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re: advice sought on passive solar design
11 feb 2003
doug wrote:
>> the slab has about 1700x6/12x25 = 21.3k btu/f of heat capacity, enough
>> to store 213,000 btu of overnight heat with a 10 f daily temp swing.
>i'm wondering if all of the slab can be considered as the solar mass.
that's a first approximation.
>from what i've read, only the mass that is exposed to the sun's rays
>should be considered. of course, this doesn't make perfect sense
>since cement will conduct the heat away from the sunny areas and into
>the recesses of the house.
k = 0.54 btu/h-ft-f and c = 0.156 btu/lb-f and rho = 144 lb/ft^3 make
alpha = k/(crho) = 0.024 ft^2/h for concrete, so a slice x feet into an
infinite slab would change halfway from an initial temp to a new surface
temp in time t = x^2/alpha = 41.6x^2... t = 6 hours makes x = 0.38', ie
4.6"... t = 24h makes x = 0.79', ie 9".
>i'm just wondering how effective this will be.
you might just count the mass exposed to the sun...
>i was considering designing the radiant floor heating so that the pumps
>can run even when the heater is off. that way the water in the pex
>tubing can distribute the heat throughout the slab. do you think
>this is a good idea?
yes. the pump might not have to run full time. you might bury temp sensors
all over the floor and run the pump until the floor temps are close to each
other, or put a fast temp sensor in the loop output and run the pump for
a minute every hour and note the water temp changes during that minute and
run the pump more frequently if they are large, or put a few floor temp
sensors near south windows and run the pump if any sense more than 80 f,
or use a differential thermostat between the warmest and coldest room,
or start the pump with a thermostat in the coldest room, or just run the
pump on a timer.
>...one thing i was wondering about is the amount of insulation i should put
>under the cement slab. i've read that the insulation should only be
>from the perimeter walls in towards the centre but the centre of the
>slab should rest on the ground...
i'd vote for insulation under the whole slab. kreider and rabl's "heating
and cooling of buildings (mcgraw hill, 1994) has a good explanation of
how to calculate heat loss from slabs. you might also look at hud's 1994
"design guide for frost-protected shallow foundations," which you can
download from www.huduser.org/publications/destech/desguide.html as two
wordperfect files by clicking on the full-text executable link to the right.
you can see a copy with drawings that do not print well at
http://cs.arizona.edu/people/jcropper/desguide.html.
>...it's -28 degrees c as i type.
one simple ashrae method assumes a basement floor slab loses heat to the
deep ground temp (about 41 f in ottowa) through an r10 ground resistance.
an 80 f with no insulation might lose (80-41)1700ft^2/r10 = 6630 btu/h.
r10 (2") styrofoam might halve that.
>> how about making the house squarer, say 36'x48', with us r48 12" sips with
>> 0.1 ach and a ft^2 of u0.2 fiberglass windows with 50% solar transmission
>> from in-line (michaelkdorgan@hotmail.com) in toronto? with no other windows,
>> the house thermal conductance g would be 1728ft^2/r48 = 36 btu/h-f for the
>> ceiling plus 0.2a for the windows plus (1344-a)/r48 for the walls plus about
>> 0.1x1728x8/60 = 23 for 23 cfm of air leakage, ie g = 87+0.179a.
>>
>wow...
simple, huh? :-)
>> >do you think we have too much south facing window?
>>
>> maybe. they account for more than half of the house thermal conductance.
>> fewer windows would lower the need for backup heat on cloudy days. on an
>> average day, the greenhouse (or a sunspace, or passive air heaters built
>> into the south wall) could provide house heat for about 6 hours, making
>> 0.5x715a+34k = 18h(65-17.6)g, so a = 196 ft^2. a square foot of greenhouse
>> or sunspace or air heater r1 south glazing with 90% solar transmission
>> might gain 644 btu on an average day and lose about 6h(80-17.6)1ft^2/r1
>> 374 btu, for a net gain of 270 btu (or maybe 400, with whitewashed stone
>> on the ground to the south). g = 122 btu/h-f means the house needs about
>> 6h(65-17.6)g-6hx34k/24h = 26.2k btu over 6 hours, which might come from
>> 26.2k/270 = 97 ft^2 of greenhouse glazing.
>>
>in our design, the greenhouse accounts for about 140 sq ft of windows.
good, especially if most of the greenhouse gets cold at night...
canadians bathe, right? you might make the roof a trickle collector...
nick
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