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re: hydronic solar closets
6 apr 2002
michael d  wrote:

>i've read that hydronic heating systems can be more efficient than
>forced air systems. 

hydronic slabs can be very efficient air-water heat exchangers, with lots
of surface and a small water-air temp diff and very little pumping power.

>how about hydronic solar closets?

there may be better ways to feed a hydronic slab. a solar closet can make
hot water, but it seems more efficient to let the sun shine on a heater
directly than to make hot air that heats water indirectly through the walls
of a sealed container. and plumbing a bunch of drums together with lots 
of connections can become expensive and invite leaks.

i'd say put big fins or black "pig mat" collectors in a sunspace instead,
thermosyphoning hot water through an insulated tank above, with no pumps
or controls or antifreeze. the tank might be a large unpressurized plywood
box lined with epdm rubber and surrounded by lots of insulation. if a
sunspace has any usefulness for people, for growing plants or drying clothes
or storing lawn mowers or heating a house with warm air during the day or
escaping sad feelings or reading a sunday paper on a sunny afternoon, that
gives it another economic advantage over a heat pump. 

we might make big fin type collectors by bending an 8' piece of aluminum
flashing into a u shape around a 10' copper pipe in the middle, bolting
it through to make a good thermal connection, then opening it back up
to make wings for solar collection. home depot sells flashing in 2'-wide
rolls painted white on one side and dark brown on the other for about
50 cents/ft^2. 

zomeworks should soon sell plastic freeze-tolerant "pig mats" redesigned
for solar water heating in simple thermosyphoning systems... 

nick




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