sq M of underfloor heating).
The system only has a standard expansion vessel fitted of 12L. Obviously this is way undersized and probably the cause of the topping up although I have never seen high pressure in the system.
I am looking at increasing the expansion to 24L and the obvious way would be to add an additional 12L vessel which seems to be standard practice when more expansion is required. However is there any reason why I shouldn't remove the existing 12L vessel and add an external 24L vessel? I could use a long SS flexi to tap into the system at the existing expansion vessel point.
Does that sound like a good plan?
Any help or suggestions appreciated.

 

[SimonG]

Fit an external 24 litre vessel and get shot of the 12. Hard pipe it with no flexi.

 

[Shower Engineer]

Fit an external 24 litre vessel and get shot of the 12. Hard pipe it with no flexi.

Would you hard pipe it back to the existing take off or cap off the existing and go into the 28mm return?

 

[SimonG]

tbh whichevers easiest. Last one I did went back to the old vessel connection.

 

[Bunker]

If you call Zilmet, they will post you a free expansion vessel sizing calculator. There is a link you can order one but I cant find it right now.

 

[Nostrum]

Are you including the dhw cylinder into the calculations?

 

[Ermintrude]

Ps u need to use f10 if u have underfloor heating to prevent bio growth. One tin per 100l

 

[PlumbBobBob]

zilmet calculator here Zilmet : Home

 

[Shower Engineer]

Thanks to everyone for their help.

If you call Zilmet, they will post you a free expansion vessel sizing calculator. There is a link you can order one but I cant find it right now.

I have used the Mr Combi Android app and have now more accurately calculated the system volume to approx 260L. Ordinarily that would give an expansion of about 28L, however 200L of the system water is in the underfloor heating which has a low temperature so I would think 24L would be fine, any comments? Should I go for next size up to be on the safe side?

Wall space is at a premium so I was thinking of floor mounting the vessel next to the boiler, would something like this 24l Pressure Vessel/ Expansion Tank with replacebale EPDM - Membrane | eBay be ok or should I go for a more conventional wall mounted vessel from Zilmet?

Are you including the dhw cylinder into the calculations?

No only flow and return plus indirect coil.

Ps u need to use f10 if u have underfloor heating to prevent bio growth. One tin per 100l

No that is one I hadn't heard of before. Looks like it might be called AF10 and 500ml treats 200L Is this the right stuff? http://www.fernox.com/files/Fernox/Content/PDF/English/AF10.pdf