"IMM" wrote in message
...

"Christian McArdle" wrote in message
. net...

Yes, if you fill the bath quicker than
the combination tank fills up. The
header tank will be much smaller
than the bath capacity.

Not so. The average bath is 100 litres and that is mixed hot and cold, so
80
litres of hot is about average. The average cold section on a combination
unit is, eh presto...80 litres. Also as water is being drawn off it is
being filled by the mains, so the cold section will not dry up. Fit a
Torbeck valve which is either on or off, in place of the brass ballcock
and
it is highly unlikely you will run out of water in the cold section. Fit
a
22mm mains pipe to it and two Torbecks and you will not run it of water in
the cold section. Fit a quick recovery coil that takes all the boilers
output, and a priority system on the heating system and you will never run
out of hot water in normal use.

By all means fit a replacement gravity system
if you want to save cash, but a mains pressure
system is vastly better and weighs less, too,
which can be a factor when installing in a loft
with 3 inch joists.

If a venturi high pressure shower mixer is also used (no power shower pump
needed) then using a combination unit is a cheaper and better alternative
to
a heat bank or unvented cylinder. Low pressure water storage, cheaper,
simple solution, no: complexity, elecricity needed, large bore blow-offs,
self venting if mains failure, about the same size as an unvented cylidner
or heat bank for equiv performance, and the same overall performance.

If you want a fully sealed system so that the loft space has no water open
to atmosphere up there, then unvented cylinders or sealed heat banks is
the
way.

Look at:
http://www.telford-group.com/trident.htm

This is an example. All are lagged; both the hot and cold sections.
Combination units can be made to order, say: 115 litres in each section,
high insulation values, quick recovery coil and a cold feed connection on
the bottom of the cold section. Using the cold section for both hot and
cold feeds, then it would be advisable to have a 22mm mains pipe to the unit
(if possible) and two Torbecks fitted to ensure maximum re-fill (the unit
may be filling faster than what is being drawn off at times, depending on
the incoming mains flow.