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glass/mass ratios
29 sep 2003
doug  wrote:

>> >i've applied some rules of thumb to see if we're overglazed or not. 
>> >it may be an exact science to nick pine or david delaney, but not to me.

exact ways involve charts and tables or a tmy2 or sbic simulations, but
why bother? direct gain (aka "direct loss") houses are such poor performers,
compared to other passive solar heating configurations. for one thing, the
thermal mass has a small temp swing, so it doesn't store much heat, compared
to the same mass with a larger temp swing. for another, a lot of heat leaks
out of the glazing at night and on cloudy days. isolated sunspaces and air
heaters can gain more heat during the day and lose a lot less at night or
on cloudy days.

>> a low-performance mass and glass passive solar house has an optimal
>> glass/heat load ratio: too few south windows means too little solar heat.
>> too many means too much backup heat on cloudy days...

you don't want to collect more heat than you can store, because extra
glazing adds to the heat loss but doesn't help much on an average day.

>the house has been designed with approx 360 sq ft of south facing
>windows.  the floor area is approx 1800 sq ft... on the south east
>corner is a sunroom about 3 ft lower than the livingroom.

you might remove its thermal mass and add an insulated wall to close it off
from the rest of the living space at night and on cloudy days. 

>all the floors are cement with no covering that would impede heat flow.
>under the 6" of cement is approx 3" of polystyrene insulation.

how about some thermal mass in ceilings, eg a layer of sand above the
ceiling drywall or plywood and below the attic floor insulation, where
it can be warmer than the living space?

>the windows... have a shgc of .51 and a u factor of .32... the house
>is just outside of ottawa.

the 1979 nrc solarium design guide for canadian homes implies december is
the worst-case month for solar heating in ottowa, when 2.257 kwh/m^2 (715
btu/ft^2) of sun falls on a south wall on an average -8 c (17.6 f) day, so
your 360 ft^2 of windows would admit 360x0.51x715 = 131.3k btu of solar heat
on an average day. a frugal 600 kwh/mo of indoor electrical usage would add
2842 btu/h to that, even on cloudy days. with a g btu/h-f house conductance
to outdoors, electrical usage has the effect of raising the outdoor temp to
17.6+2842/g, even on cloudy days. for instance, 200 btu/h-f effectively
raises the outdoor temp to 17.6f+2842btu/h/(200btu/h-f) = 31.8 f. 

you might imagine the house is 70 f at dusk on an average day and 60 f at
dawn, and figure out how much thermal mass with a 10 f day-night swing is
needed to store 18 hours of heat. the house might be 80 f at dusk on a
clear day. you might weight the mass based on its position and depth. dan
chiras says each square foot of glazing in excess of 7% of the floorspace
requires 5.5 ft^2 of uncovered sunlit floor mass or 40 ft^2 of unlit floor
mass or 8.3 ft^2 of wall mass... then again, that's just a rule of thumb.
it really depends on the amount of insulation and the climate. you might put
5 vertical 8'x4" sealed pvc water pipes in each 2'x2x6 inside wall cavity,
with holes at the top and bottom for ventilation...

you might figure cloudy days are like coin flips, with 2x715 btu/ft^2 on
sunny days and 0 on cloudy days. a house that can store 1 day's worth of heat
can have a 50% max solar heating fraction, with 75% for 2 days, 88 for 3, and
100(1-2^(-n))% for n. but real weather changes more slowly, so you might use
a die instead of a coin and toss for cloudy days (1 or 2), average days (3
or 4), and clear days (5 or 6), remembering that more glass loses more heat
on cloudy days.

good luck.

nick




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