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.

David