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re: concrete slab
21 feb 2001
bill kreamer  wrote:

>could i prevail on you to do either a numerical or a features/benefits
>comparison of sunspaces and active heating collectors?

sure.

>near the end of the analysis below, your thinking seems to be trending
>toward recommending the characteristics of an active air-heating collector
>over those of a sunspace.

oh?

>it has the least mass (lowest thermal capacity), the quickest response
>to changing conditions (quickest turnover to the heated space), the
>least loss at night, and the least expense.

i wouldn't say "least expense." serving more than one purpose can change
the economics, even with sunspaces that cost more than mine...

>unless one saw a specific secondary benefit to "living inside the solar
>collector" (there could be some), why would one choose to build a sunspace?

sunspaces can thermosyphon better than panels, with more height and depth
and less air velocity near the cold glazing... i have some dark shadecloth
hanging near the north wall of my sunspace to keep warmer air closer to
the house... my cat enjoys it...

i don't do much living inside my sunspace, but i store some stuff there
and i sometimes work in there, since it has lots of light and a 16' ceiling.
it can also be a nice place to read on a sunny afternoon, and i could grow 
things there if i wanted to. my sunspace is half of a gothic-arch commercial
plastic film greenhouse turned on the side, with the straight parts of the
bows supported by ground stakes (the "foundation" and the curved ends hinged
to the house under the soffit. it's 32' long x 12' deep x 16' tall, and the
parts cost about $500. it's cheap "living space," at $1.30/ft^2 :-)

it has an oak floor--shredded wood playground mulch over black plastic film. 

our boca-township building inspector gave me a ($150 :-) permit to build
this thing, and classified it as "hvac space," which doesn't raise real-
estate taxes. it looks a bit strange, facing the road 100' away, but so far
none of my neighbors have threatened to kill me. (i also have a couple of
100' greenhouses and a 6-ton pile of unistrut on my front lawn.)

i sometimes wish i'd covered it with $1.50/ft^2 20 year clear replex
polycarbonate vs 5 cent/ft^2 4-year cloudy greenhouse polyethylene film,
but the polycarbonate only comes in 4' widths (vs greenhouse poly in
folded rolls up to 40' wide and 100' long :-), so it would need attaching
to every curved metal pipe bow, and i have 300 more neighbors now than
a couple of years ago, so the privacy isn't unwelcome. then again, it would
be nice to be able to see clearly out of the south windows of my house... 
most people wouldn't want to obscure their house window views with cloudy
film. clear plastic might raise the cost of a sunspace to $5/ft^2. 

>i have a unambiguous opinion, of course -

of course :-)

>our shvc (solar heating/ventilation cooling) solar system is 72% efficient,
>and never causes discomfort (it ventilates in warm weather).

norman saunders used to say "sunspaces are always too hot or too cold."
mine tends to overheat in summertime, even with 80% shadecloth over the
outside and a 6" slot at the top that vents into my solar attic, which
has 2 turbine vents and 4 small windows, open in summertime. (the south
roof of the attic is covered with corrugated polycarbonate, at a 45 degree
pitch, with serious summer overheating. i've measured 143 f up there.) 

honestly speaking, my gut feeling about factory-built air heating panels
is that they are very expensive, and people often buy them in 1s and 2s
as toys, vs trying to heat a whole house that way. i reviewed one design
(wolfe?) that appeared to consume more electrical energy than the solar
heat it collected :-) i realize yours works better, but it still seems
expensive. a new house might have economical corrugated polycarbonate
"solar siding." then again, there's the big problem of storing heat from
hot air overnight, and for a few cloudy days in a row. not everyone lives
in a supermarket full of canned goods on shelves.

>> ...you might replace the r4 windows with cheaper r2 windows with 80%
>> transmission to raise the solar input to 478k btu/day while raising
>> the solarium loss to 6h(80-14.7)228 = 89.3k, for a net gain of 388k
>> btu/day, with 65% efficiency, or more, if you redo these calcs using
>> a (14.7+33.3)/2 = 24 f outdoor temp during the warmest part of an
>> average day.

let's try that... 478k-6h(80-24)228 = 401k btu/day with 67% efficiency,
or about 800 btu/dollar-day or 50 peak watts per dollar, using a plastic
film sunspace.

nick




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