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re: advice sought on passive solar design
10 feb 2003
doug wrote:
>we are in the process of having a house designed and want to take
>advantage of passive solar gain. it will be built on a south facing
>knoll near ottawa, ontario.
the nrc solarium workbook indicates that december is the worst-case month
for solar house heating in ottowa, with 2257 wh/m^2 (715 btu/ft^2) of sun
on a south wall on an average -8 c (17.6 f) day.
>it will be slab-on-grade with an exposed cement floor (~ 6" thick with
>2" insulation under) with radiant floor heating. the house is ~1700 sq ft
>- 50' by 34' with a central loft area. there is a green-house built on
>to the south east corner.
you might enclose plants in a smaller structure within the greenhouse and
circulate warm dry air between the greenhouse and house during the day...
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. you
might use part of the roof as a trickle collector.
>essentially, all the south side of the house is window -almost 500 sq ft.
>we've had a solar assessment of the lot - it is pretty clear -
>very few obstructions... we hope to use triple glazed low-e windows.
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.
with a 300 kwh/mo (34k btu/day) electric bill, 0.5x715a+34k = 24h(65-17.6)g,
so a = 422 ft^2, with 84 btu/h-f (52%) of the 162 btu/h-f house conductance
as south windows collecting 151k btu of sun on an average december day.
>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.
>will the cement slab be able to sink all the heat gained throughout the day?
seems likely, if the sun shines on the dark colored slab.
nick
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