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re: home design and ac load calculation
28 jun 2003
brent geery   wrote:

>the structure will be about 450 sq. ft. one story structure, about
>25'x20, using a slab-on-grade foundation, with a double-wall/staggered-
>studs metal design, to give me the depth necessary for the insulation,
>and act as a thermal break between the outside and inside.

you might do the same thing cheaper with 2 1/2" x 11 7/8" i-joists as
studs on 2' centers (fingerle lumber sells gp's wis for $1.45/ft)
and sawdust(?) or cellulose fill insulation with fewer voids.

>i will be using metal studs throughout, on 24" centers (between the inner
>and outer wall, give effectively 12" spacing.)  r-42 insulation in the
>walls, and r-84 in the attic. 

...500ft^2/r84 = 6 btu/h-f for the attic? 

>windows being one each on the north and south side, 3'x3' double
>glazed low-e with argon fill, and one on the east side, 1'x2' double
>glazed low-e with argon fill.

say 20ft^2/r4 = 5 btu/h-f? with (2(20+25)8-20)/42 = 17 for the walls... 

>extra special care will be given to eliminating all breaks in the vapor
>barrier envelope.

with excruciating care and lots of well-taped plastic film you might end up
with 15 cfm of natural air leakage, another 15 btu/h-f or so, making the
total thermal conductance about 6+5+17+15 = 43 btu/h-f.

>...i am thinking of having a double-entry door, to decrease the heat
>transfer through the entryway.

the energy savings are probably not worth the expense.

>the nearest solar data for my construction site is dagget, ca...

nrel says july is the warmest month, with min, 24h and max average
daily temps of 74.0, 88.9 and 103.9 f and humidity ratio w = 0.0071.
pa = 29.921/(1+0.62198/w) = 0.338 "hg, with a 9621/(17.863-ln(pa))-460
= 48 f dew point. the deep ground temp is 67.7. 

>originally, was going to go with a cool tower/thermal chimney combo
>for cooling, but with the low cost of off-spec pv on a two-axis
>tracker, i'm thinking of going with a conventional ac... i'm hoping
>a 5000 btu ac will be more than enough capacity, and only have to run
>a fraction of the time, even on a record-setting 115-120 degree day.

nrel's 30 year record high is 116.1... (120-70)43 = 2150 btu/h, but
evaporative cooling should work well most of the time. you might have
a 4'x25' shallow pond filled with rocks along the north edge of the
house, with water close to the dew point. with an 800 btu/h-f auto
radiator, you might have something like this:

       1/800  ti  1/43
48 f ---www---*---www--- 120 f   
            <-- i              

i = (120-48)/0.0245 = 2938 btu/h, so ti = 48+2938/800 = 52 f :-)

nick




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