On Wed, 10 May 2006 20:02:54 +0100, 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 ?

You should be looking for around 10% of total system volume.
That's about 4 times the expected expansion of water from 25-75C.

Allow 10 litres per rad + say 10 litres for a modern boiler.

A 12 litre standard expansion vessel will do 10 rads.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html