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re: proper dehumidifier and desiccant placement?
13 sep 2003
lew harriman  wrote:

>1. nick pine is almost certainly right...

don't hear that often :-)

>3. given that you apparently can get enough money for five small
>dehumidifiers, five small hepa recirc units and five shelves and
>probably drains for the dh units, i'll bet you might actually be able
>to provide a better solution for less money if you installed a single
>thermastor semi-commercial dehumidifier: 

nice... 610 w, 85 pints/24h at 5.3 pints/kwh from 70 f air at 60% rh.
optional muffler. (800) 533-7533. moneyback guarantee. how much does it cost?

but venting with outdoor air when it's drier still seems a better solution.
if it's say, 75 f and 70% rh indoors, pi = 0.7e^(17.863-9621/(460+75))
= 0.621 "hg, so wi = 0.62198/(29.921/pi-1) = 0.0132 pound of water per
pound of dry air (vs phila's 0.0130 outdoor average in august.) we might
run the fan whenever wo < 0.9wi = 0.0119, ie po < 0.560 "hg, ie
ro < e^(9621/(460+to)-18.44)), eg 

to      ro

90 f   <39%     we might remove an average of 40 pints of water per day
85 f    45%     by ventilating with 40/(0.0132-0.0119)0.075) = 410k ft^3 of
80 f    53%     outdoor air using grainger's $183 20" 8900 cfm whole house
75 f    63%     fan for an average of 46 minutes per day, consuming about
70 f    75%     46/60x180w = 138 wh of electricity worth about 1.3 cents,
65 f    89%     at 290 pints/kwh.
62 f   100%    

can we use recent ashrae temp/humidity percentiles to figure out how well
this might work? what's the standard deviation of the humidity ratio over
time, for a given location? how often do houses need dehumidification, if
it occurs intermittently? how much water can house materials safely store?

given a choice, we'd like to ventilate with cooler air in the summertime
and warmer air in wintertime, using a "smart-whole-house-fan controller."
removing condensation in summertime can cool a house instead of warming it
with a dehumidifier. we might choose to condense some water on walls for
heating in spring and fall... 

>also, you won't want to use this system when the temperature outdoors
>drops below about 55f, because the dh unit can freeze-up...

not a problem when venting with outdoor air, altho we might save more
energy by using a small periodically-reversing fan in a partition that
divides a house into two parts, turning all the house envelope cracks and
crevices into efficient low-rate air-air bidirectional heat exchangers.


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