Heating system expansion vessel sizing

[Handyman]

Proposed System: Sealed, with 10 rads, all new (so low volume), one large (300x2000), one small (400x300), remaining 8 average out at about 600H x 800W, P+ type.
Question: is an 18L expansion vessel correct?
Not an exact science, I know, but would welcome confirmation of my calculations.

 

[chrishutt]

For an average domestic system 18 litres is far bigger than you need, although no harm in it being oversized. 8 litres would probably be fine, although you can work it out exactly if you like. Most of us just use 8 litres as the default size unless it's an exceptionally large system.

 

[Handyman]

What would happen if it's too small? And I don't mean ridiculously too small. For example, my rough calcs suggest around 14L. My plumber reckons 16L (better to be too large than too small, he says). But Altecnic (whose Robokit product I want to use 'cos I get a good deal from Plumbase) manufacture 12L and 18L sizes. So, what would be the consequences in this instance if I used 12L instead of 18L? Are we talking "mushroom cloud" if I wack the heating up too much??

P.S. I have a 3 bed det house, with one bathroom, and another on the way, and a large kitchen with 2 rads,

 

[gasgeezer]

if your expansion vessell is to small your prv will lift as the preeure cant be accommodated by the vessell from what you describe 12 litres should be adequate

 

[Handyman]

Gasgeezer, by "adequate", do you mean "just adequate", or "litres to spare adequate".

As a matter of interest, how do you calculate it exactly?

 

[kevplumb]

work out how much water is in the system

 

[chrishutt]

my rough calcs suggest around 14L. My plumber reckons 16L

How do you work that out? It sounds way too high.

 

[corgiman]

mm a bit high as chris says

i would say 12 litre would be ample

 

[noonespecial]

Have a look on www.mikrofill.co.uk.

They have a simulator for their EFD unit which is fun to play with and it shows you what happens when things go wrong with sealed systems. You wont have a Mikrofill unit but the rest of it will be applicable.

Also in the simulator they have a vessel sizing programme which you can play tunes with and see what happens if you reduce or enlarge the vessel size.

On a side note for anyone who fits pumped pressurisation units I would recommend you give the EFD unit a try. Much cheaper and seems to work very well. Also they are nice people to do business with.

 

[chrishutt]

Great link, noonespecial.

I think it confirms that for the typical domestic system (max 5 metre height, 100 litres water content) 8 litres is correct, and even works for 150 litres content - the final system pressure is a little higher but still below 2 bar.