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re: where can i find?
14 may 1998
kevin j dressel   wrote:

>where can i find a low power consuming exhaust fan?  i need one for my
>greenhouse which is fairly small (about 500 cubic feet).

like, 8x8x8', with a peak solar input of about 128ft^2x300 btu/h-ft^2
= 20k btu/h? many growers use exterior whitewash or greenhouse shadecloth
to prevent overheating in summer; 80% black shadecloth would reduce peak
sun to 4,000 btu/h. some plants like less sun, but a few like full sun with
lots of venting to keep the greenhouse cooler. you might move the plants
outdoors, or put lots of thermal mass in the greenhouse to smooth out
day/night temperature swings and leave the door partly open 24 hours a day.

that's what david boyer does in the summer. his 20x100' greenhouse contains
200 55 gallon drums full of water. with 44 tons of water, its temperature
changes so slowly that there isn't much need for automatic control, just
a little door adjustment every few days. this also helped winter heating,
reducing the propane bill from $8,000 to $1,500 per year. 

>i was hoping to be able to utilize my solar panels so i would like
>the fan to run off 12-15volts dc.  any ideas??
here's an empirical formula for chimneys with no airflow obstructions:
q = 16.6 av sqrt (h dt), where q is the airflow in cfm, av is the area
in ft^2 each hole at the top and bottom, h is the chimney height in feet
and dt is the farenheit temperature difference. one cfm with a temperature
difference of 1 f moves about 1 btu/h of heat, so an 8' "chimney" moves
roughly 47 av dt^1.5 btu/h. removing, say 20k btu/h with an indoor air
temperature 10 f above outdoor air requires vent area av = 20k/(47x10^1.5) 
= 13.5 ft^2 at the top and bottom, eg an open door and a 4'x4' gable end
window, which might have a counterweight.

the window might open with one of jade mountain's $54 thermofor solar vent
openers, cat #fc115, which requires no electrical power and can be set to
open between 55 and 85 f. if you are philosophically compelled to use pv
panels, they also sell a $96 10 watt 12v gable fan (optimistically?) rated
at 1,000 cfm, to limit the greenhouse temperature rise to about 20 f.
cat # fc127, (800) 442-1972. 

alternatively, for less average electrical power, you might use
grainger's $31 17 rpm 1.4 amp 30 in-lb 2l008 reversible gearmotor and
a couple of their $15 2e158 spdt thermostats and limit switches and
diodes to open the vent window with a circuit like this:

       +12   -----------------------------------------
                    |                                 |
                    c           close <---            o
                    |     -|<-    -------    ->|-     |
                    |----|    |--| motor |--|    |----|
                    |     -vo-    -------    -vc-     |
                    o           ----> open            c
                    |                                 |
       gnd   -----------------------------------------

c are thermostat contacts that close on temperature rise.
o are thermostat contacts that open on temperature rise.
vo opens when the window is fully open.
vc opens when the window is fully closed. 

this requires a 12v battery as well as the pvs. 

or, you could add an inverter and 24 vac transformer and one of honeywell's
$50 ml6161a 2 watt direct-coupled damper actuator motors,
with their $15 limit switch kit.


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