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re: utility costs/ environment gain
11 apr 1999
m.f.  wrote:

>i want to heat the house and the domestic hot water with a solar thermal
>collector, and with use of only minimal auxiliary energy.

sounds nice. how much heat do you need? that depends on where the house is
and how it is built and how many teenagers live in it. that determines
how big the "collector" needs to be, and how much it costs...

has the house already been built?

>i want to use a solar array to heat two tanks, one tank on top of the
>other.

why two tanks? that may be overcomplicating things.
and why one on top of the other, when water is so heavy? 

>the top one would be filled with domestic hot water (drinking water
>quality) and would be separated from the primary circuit by a simple
>heat-exchanger. the second tank would be in open loop with the primary 
>collector fluid circuit and be filled with anti-freeze; this tank would
>serve the hydronic floor heating.

a lot of people seem to like hydronic floor heating... maybe there's
some easier way to do this, with one tank, and no antifreeze or heat
exchangers. 

>by guiding the heated fluid from the primary circuit through the top tank
>first, i hope to achieve higher temperatures in the domestic hot water,
>since temperatures over 65 c are required to prevent legionella.

sorta makes sense, altho 65 c seems very warm, and people don't usually 
worry about legionella in water supplies. what would they eat?
should we all be heating our cold water pipes to 65 c too?

>the rest of the energy from the water+antifreeze mixture that has passed
>through the primary circuit would all be transferred in the second tank,
>without loss of efficiency caused by an additional intermediary heat
>exchanger.
 
you might consider heating the house with warm air from an inexpensive 
sunspace and solar closet, and heating water for showers with a few bare
overhead solar collectors in the sunspace, with no glazing or insulation
of their own. something like big fins by zomeworks, metal extrusions that
snap on to copper pipes. the copper pipes carry pressurized water, with a
thermosyphoning loop through an insulated tank above. no pumps or controls
or antifreeze or heat exchangers. you do have to keep them from freezing
somehow, eg by keeping the sunspace from freezing. 

>do you know that it has ever been done before in this way,
>and all the gorey details, please.

it's probably been been done at least once. i don't know the details.
the configuration you suggest has some advantages, but it sounds like
you need to explore the rest of the system, eg all those "collectors."

nick




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