On Sun, 09 May 2004 20:45:09 +0100, Vortex wrote:

Hi,

Shortly I want to upgrade my gas central heating from "vented" to "sealed"
(primary reason to eliminate the loft tank to make way for a future loft
conversion.....secondary benefit is that I'll be able to put a rad up
there.)

On investigating the "tide marks" in my header tank I calculate system
expansion to be about 3 litres in normal use. This is consistent with a
system volume of about 100 litres...which seems plausible to me (4 bed
house).

Good old bes.ltd.uk sell expansion vessels in 8, 12, 18, 24 litre (and
larger) sizes.

Is there a rule of thumb for specifying the correct expansion vessel? At
first glance an 8 litre one would seem adequate...but the price difference
is so minimal I'm inclined towards an 18 litre one (there's plenty of
space). Is there any penalty in using a vessel that's too large?

While I'm at it I want to put in "bleed valve" on a spur off the main boiler
flow (actually this is where the fill and vent pipes will be removed) for
easy introduction of Fernox. Is this normal practice? Seems to make a lot
of sense to me.


Sizing the vessel is one of the subjects I did not include in the FAQ
below. Some boiler manufactuers suggest about 10% of total system volume.
Set the initial dry side pressure to 1 bar. A 12 litre vessel wil then go
to 1.5 bar with and expansion of 3 litres. That's about right.

3 litres is about the expansion you would get on a 'typical' house with
around 100 litres total circuit contents.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at 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