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re: dehumidifier
16 nov 2004
dominique cormann   wrote:

>we live in a 3 year old townhouse. its very well insulated ( ontario 
>canada is where we are ), and so we have humidity problems.

good airsealing...? a damp basement? downspout problem? leaky roof?

>even more so after we have been cooking
>( making soup - boiling on the stove ).

pot lids can help.

>by code, all new houses here must come with a dehumidifier, so we have 
>one. its a portable unit. now that its getting colder and dew is 
>settling on the windows,

on the room side...?

>i have been running it during the day. the bucket inside the unit is
>usually half full by the time i come home from work.

maybe 1 gallon per day? not much.

>the house was at 80% humidity at the beginning of this week, and 
>with it running during the day it has gone down to 70%.

...60% seems better.

>i did some googling and found a suggestion that a less expensive way of 
>getting rid of the humidity would be to run the central air conditioner.

maybe not, in wintertime.

>any tips on what would be the least expensive way to decrease the 
>humidity in the house?

the moisture is coming from somewhere. track down the sources and fix them.
unvented clothes dryers, kerosine heaters, indoor firewood storage, overly
frequent showers, a forest of green plants, new lumber, plaster, paint,
concrete... i've heard "1-2 years" for a new house to dry out.

>we are also selling this house now, and are building a new house. we 
>haven't finalized the options on the new house yet. one of the options 
>is a van ee air exchanging system ( supposedly 95% efficient ). its 
>$2500 (canadian). we are considering this option. would that be 
>cheaper/more effective to use then running a portable dehumidifier 
>during the day?

i don't think so.

>so basically its a question of whats cheaper.

stop the moisture sources.

>running the dehumidifier at 4.5 cents per kwh or running the ng furnace
>to heat the house on 26 cents per cubic meter.

...26 cents/m^2 is $0.7366/100 ft^3 or $7.366x10^-6/btu, if 100 ft^3
is a therm, ie 10^5 btu.

my portable kenmore dehumidifier acts like a heat pump with a 1.6 cop, ie
for every 1 kwh of electricity consumed it produces 1.6 kwh of heat, ie an
excess of 0.6 kwh or 2047 btu, as it condenses about 2.047 pounds of water
vapor, so condensing 1 pound of vapor requires 1/2.047 = 0.4885 kwh worth
4.5x0.4885 = 2.198 cents, but that also produces 1.6x0.4885 = 0.7816 kwh
or 2667 btu of heat, which may offset 1.965 cents of natural gas used for
heating, making the net cost of removing a pound of water vapor with a
dehumidifier 0.2334 cents.

the outdoor humidity ratio wo = 0.0025 pounds of water per pound of dry air
on an average 30 f january day where i live near phila. indoor air at 70 f
and 60% rh has wi = 0.009476, so removing a pound of water requires moving 
1/(wi-wo) = 143 pounds of fresh air through the house. warming it from 30
to 70 f requires 0.24x143(70-30) = 1376 btu, ie 1.01 cent's worth of gas.

removing a gallon of water per day requires moving 8.33x143/24/60 = 0.827
pounds per minute of fresh air through the house, ie 0.827/0.075 = 11 cfm.
not much. 

so it looks like the dehumidifier is 4 times cheaper, at 2 vs 8 cents per
day, altho fresh air has other benefits... 


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