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winter humidification wastes energy
16 nov 2004
hvactech2  wrote:
>> z> here's a link for you to read up on...
>>it doesn't do any good to show him the truth. this guy is a legend in his
>>own mind and doesn't believe anything other then what he thinks.
>it's surprising how little hvac installers know about physics and comfort. 
>you'd think they'd be curious, beyond the everyday nuts and bolts, or feel
>some sense of responsibility.
>we might try this in two steps:
>1. the ashrae 55-2004 comfort standard says 69 f at 39% rh and 69.8 at 19%
>are equally comfortable (pmv = -0.542.) do you disagree with this?
>2. keeping a house 69 f at 39% rh uses more energy than keeping a house
>69.8 at 19% in wintertime. do you disagree with this?
>if so, where are your calculations?

no disagreements? no calculations? :-) let's try again, from the top:

i suspect that winter humidification wastes vs saves heating energy, and
the savings claim is an energy myth. people tend to forget that evaporating
water takes heat energy, and that heat energy has to come from somewhere,
even if something like a humidifier belt motor uses little energy by itself.

the heat saved by turning a thermostat down appears to be far less than
the extra heat used to evaporate water, in all but extremely tight houses,
eg submarines. claims
that 69 f at 35% rh and 72 f at 19% rh are equally comfortable, but the
basic program in the new ashrae 55-2004 comfort standard predicts that
69 f and 35% rh and 69.8 at 19% rh are equally comfortable (pmv = -0.542.)

if a 2400 ft^2 tight house has 0.5 ach and say, 400 btu/h-f of conductance,
turning the thermostat down from 69.8 to 69 saves (69.8-69)400 = 320 btu/h.

air at 69 f and 100% rh has humidity ratio w = 0.015832 pounds of water per
pound of dry air, so 19% air has wl = 0.00301, and 39% air has wh = 0.00617.
raising 69 f air from 19 to 39% requires evaporating wh-wl = 0.00316 pounds
of water per pound of dry air. dry air weighs about 0.075 lb per cubic foot.

with 0.5x2400x8/60 = 160 cfm or 9600 ft^3/h or 720 pounds per hour of
air leakage, raising the indoor rh from 19 to 39% requires evaporating
720x0.00316 = 2.275 pounds of water per hour, which requires about 2275
btu/h of heat energy.

humid air seems healthier and may prevent furniture cracking and static
electricity, but serious air sealing seems like a better solution, with
blower door testing. houses have natural humidity sources. andersen says
an average family of 4 evaporates 2 gallons per day of water by breathing,
cleaning, cooking, and so on. unlike humidification, more air sealing can
raise the rh while saving vs wasting heat energy.


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