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Posted by Abby Normal on February 11, 2005, 12:32 pm
Turning down from 69.7 to 69 is not going to do much, lets get frugal
and look at keeping a home at 69 instead of 72.
Compare 72 @ 29% to 69 @ 36%, similar enthalpy. Almost think Lennox had
a typo. Look forward to seeing their new brochure, to see if in fact
you have basically pointed out a typo. 72 @ 29% vs 69 @ 35%, or 72 @
19% vs 69 @ 25%
2400 sq ft with a conductance of 400 btu/(hr F), again this is heat
conducting out of the house.
Air infiltration equivalent to 160 CFM.
So setting thermostat down from 72 to 69 saves 3 x (400 +1.08 x 160) =
1718.4 Btu/hr.
As a check, assuming 70F indoor temp, 0F outdoor temp, heatloss of home
in the ball park of 70x (400 + 1.08 x 160)= 40,096 Btu/hr. Wow a 45 MBH
90% eff gas furnace would be right on the money, and this is typically
the smallest size condensing furnace on the market, so this scenario
sounds realistic.
The house volume is about 19,200 cubic feet so the difference in the
amount of water held in the air is a little under one pound and the
heat to evaporate this moisture will be a maybe 900 Btu.
Save 1718 Btu then waste 900 to evaporate some water.
So there is a 'savings' of 818 Btu.
Yes the motors that turn humidifier drums use energy, but energy is
conserved and ultimately this energy creates heat in the home as well,
so it is not wasted.
nicksanspam@ece.villanova.edu wrote:
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> >OK Nick, care to elaborate on what your theory is?
> Sure. Here's the email I sent to Lennox...
> Sent: Monday, January 17, 2005 8:58 AM
> Subject: Attn: president/legal--Winter humidification wastes energy
> Gentlemen,
> I suspect that winter humidification wastes vs saves heating energy,
and
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> the savings claim is an energy myth. People tend to forget that
evaporating
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> water takes heat energy, and that heat energy has to come from
somewhere,
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> even if something like a humidifier belt motor uses little energy by
itself.
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> The heat saved by turning a thermostat down appears to be far less
than
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> the extra heat used to evaporate water, in all but extremely tight
houses
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> with little insulation, eg submarines.
> http://lennox.com/pdfs/brochures/Lennox%20WB2-WP2%20Humidifiers.pdf
claims
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> that 69 F at 35% RH and 72 F at 19% RH are equally comfortable, but
the BASIC
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> program in the new ASHRAE 55-2004 comfort standard predicts that 69 F
and 35%
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> RH and 69.7 at 19% RH are equally comfortable (PMV = -0.537, see
below.)
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> If a 2400 ft^2 tight house has 0.5 ACH and say, 400 Btu/h-F of
conductance,
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> turning the thermostat down from 69.7 to 69 saves (69.7-69)400 = 280
Btu/h.
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> Air at 69 F and 100% RH has humidity ratio w = 0.015832 pounds of
water per
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> pound of dry air, so 19% air has wl = 0.00301, and 39% air has wh =
0.00617.
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> Raising 69 F air from 19 to 39% requires evaporating wh-wl = 0.00316
pounds
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> of water per pound of dry air. Dry air weighs about 0.075 lb per
cubic foot.
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> 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
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> 720x0.00316 = 2.275 pounds of water per hour, which requires about
2275
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> Btu/h of heat energy, so it looks like humidifying this fairly
airtight
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> house wastes 2275/280 = 8 times more energy than it "saves." And many
> S houses are less airtight, so humidification would waste more
energy.
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> Please modify your energy-savings claim.
> Thank you.
> Nick Pine
> 10 SCREEN 9:KEY OFF
> 20 CLO=1'clothing insulation (clo)
> 30 MET=1.1'metabolic rate (met)
> 40 WME=0'external work (met)
> 50 DATA 69,35,69.74,19
> 60 FOR CASE=1 TO 2
> 70 READ TC,RC
> 80 TA=(TC-32)/1.8'air temp (C)
> 90 TR=TA'mean radiant temp (C)
> 100 VEL=.1'air velocity
> 110 RH=RC'relative humidity (%)
> 120 PA=0'water vapor pressure
> 130 DEF FNPS(T)=EXP(16.6536-4030.183/(TA+235))'sat vapor pressure,
kPa
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> 140 IF PA=0 THEN PA=RH*10*FNPS(TA)'water vapor pressure, Pa
> 150 ICL=.155*CLO'clothing resistance (m^2K/W)
> 160 M=MET*58.15'metabolic rate (W/m^2)
> 170 W=WME*58.15'external work in (W/m^2)
> 180 MW=M-W'internal heat production
> 190 IF ICL<.078 THEN FCL=1+1.29*ICL ELSE FCL=1.05+.645*ICL'clothing
factor
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> 200 HCF=12.1*SQR(VEL)'forced convection conductance
> 210 TAA=TA+273'air temp (K)
> 220 TRA=TR+273'mean radiant temp (K)
> 230 TCLA=TAA+(35.5-TA)/(3.5*(6.45*ICL+.1))'est clothing temp
> 240 P1=ICL*FCL:P2=P1*3.96:P3=P1*100:P4=P1*TAA'intermediate values
> 250 P5=308.7-.028*MW+P2*(TRA/100)^4
> 260 XN=TCLA/100
> 270 XF=XN
> 280 EPS=.00015'stop iteration when met
> 290 XF=(XF+XN)/2'natural convection conductance
> 300 HCN=2.38*ABS(100*XF-TAA)^.25
> 310 IF HCF>HCN THEN HC=HCF ELSE HC=HCN
> 320 XN=(P5+P4*HC-P2*XF^4)/(100+P3*HC)
> 330 IF ABS(XN-XF)>EPS GOTO 290
> 340 TCL=100*XN-273'clothing surface temp (C)
> 350 HL1=.00305*(5733-6.99*MW-PA)'heat loss diff through skin
> 360 IF MW>58.15 THEN HL2=.42*(MW-58.15) ELSE HL2=0'heat loss by
sweating
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> 370 HL3=.000017*M*(5867-PA)'latent respiration heat loss
> 380 HL4=.0014*M*(34-TA)'dry respiration heat loss
> 390 HL5=3.96*FCL*(XN^4-(TRA/100)^4)'heat loss by radiation
> 400 HL6=FCL*HC*(TCL-TA)'heat loss by convection
> 410 TS=.303*EXP(-.036*M)+.028'thermal sensation transfer coefficient
> 420 PMV=TS*(MW-HL1-HL2-HL3-HL4-HL5-HL6)'predicted mean vote
> 430 PPD=100-95*EXP(-.03353*PMV^4-.2179*PMV^2)'predicted %
dissatisfied
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> 440 PRINT TC,RC,PMV
> 450 NEXT CASE
> 69 35 -.5376486
> 69.74 19 -.5372599
> Engineering VP Mark Hogan said Lennox was embarrassed by all this and
> he didn't know where their numbers had come from, and he thanked me
> for bringing this to their attention and said they are changing their
> printed brochures and Aprilaire web site energy-savings claim.
>
> This reminds me of David and Goliath :-)
>
> Nick
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