In an earlier contribution to this discussion,
Andrew Mawson wrote:

What is the rule of thumb for sizing an expansion vessel for a sealed
heating system?

My application is not a domestic heating system but rather an unvented
cooling system cooling a large industrial machine, but the principal
is the same. The 'boiler' equates to the machine being cooled. the
'radiators' are a fan cooled bank of tubing and the water based
coolant is circulated by a pump.

Given the system volume = V and coefficient of expansion of water per
degree is E and temperature rise is deltaT presumably :

(V(orig) x E x deltaT) - V(orig) = the max increase in volume

But what safety margins are applied in domestic heating systems? Do
plumbers really calculate system volume ?

AWEM

I assume that your system is pressurised, so similar considerations will
apply.

With a domestic CH system, the charge pressure (air) in the expansion vessel
is typically 0.7 bar with the water system unpressurised. The water system
is then pressurised to about 1 bar, cold - which partially compresses the
air in the expansion vessel, using up some of its volume. Then the system
gets hot - and the volume of water increases by 2 or 3%. The trick is to
have sufficient volume in the expansion vessel such that it can accommodate
the water expansion without increasing the air pressure (and hence the
system pressure) by more than (say) 1 bar.

So if you work out what volume change you need to accommodate, and what
pressure rise is acceptable - and apply Boyle's or Charles' Law (forget
which!) - you should be able to work out how big the expansion vessel needs
to be.
--
Cheers,
Roger
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