Expansion vessel sizing question (because it's driving me mad)

[JLS]

Hi all, I wonder if someone can explain something to me when it comes to expansion vessel sizing.

On the basis of about 4% expansion at 100degrees one would think that for a 200l closed system an 8l expansion vessel would be sufficient, with a bit of a safety factor say 10l.

Worcester installation instructions for Greenstar CDi Classic which has a 7l internal expansion tank says a system larger than 100l needs additional expansion, assuming the above 200l system we're now at 14l

Using the following formula from Advanced Water Company


V= eC / (1- p1/p2)

Where:
V The total volume of the expansion vessel. It is not the acceptance volume.
C The total volume of water in the system (Litres).
p1 The fill pressure in Bars absolute (atmospheric or gauge pressure plus 1 Bar).
p2 The setting of the expansion/pressure relief valve in Bars Absolute (atmospheric or gauge pressure plus 1 Bar).
e The expansion factor that relates to the maximum system requirements.

Assuming I understand what they mean by p1 and p2, gives a volume of just over 201

Asking an experienced and well thought of plumber I know he says 24l......


Upshot of all this, can someone shed any light on how to properly size an expansion vessel for a combi on a larger system? An explanation would be gratefully received as "You're over thinking it, just do this..." doesn't really work with the way my mind works unfortunately.

 

[Chester]

Being slightly oversize isn't going to harm it so maybe you are over thinking it a little. 24l would be plenty.

 

[JLS]

Being slightly oversize isn't going to harm it so maybe you are over thinking it a little. 24l would be plenty.

A 24l vessel isn't exactly small though, and whilst I take your point about oversized not hurting I'd still like to understand the theory. After all For the system in question a 24 + the internal 7 gives over 15% expansion!

 

[Chuck]

The air in the vessel remains at (close to) constant temperature so one uses Boyle's law pV = constant, as follows.

It's easier to think in terms of absolute pressures.

You start with 7 litre of air at 1 bar. Fill the system with cold water at 2 bar. The expansion vessel now contains 3.5 litre of water and 3.5 litre of air compressed at 2bar. As the system water expands the air is compressed further and at maximum operating pressure, say 4 bar, will occupy 1.75 litre of the expansion vessel and a total of (3.5 - 1.75) = 1.75 litre of water has flowed into the vessel as the system water warmed up.

If you assume 4% expansion, this would correspond to 44 litre of water increasing in temperature from 20 to 100°C. In practice, the maximum flow temperature might be 85°C. and the return temperature will be about 65°C so the average temperature in the system will be 75°C, which will give you ~ 2.5% expansion in which case you can have ca 70 litre of system water.

If your average system temperature is 55°C, e.g. 65/45 F/R, then the expansion is ~ 1.4% and your 7l vessel can accommodate 125 litre of system water.

These estimates are consistent with WB's 100 litre figure. So, if you have a 200 litre system you will need to supplement the internal 7 litre with another 7+ litre external vessel.

 

[ShaunCorbs]

Easy way to size it 1l per rad

So you have 15 rads you need 15l of expansion etc

 

[oz-plumber]

Another easy way & fairly rough

Divide the total kW on the system by 3.
30 kW of rads - 10 ltr expansion tank.

 

[JLS]

Thanks all, particularly Chuck, that was just what I was after!!