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re: engineered home?
17 feb 2001
mtnguy \(co\) wrote:
>doesn't get -30f where you are, does it ... ;-)
>> > incorporate air locks on entrance areas...
>> this one makes little sense unless you have 58 children...
suppose you live in massena, ny, with record min temps of -32.1, -38.0 and
-34.1 f in december, january, and february, and 8255 dd and a 270 day
heating season, ie 8255/270 = 31 dd/d, ie an average 65-31 = 34 f outdoor
temp over the heating season. say 16.6(4ft^2)sqrt(7'x(65-34))5/60 = 82 ft^3
of air flows over 5 seconds whenever one of your 58 children opens the door
with no airlock, and heating that air requires 82/55(65-34) = 46 btu, or
270(2n)(58)46 = 1440720n btu over the heating season, if each kid leaves
the house n times per day. that's $21.6n, burning oil at $1.50/gallon and
70% efficiency.
now let's add a 4x4x8' airlock costing 4x4x$60 = $960, with an interest
cost of $96/year at 10%. smaller airlocks are more efficient. let's say
the 82 ft^3 of 65 f air from the house mixes with 4x4x8 = 128 ft^3 of 34 f
air in the lock, so the average temp of the air that enters the house is
(128x34+82x65)/(128+82) = 46 vs 34, and we only need to add 82/55(65-46)
= 28 btu of heat to the house every time the door opens, or 882318n btu
or $13.2n per year, saving $8.4n per year. the airlock only saves money
if $8.4n > $96, ie if each of the 58 children leaves the house more than
n = 11.5 times per day...
nick
that airlock entries are not cost effective has been discovered
over and over again in the energy design community. although
they may provide ancillary benefits such as "mud room" space or
protection from uncomfortable wind gusts, airlock vestibules
can never be justified on energy savings alone.
from the superinsulated home book, page 70, by
j.d. ned nisson and gautam dutt, wiley, 1985.
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