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re: question about combustion air input on a high efficienct furnace.
13 nov 2005
wrote:
>>... i remain skeptical that infiltration induced by non-sealed furnace
>>combustion will significantly decrease the overall exterior temperature
>>of conventionally built exterior walls
you may be right.
>>or make them perform like these scandanavian walls of which you speak...
>>it seems to me that even if unidirectional infiltration (i.e. enter
>>through wall, exit through furnace exhaust stack) were to reduce the
>>wall exterior surface temperature to outside temp so as to eliminate
>>heat loss from that surface, it wouldn't eliminate heat loss to the
>>outdoors altogether. the heat-losing surface has simply relocated to
>>somewhere within the wall or the living space.
i've run into that kind of problem when thinking about how to preheat the
air that flows in through the wall with outgoing air, using a large poly
film air-air heat exchanger built into the wall. maybe the heat exchanger
and incoming air filter need to be inside a 4' cube filled with poly film
"plates" on 1/2 inch centers, or several cubes or valances near ceilings.
>... if a uniform sheet of outdoor air is slowly flowing towards you from
>a wall (at a velocity less than a perceptible draft), you can't lose any
>heat to that wall by convection, because the warm air won't travel upwind.
>you can still lose heat by radiation to the wall, and it still takes power
>to heat the air that flows in through the wall, but the heat loss by
>convection through the wall can be close to zero...
http://www.cibse.org/pdfs/8cimbabi.pdf has an equation for the dynamic
metric u-value of a breathing wall, as corrected:
ud = vrhoaca/(e^(vrhoacars)-1) w/m^2k, where
v is the air velocity in meters per second,
rhoa is air density, 1.2 kg/m^3,
ca is the air's specific heat, 1000 j/(kg-k), and
rs is the wall's static thermal resistance in m^2-k/w.
using v = 1/3600 (1 meter per hour :-), and rs = 5.7 m^2k/w (a us r32 wall),
ud = 0.058 w/m^2, like a us r98 wall. a more typical v = 10 meters per hour
makes ud = 1.7x10^-8 w/m^2k, like a us wall with an r-value of 334 million :-)
ud becomes infinite as v becomes 0, which seems odd. and ud approaches 0 as
v increases, no matter what kind of porous insulation is used. i'm surprised
to see 40 mm (about 1.6 inches) in the pdf. that seems very thick. i thought
scandinavian breathing walls used something closer to 1/4 inch felt. would
a single layer of tyvek or typar wind barrier work, with a replaceable air
filter? typar seems to have too much pneumatic resistance. maybe perforated
"builder's foil." bucky fuller wanted to minimize building materials...
with an air-air heat exchanger, we might move, say 240 cfm through 4000 ft^2
of walls and ceilings of a 40x60' house at v = 240/4000 = 0.06 fpm, ie 3.05
x 10^-4 m/s, giving a us r20 (metric r3.5) wall ud = 1200v/(e^(1200v3.5)-1)
= 0.141 w/m^2k, like a us r40 wall. doubling v might make it an r93 wall, if
the house were otherwise airtight (a big if, in the us.)
nick
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