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

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

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
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

The message . uk
from Ed Sirett contains these words:

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

A 12 litre standard expansion vessel will do 10 rads.

Eh? 10 litres per rad and 10 for the boiler is 110 litres, for a 10 rad
system, surely?

--
Skipweasel
Pay no attention to that man behind the curtain.

In an earlier contribution to this discussion,
Guy King wrote:

The message . uk
from Ed Sirett contains these words:

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

A 12 litre standard expansion vessel will do 10 rads.


Eh? 10 litres per rad and 10 for the boiler is 110 litres, for a 10
rad system, surely?


The *water* volume is about 110 litres. The capacity of the *expansion
vessel* needs to be about 10% of this - which is 11 litres - so 12 is ok.
What's your problem?
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

The message
from "Roger Mills" contains these words:

What's your problem?

Failure to correctly read your post!

--
Skipweasel
Pay no attention to that man behind the curtain.

replying to Roger Mills, John wrote:
Be careful with Boyle's and Charles Law These relate to ideal gases and do not
work for liquids
See Flamco website for a good method to calculate required vessel

--
for full context, visit https://www.homeownershub.com/uk-diy...ls-246893-.htm

replying to Andrew Mawson, John wrote:
normal safety margin applied to heating systems is 25%

--
for full context, visit https://www.homeownershub.com/uk-diy...ls-246893-.htm

John m wrote:

replying to Roger Mills, John wrote: Be careful with Boyle's and Charles
Law These relate to ideal gases and do not work for liquids


Shame that; it would make cooking really exciting if they did.




See Flamco website for a good method to calculate required vessel


--

Roger Hayter