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re: cabin heating
12 mar 1998
sandy kear wrote:
>crthurs@ibm.net wrote:
>>the cabin i am building... is in a mountain "development" in nc... and
>>the building code for the county says that such a structure must have a
>>source of permanent heat; primarily to keep pipes from freezing...
since this is new construction, i wonder if wrapping every pipe with
electrical heat tape under pipe insulation, with a separate thermostat,
counts as "permanent." or using copper pipe itself as a heater, via a
low-voltage transformer. or putting all the piping in a crawl space with
perimeter insulation, kept warm by the soil below, with a warm water or
warm air thermosyphoning loop, or an active loop with a thermostat and
a small pump or a fan to keep the fixtures above from freezing.
or... solar heating the cabin, using a transparent steep south roof with
insulation in the attic floor, and a fan to bring down heat to the house
when the attic is warmer, or some transparent siding over insulation in
the south wall to make a passive or active air heater. a single layer of
replex (800) 726-5151 or dynaglas corrugated polycarbonate plastic costs
about $1/ft^2, comes in 4'x12' sheets from greenhouse suppliers, has a
10 year guarantee, and goes on quickly compared to osb + felt + shingles.
the worst-case month for solar heating in ashville is january, with a
24-hour average air temperature of 35.7 f and average daily max of 46.5.
on an average january day, 1180 btu/ft^2 of sun falls on a south wall.
single glazing might transmit 0.9x1180 = 1062 btu/ft^2 of that. collecting
solar heat for 6 hours at 80 f makes for an average daily glazing loss
of about 6h(80f-40f)1ft^2/r1 = 240 btu/ft^2 and a net solar sunspace gain
of about 800 btu/ft^2/day. a 32x32' cabin with average us r20 walls and
ceiling and thermal conductance of 2000 ft^2/r20 = 100 btu/h-f needs
about 24h(68f-35.7f)100 = 78k btu to stay warm on an average january
day, ie about 78k/800 = 100 ft^2 of low-thermal-mass sunspace glazing.
with suitable thermal storage for multiple cloudy days in a row, you might
interpret this as "permanent," in the sense that it can surely keep pipes
from freezing. norman saunders, pe, has been designing solar houses in
cold cloudy new england since 1946. he calculates how often their thermal
stores will fail to keep the house at 68 f using statistical methods like
those used to predict 100 year floods :-) some of his clients have decided
not to install any backup systems at all, on hearing they might have to
wear a sweater indoors once every 35 years. a horizontal surface in ashville
receives an average of 790/1110 = 71% of the possible clear-day sun in
january (with a standard deviation of 62 btu/ft^2-day). we might estimate
the chance of 2 cloudy days in a row as (1-0.71)^2 = 8%, with 2% for 3
cloudy days, 0.7% for 4, 0.2% for 5, and 0.005% for 8, ie we might have
8 cloudy days in a row, once every 57 years.
water with heat capacitance c cooling from t = 120 f to 80 f could supply
8 days x 78k = 624k btu of useful space heat if 624k = (120f-80f) c, ie
c = 15,600 btu/f equivalent to 15,600 pounds of water or about 32 55 gallon
drums full of water completely surrounded by r20 insulation inside an 8'
cube with an 8x8' solar air heater on its south side inside a 12' deep
sunspace. after a long string of average days, if the solar energy that
flows into the cube equals the heat energy that flows out, we might have
8x8'x1180x0.9x0.9 = 61k btu/day
= 6h(t-80)64ft^2/r1 [daytime air heater loss] +
18h(t-35)64ft^2/r20 [nighttime air heater loss] +
6h(t-80)4x64ft^2/r20 [daytime loss from other sides] +
18h(t-35)4x64ft^2/r20 [nighttime loss from other sides]
= (384+57.6+76.8+230.4)t - (30720+2016+6144+8064)
= 748.8t - 46,944,
making the steady-state water temperature t = (61k+46,944)/748.8 = 144 f,
warmer than 120 f, so the 120 f assumption looks ok... with more glazing,
and some fin tube pipe near the ceiling, the cube might also make hot
water for showers.
>...your alternatives are gas/oil forced air ($1000+)...
how about electric resistance/forced air? that would be cheap to begin
with, and cheap to operate if rarely used, and the system might be used
to evenly distribute solar warmed air throughout the house with the
electric heating element off. one might buy a new electrically-heated
house with certified blower door testing, add-on a $2/ft^2 lean-to
sunspace containing some insulated thermal storage, and disconnect
the heating element in the furnace.
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.eedeeyou
computer simulation and modeling. high performance, low cost, solar heating and
cogeneration system design. bsee, msee. senior member, ieee. registered us
patent agent. hi/dvc board member. web site: http://www.ece.vill.edu/~nick
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