Expansion Vessel Sizing

[LA3222]

How to size an expansion vessel is a question which comes up every now and then and I don't believe this graph has been shown before, I came across it whilst researching ASHP/UVC and figured it would be useful information to have on here:

 

[PeterW]

UVC is 10% and 99% of suppliers will provide you with the whole installation set of tank, stats, control group and expansion vessel. It looks like that graph is for heating systems which follows a rough 6% at 70°C and 4% at 50°C. Most tend to oversize them and it’s also worth understanding the minimum pressure the heat provider will run at as a lot of ASHP will run happily at 1Bar (0.1Mpa) and don’t need higher pressures. Putting 2-3Bar into a heating system isn’t really a necessity. 

[LA3222]

I figured it is a useful starter for ten when people are trying to work out how to size them, even if it leads to oversizing that can't be a bad thing as opposed to under sizing.

[PeterW]

12 minutes ago, LA3222 said:

I figured it is a useful starter for ten when people are trying to work out how to size them, even if it leads to oversizing that can't be a bad thing as opposed to under sizing.

Yep oversize is better than undersized but a lot of modern boilers already have them fitted.
 

The issue of system volumes comes more into play when you compare rads with UFH. Standard 600mm high double rad contains about 7 litres per meter of radiator, so a standard 12 rad set up (4 bed / 130sqm) is about 18m of radiator or 120 litres - expansion vessel for that is most likely an 8 or 12 litre vessel. Compare that to UFH and 65sqm of pipe at 150mm centres is around 500m - that is just 8 litres of water per floor, or 16 litres for the whole house. The you now only need a 1 litre expansion vessel ..! Add in a buffer tank of 60 litres and you’re adding 4 times the volume to the system. 

This is also the reason some installers don’t like adding buffer tanks as it vastly increases the volume of antifreeze required in an ASHP system - at 25% volume, a direct to floor install will need around 5 litres, add the buffer and that jumps up to 20 litres and it’s not cheap. 
 

This is one single component you need to consider in a heating design - there are plenty more !!