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re: how to design an air convection heating?
5 feb 1999
phil kabza, aia wrote:
>...in order to predict the heating load on a superinsulated house,
>you need to use outdoor temperature data built around a different
>degree day base - say, 45 or 50 degrees f, instead of the standard
>65 degree f commonly used.
finding that data doesn't seem easy. the ashrae handbook of
fundamentals has a complex way to estimate it. a sinewave
curve fit might be simpler.
>the actual difference is based upon your internal heat loading
>(number of occupants, cooking, hot water heat, lights)
suppose a house has a thermal conductance of g btu/h-f and
an internal heat generation of p btu/h. at the outdoor balance
point temperature t, where the house needs no additional heat,
t+p/g = 68 f, using "ohm's law for heatflow," so t = 68-p/g.
for example, a house with conductance g = 200 btu/h-f and
400 kwh/mo of electrical use, an average of about 1,900 btu/h,
would have t = 68-1900/200 = 58.5 f.
>and the amount of passive solar gain.
i usually don't include that, because i like to estimate how much
heat a house needs on an average cloudy winter day.
>superinsulated houses respond quickly to very modest amounts
>of passive solar gain;
an unusual one with lots of thermal mass inside might respond slowly.
>...a gravity system based upon natural air flow, possibly enhanced
>with a couple of small room-to-room fans or the fan from your backup
>heating system, is quite workable in a superinsulated house, especially
>if the occupants can tolerate 1) differences in temperature over time
>and 2) differences in temperature between the area surrounding a
>woodstove and the more distant parts of the interior.
we might think of natural convection as having certain thermal
conductances from the woodstove to other parts of the house which
tend to make the inside of a house nearly the same temperature
all over. if the house is very well insulated, the high resistances
of the walls make those fixed thermal conductances more effective
in keeping the house temperature spatially constant, in a dc
circuit analogy.
>...use the stairs to the upstairs as the major supply and return
>"duct" and large floor/ceiling registers to return cooler air
>from distant corners of upstairs rooms...
that seems like a good idea, although going back to those registers
implies a loss of sound isolation. how big should they be?
a 2 story 32x32' house with r30 walls and an r60 ceiling and 4% of
the floorspace as r4 windows and 0.25 ach of air infiltration has
a thermal conductance of 1024ft^2/r30 = 68 btu/h-f for the walls,
1024ft^2/r60 = 17 for the ceiling, 41ft^2/r4 = 10 for the windows
and an air volume of 16,384 ft^3 and an air infiltration rate of
0.25 times that, ie 4,096 ft^3/hour or 68 cfm, which adds about 68
btu/h-f to the thermal conductance, making a total of 163 btu/h-f,
if we ignore the heat loss from the floor. keeping it 70 f indoors
on a 30 f day requires (70f-30f)163btu/h-f = 6,520 btu/h, if
there's no internal heat generation.
suppose we want to keep the house 70+/-5 f all over, and it has
perfectly insulating floors, and hot air from a woodstove travels
up the stairway, an 18 ft^2 opening, but the limiting factor in
this air convection loop is the size of the floor registers in
the 2nd floor rooms. how big do they need to be? the second floor
has a thermal conductance to the outdoors of 17 btu/h-f for the
ceiling, about 5 for the windows, 34 for the walls and 34 for air
infiltration, a total of about 90 btu/h-f, so it needs about
(70-30)90 = 3,600 btu/h to stay 70 f on a 30 f day.
with 4 equally-sized rooms, each register needs to supply
900 btu/h of heat. with a 10 f temperature difference and a
16' height and a register area a, we might have a natural
airflow of 16.6asquare_root(16'(10f)) = 210a cfm, using one
empirical chimney formula. heatflow would be approximately
10f(cfm) = 2100a = 900 btu/h, so a = 0.43 ft^2, like an 8"
square register.
but if the second floor were uninsulated, it might have a
thermal conductance of 1 btu/h-f-ft^2, or 1024 btu/h-f, so
the downstairs-upstairs temperature diffference might only
be 3600btu/h/1024btu/h-f = 3.5 f, even without the stairway,
or with all the bedroom doors closed. so maybe we don't need
any registers at all...
>there's nothing terribly high-tech about all of this... what is
>required is somewhat fanatical attention to the workmanship in
>framing insulation cavities, installing the insulation, and
>installing a proper vapor retarder.
especially that vapor barrier. and avoidance of thermal bridges.
how can we build houses like this inexpensively, without all that
fanatical attention. modular homes with good rubber gaskets?
>a proper fresh air system, such as an air exchanger, is a requirement.
i like the idea of a double-walled woodstove chimney for that,
with warm indoor air traveling up the middle and cold outdoor air
traveling down the outside cavity, even when there is no fire.
nick
nicholson l. pine system design and consulting
pine associates, ltd. (610) 489-0545
821 collegeville road fax: (610) 489-7057
collegeville, pa 19426 email: nick@ece.vill.edyou
computer simulation and modeling. high performance, low cost, solar heating
and cogeneration system design. bsee, msee. senior member, ieee. registered
us patent agent. web site: http://www.ece.vill.edu/~nick
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