System volume for expansion vessel size

[Sillychuckie]

Hi. I need to purchase a hot-water expansion vessel for my system, and understand I need to size it, based on "10% of the system volume" (based on a Google).

I'll be fitting a sealed system with under floor heating. Can someone clarify which of these items would contribute towards the system volume?

 

- All Radiators (total volume).    [I'm assuming 'YES']   : 72 litres.

- UFH Manifold 1 & Loops.         [I'm assuming 'YES']   : 43 litres

- UFH Manifold 2 & Loops:        [I'm assuming 'YES']    : 72 litres

- Hot Water Cylinder:                 [I'm thinking NO, as the cylinder contents isn't actually part of the 'sealed' system?]  :  180 litres.

 

A website also suggested adding another 10 litres for good measure for the boiler, and perhaps another 10 for the pipe runs.

My total of the above is coming in at around 386 litres, so I was going to purchase a 35 litre expansion vessel.

 

Then again, if this vessel is needed to take up expansion as I heat up the hot water cylinder, maybe it is actually *ONLY* the hot water cylinder volume that matters (180litres), and the rest is handled by the expansion vessel in the boiler?

 

Can someone provide a little advice here so I'm as well prepared for when the plumber comes?

Its one of the sealed red things (wall mounted) he needs. I know I could ask him, but for reasons I wont go into, its not so easy for me to reach him until he comes on Thursday to do the work (so I'm trying to get in as much of the stuff he will need ready for him in advance).

 

I'm just trying to figure out which of the things above 'matter'. Perhaps there are 2 different sorts (one of the sealed system items like Rads & UFH), and then another for the cylinder (if not 'built in'). If thats the case, then I don't understand why system volume matters at all here. Surely it would directly based on the volume of your cylinder only.

 

Thanks.

SC.

[JohnMo]

Your unvented cylinder is not included, but the coil volume is.

 

If you have a plumber coming, get him to do sizing and purchase. He will be going to the plumbers merchants anyway I suspect or they will deliver to him.

 

You have to be suitably qualified to work on Unvented cylinders, not a DIY job. The cylinder will have all the parts required including its own expansion vessel.