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re: humidifier to reduce need for heating
1 dec 2003
chris hill   wrote:

> wrote:

>>i recently read an article on msn about increasing humidity to save on
>>heating...
>
>i've heard of this.  the problem is that after trying it, i believe it
>doesn't work.  i'm sure less humidity makes it feel cooler if you're
>sweating, but when you aren't sweating, increasing humidity wouldn't
>make sense.

it takes about 1000 btu to evaporate a pound of water. we lose heat as water
evaporates from the skin, even without sweating. some hunters keep warmer
outdoors by wearing latex close to the skin. we also lose heat as our lungs
humidify air we breathe. mufflers can keep us warmer by condensing water
vapor as we breathe out and evaporating it as we breathe in.

can we save house heating energy by using a humidifier, so we can turn the
thermostat down? it looks like the answer is yes, for a very airtight house
without too much insulation, but few houses are sufficiently airtight. they
usually leak far more air than the ashrae 15 cfm/occupant standard. a 2400
ft^2 house with r20 walls has a conductance of about 200 btu/h-f, excluding
air leaks. with 1 air change per hour, it leaks 2400x8/60 = 320 cfm, so we
use less energy by leaving it dry and turning up the thermostat. 

the ashrae comfort zone has a "constant comfort line" with t = 89.4-1867w,
where w is the humidity ratio, ie the number of pounds of water per pound
of dry air, within the limits of t = 67 to 81 f, but a lot of people are
comfortable outside of these limits. i extended this line down to 62 f in
the program below. air sealing this house could reduce the heating bill by
a factor of five...

nick

10 w0=.0025'outdoor humidity ratio
20 g=200'house thermal conductance (btu/h-f, excluding air leakage)
30 print "t =3d    62      64      66      68      70      72      74
40 for fe=0 to 5
50 f=15*2^fe'air leakage (cfm)
60 print f;
70 for tf=0 to 6
80 t=62+2*tf'house temp (f)
90 w=(89.4-t)/1867'humidity ratio needed for comfort at temp t
100 heatcost=(t-30)*(f+g)'heat power required (btu/h)
110 humcost=60*f*0.075*1000*(w-w0)'humidification power required (btu/h)
120 cost=heatcost+humcost'heat power required (btu/h)
130 print tab(7+8*tf);int(cost+.5);
140 next tf
150 t=76
160 w=(89.4-t)/1867
170 cost=(t-30)*(f+g)+60*f*0.075*1000*(w-w0)
180 print tab(7+8*7);int(cost+.5)
190 next fe

t =   62      64      66      68      70      72      74      76

15   *7702    8060    8417    8775    9133    9490    9848    10206
30   *9004    9319    9635    9950    10265   10581   10896   11211
60   *11608   11838   12069   12300   12531   12761   12992   13223
120  *16815   16877   16938   17000   17061   17123   17184   17246
240   27230   26953   26676   26399   26122   25845   25568  *25291
480   48060   47106   46152   45198   44245   43291   42337  *41383
 ^
cfm  * least cost     

it's a snap to save energy in this country. as soon as more people
become involved in the basic math of heat transfer and get a gut-level,
as well as intellectual, grasp on how a house works, solution after
solution will appear.
                                        tom smith, 1980




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