David Hansen wrote:
On Fri, 10 Mar 2006 09:54:57 +0000 someone who may be AJH
wrote this:-

Once the option for sealed
system is taken I can see little advantage in drainback over a glycol
or food grade antifreeze system.

When you choose sealed you're choosing not drainback, so the above
doesnt seem to make sense, unless you know of a sealed drainback
design?


However, for solar water heating the disadvantages of an open vented
system include the trouble of placing the header tank high enough,

I cant see how this is a problem. The loft is normally fine, sitting on
the usual dividing wall. The tank is below the panels for drainbacks,
it only needs to be above them for thermosyphons, which are not the
usual choice.

then maintaining it

again I dont see any downside with vented system maintenance. Sealed
system maintenance does introduce extra issues, and for CH seems to
result in more maintenance need in practice, as well as being less easy
to work on.

and the inability to heat water above 100C.

That is one of the plusses for vented systems, that they will never go
into that dangerous region of pressurised steam. Any sealed system that
does requires extra certification, which isnt free, and must be
designed to withstand much higher pressures. If it is not pressure safe
then its just dangerous. Vented eliminates any such risk. Worst case
for vented is steam coming out the overflow pipe, a non issue, or in
the highly unlikely event of both a system boiling and a blocked
overflow pipe, the steam simply comes out from around the tank lid.
Safely.


The system at http://www.imaginationsolar.com/system.htm has a
number of advantages. One of these is that it does not use mains
electricity, which can exterminate 20% of the savings gained by
using solar hot water heating [1]. The control system is very
simple, if the sun is shining it pumps water into the panel and
starts warming up the cylinder. If the sun is not shining, or there
is snow on the PV panel, then the pump is not running and the panel
is drained of water.

Imho this is more the hype than the reality. The one advantage of a
solar panel pump supply is simplicity, but thats all. The electricity
created by the solar panel is the opposite of a worthwhile investment
for 99% of end users. The control system is a bit crude, and will
result in the system actively cooling your HW some of the time instead
of heating it. In short it isnt worth it, unless you're just not
skilled to do anything better.


NT