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Posted by on April 24, 2007, 1:00 am
>nicksanspam@ece.villanova.edu wrote:
>
>>
>>>I've been looking for a good dehumidifier and I notice that each big
>>>box store only really carries one brand and there really isn't much
>>>online as far as reviews. I know some dehumidifiers are energy hogs...
>>
>>How about a Crawlspace Smart Vent?
>>
>>http://www.smartvent.net
>>
>>They cost $365, but they only use 40 watts when moving 290 cfm of air
>>out of a basement when the absolute moisture content of basement air
>>is greater than the absolute moisture content of outdoor air.
>
>But that isn't what he asked about. If he has a serious humidity
>problem as I did then venting won't do a thing.
Au contraire. It will, with suitable weather conditions and controls
and building materials that can store moisture and dryness, eg paper
and wood and clothing and concrete with suitable sorption isotherms.
Concrete stores about 1% moisture by weight as the RH of the surrounding
air increases from 40 to 60%, and it weighs about 150 lb/ft^3, so
a 4"x1000ft^2 50K pound floorslab might store 500 pints of water as
a basement RH increases from 40 to 60%.
Smart Vent's 12/19/2000 US patent no. 6,161,763 "Module-controlled building
drying system and process" at http://www.freepatentsonline.com describes
"...drying air circulation between inside and outside the building based
on absolute humidity and temperature sensor measurements... the input ports
are connected to... outside absolute humidity sensors... [and] inside
absolute humidity sensors [and] the output ports are connected to...
[a fan system.] ...if the outside air has a lower absolute humidity than
the inside air... the fan system output will be activated... if the outside
air has a higher absolute humidity than the inside air... the fan system
will be shut down."
>Last year when I turned my vacation home into my residence and moved a
>lot of possessions into my basement, I bought this dehumidifier:
>
>http://www.samsclub.com/shopping/navigate.do?catg=535&item=266795&prDeTab=2#A
>
>The way to evaluate the efficiency of a dehumidifier is its rating in
>pints removed per KWH consumed. Using the spec sheet of 580 watts
>draw and 40 pints/day, that works out to 2.9 Pts/KWH.
>
>It turns out that this is worst-case, for I just checked my unit with
>a Kill-A-Watt and measured 450 watts. That makes the Pts/KWH a much
>better 3.7...
Yesterday it was 67.8F with 41% RH in my house with some windows open,
so the vapor pressure Pi = 0.41e^(17.863-9621/(460+67.8)) = 0.284 "Hg.
The indoor humidity ratio wi = 0.62198/(29.921/Pi-1) = 0.00597 pounds
of water per pound of dry air. The outdoor sensor in partial sun read
84.0 at 19%, so Po = 0.181 "Hg and wo = 0.00379, so every pound of air
that flowed through the house removed wi-wo pounds of water.
A Smart Vent could have removed 290x60x0.075(wi-wo) = 2.8 pints of water
per hour, at 70 vs 3.7 pints per kWh.
To also heat (cool) a house in a cool (warm) season, we might power up
a Smart Vent with a differential thermostat only when outdoor air is
warmer (cooler) than house air.
And we could hook a relay in parallel with the Smart Vent fan to power
a whole house fan, and add house room air thermostats to shut off the fan
if the house becomes too warm or too cool.
Nick
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