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A wall warmer with a fan
A local siding contractor gave me a verbal quote of $2 per square foot
including materials, to build my passive wall-warmer, which I thought was
pretty good... But when I asked for that in writing, he came back with
a reluctant $5/ft^2, and other contractors wanted more like $10 or $12. Yipes.
This is supposed to replace vinyl siding. Who woulda thunk it was 90% labor...
So I've redesigned it now, adding an $11 fan and changing the 4 32' long slots
along the top and the bottom and the 2 long plywood doors and 32' piano hinges
at the top for passive airflow, to a couple of 2' x 2' holes in the wall at
the top and the bottom diagonal corners. I guess it can't be called passive
anymore (sniff), but that doesn't seem too important. The wall can now be
controlled by thermostats, which is nice. The total cost of materials is
about $800, or about $1.50/ft^2, and the wall should save about 600 gallons
of oil per year, at a cost of about (63 cents + $12 (Gulf War)) per gallon,
as well as a few human lives. Below is an ascii picture.
32'
s4
--------------------------------------- --------------------
| | | | | | | |fvfv| plywood / f. f .6T2 g fv
| s3 | fan| Door / f . a .7\ g
|----|----|----|----|----|----|----|----| / f -n--2 g
| <-4'-> | w w c g
| s2 | T1 w w c g
16' |----|----|----|----|----|----|----|----| (inside) w w c g S ->
| | w w c g
| s1 | w w c g
|----|----|----|----|----|----|----|----| w w c g
| | | .f 6 g
|vent| s0 | . f 7 / g
|____l____|____|____|____|____|____|____| _________f_2______
f is plastic film
S4: wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww c is shadecloth
| | | | | | | | | g is rigid plastic
| | | | | | | | | . is hardware cloth
3.5" | | | | | | | | | w is the original wall
| | | | | | | | |
cccccccccccccccccccccccccccccccccccc The fan is a Holmes
ggggggggggggggggggggggggggggggggggggggggg 20" Slimline, 3-speed
HABF-20 box fan,
(views from above) $11 from K-Mart.
S3: wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww
| | | | | | | | | fv are Leslie-Locke
2.6" | | | | | | | | | FV-1B automatic
| | | | | | | | | foundation vents,
ccccccccccccccccccccccccccccccccccccccccc $10.83 each from
0.9" | | | | | | | | | Home Depot.
ggggggggggggggggggggggggggggggggggggggggg
T1 is a an attic fan
thermostat, 2E340
S2: wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww $15.48 from Grainger
| | | | | | | | |
1.75"| | | | | | | | | T2 is a house
ccccccccccccccccccccccccccccccccccccccccc thermostat, 2E158
| | | | | | | | | $14.05 from Grainger
1.75"| | | | | | | | |
ggggggggggggggggggggggggggggggggggggggggg To the left of these
words, air rises up
between the shadecloth,
S1: wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww c and the glazing g,
0.9" | | | | | | | | | moves sideways from
ccccccccccccccccccccccccccccccccccccccccc South to North through
| | | | | | | | | shadecloth c and then
2.6" | | | | | | | | | further up and sideways
| | | | | | | | | from West to East until
ggggggggggggggggggggggggggggggggggggggggg it is pulled into the
house by the fan.
S0: wwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww At the bottom of the
cccccccccccccccccccccccccccccccccccc wall, the shadecloth
| | | | | | | | | is stapled to the
3.5" | | | | | | | | | original wall sheathing
| | | | | | | | | At the top, it is
| | | | | | | | | stapled to the vertical
ggggggggggggggggggggggggggggggggggggggggg spacer just under
the glazing.
The glazing could be Dynaglas (408) 997-6100, or Replex (800) 726-5151
clear corrugated polycarbonate plastic, which comes in standard lengths,
12' long and a bit over 4' wide, eg from D & L Grower Supplies in Leola, PA
(800) 732-3509, and custom lengths up to 38' long in large quantities. It
costs a bit over $1 per square foot in small quantities and overlaps and
requires support on 4' centers. Replex also makes this in a flat form,
0.20" thick, sold in rolls 4' wide and 50' long. This material has a 10 year
guarantee against yellowing and an expected solar transmission lifetime
of at least 25 years. It is widely used in commercial greenhouse roofs.
In fact, the South roof of my house is now covered with this stuff, and
it just survived some golf-ball-sized hail with no damage. You could also
use glass for the glazing...
Shadecloth is available from Stuppy Greenhouse Mfg. at 1212 Clay Street,
North Kansas City, MO 64116, (816) 472-1512 or (800) 877-5025 voice/
(800)423-1512 fax. They sell black polypropylene Sundown 80%-absorbing
shadecloth as item number 23691 for a basic price of $0.139 per square
foot. Shadecloth also comes in other colors, like red and green. (Stuppy
also sells plastic film greenhouses for 55 cents per square foot :-)
The vertical spacer/sleepers could be ripped from 2 x 4s. I would make
them about 1' long, with a 3' gap between them. The items marked 672
are Hart and Cooley 672 30" x 30" return air grills, $24.05 each, wholesale,
installed with the 40 degree vanes pointing up towards the outside at the
bottom of the wall, and up towards the inside of the house at the top of
the wall. Each 30" x 30" vent hole in the wall would have one stud of the
original house wall running through it vertically, up the center.
Here is a wiring diagram for the fan and thermostats:
T1, 70F T2, 100F
---------|--------------X-----------------
house fan |
thermostat thermostat ---
110 VAC opens on closes on |fan|
temperature temperature ---
rise rise |
------------------------------------------
I would guess that the free air delivery of the fan is about 2000 CFM,
so in full sun, with the wall receiving 300 x 512 = 150K Btu/hour (that's
about 44 kW, or 18 cents per peak watt, for you PV people :-), if the air
goes into the wall at 68F, it will emerge at about 68 + 150K/2K = 140F.
When the house is warm enough, the fan will stop and the wall will continue
to heat up, until perhaps the automatic foundation vents open, if their
bimetallic springs have a little bit of wall air flowing over them. The
polycarbonate plastic can easily withstand 130F on a continuous basis,
with higher peak temperatures. In the summer, the vents will open to let
warm air flow out of the wall to the outside. Each vent is about 8" x 16",
not enough area to cool the wall very well, so it might be a good idea to put
some screw eyes through the glazing or hang some ropes or wires from the eaves
and grow some grapes, runner beans, trumpet vines, or clematis in front of
the wall. Or add an overhang. Or hang another piece of shadecloth over the
front, perhaps as an awning that goes to some guyed poles on the lawn...
Who will be the first to build one of these?
Nick