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re: venta airwasher report
5 nov 2002
tono   wrote:

>> >hot-air heat:  very drying.  no matter how well insulated or
>> >caulked that apartment was, the hot air heat would still
>> >have made it very dry.
>> 
>> in myth.
 
>myth indeed!
 
myth indeed. consider a typical family of 4 putting 14 gallons of water
vapor per week into house air via breathing, washing, cooking, and so on.
ignoring the problem of breathing, what happens to all that water, in
a perfectly airtight apartment? in wintertime, the indoor rh rises until
some of the water vapor condenses on the inside surfaces of windows. 

>any heat (except steam with vents) will make your indoor air dry.

sure, but it won't dry it :-) adding heat to air doesn't change the
moisture content... 

>i get a kick out of people who think that their hot water
>baseboard heat doesn't lower the humidity %.

but the lower rh won't persist without air leaks from the outdoors.

it's 33.3 f outdoors here this morning, and 70 f at 66% rh indoors.
the dehumidifier (a heat pump with a cop of 1.6 :-) is running. this
house might float, after 3 blower door tests and lots of caulking.
the water vapor is just beginning to condense on the inside surfaces
of the double-glazed windows. if that happens too much, i leave a door
open for a few minutes.

>take cold air (it holds less moisture than warm air)...

another myth. warm air is capable of holding more moisture at 100% rh.
take 30 f air at 80% rh and vapor pressure pa = 0.8e^(17.863-9621/(460+30))
= 0.136 "hg and humidity ratio wc = 0.62198/(29.921/pa-1) = 0.00284 pounds
of water per pound of dry air... 70 f air at 10% has pa = 0.0748 "hg and
ww = 0.00156 < wc, so this cold air holds more moisture than that warm air.

>heat it, and the relative humidity will decrease.

heating 30 f 80% air to 70 f will lower the rh to 100x0.136/0.748 = 18%.

nick




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