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expanding ashrae comfort zones
29 jan 2005
the ashrae 55-2004 comfort standard (based on worldwide surveys of 21,000
people) defines the winter and summer comfort zones below with equal air
and wall temperatures, equal human activity levels, and a fixed 0.1 m/s air

t (f)         rh            clo          pmv            ppd

67.3          86            1            -.4778556      9.769089
75.0          66            1             .4732535      9.676994
78.3          15            1             .5239881      10.74283
70.2          20            1            -.4779105      9.770202

74.5          67            .5           -.4747404      9.706658
80.2          56            .5            .5145492      10.53611
82.2          13            .5            .5003051      10.23146
76.5          16            .5           -.4883473      9.982468

the winter zone assumes heavier clothing with more thermal resistance
(clo=1), and the summer zone assumes lighter clothing (clo=0.5). the
zones are defined by a +/- 0.5 score on a "predicted mean vote" comfort
scale that varies from -3 (very cold) to +3 (very warm.) the 4 corners
of each zone are based on high and low temperatures and humidities. 
the "percentage of people dissatisfied" (ppd) score is based on the
pmv. even with pmv = 0 ("comfortable"), about 6% of the people are

if people are willing to change clothing more than twice a year (early
pa farmers had one set of clothes for work and another for church, and
washed them twice a year, when they also took baths :-) and we vary air
velocity with a ceiling fan, these zones can be expanded, which can make
a solar house or one heated and cooled with the help of a whole-house fan
more efficient. we can also raise the upper comfort temperature limit
in air with lower humidity, and vice versa. the ashrae 55-2004 standard
contains a basic program to help do this. here are some calculations based
on nrel's long-term december and august weather averages for san diego:

20 clo = 1'clothing insulation (clo)
30 met=1.1'metabolic rate (met)
40 wme=0'external work (met)
50 ta=19.6'air temp (c)
60 tr=19.6'mean radiant temp (c)
70 vel=.1'air velocity
80 rh=0'relative humidity (%)
90 data 68.8,0.0062,0.05,1
100 data 83.9,0.0062,0.5,0.5
110 data 67.1,0.0121,0.05,1
120 data 82.9,0.0121,0.5,0.5
130 for case = 1 to 4
140 read tc,wa,vel,clo
145 ta=(tc-32)/1.8'air temp (c)
146 tr=ta'mean radiant temp (c)
150 pa=29.921*3377.2/(1+.62198/wa)'water vapor pressure (pa)
160 def fnps(t)=exp(16.6536-4030.183/(ta+235))'sat vapor pressure, kpa
170 if pa=0 then pa=rh*10*fnps(ta)'water vapor pressure, pa
180 icl=.155*clo'clothing resistance (m^2k/w)
190 m=met*58.15'metabolic rate (w/m^2)
200 w=wme*58.15'external work in (w/m^2)
210 mw=m-w'internal heat production
220 if icl<.078 then fcl=1+1.29*icl else fcl=1.05+.645*icl'clothing factor
230 hcf=12.1*sqr(vel)'forced convection conductance
240 taa=ta+273'air temp (k)
250 tra=tr+273'mean radiant temp (k)
260 tcla=taa+(35.5-ta)/(3.5*(6.45*icl+.1))'est clothing temp
270 p1=icl*fcl:p2=p1*3.96:p3=p1*100:p4=p1*taa'intermediate values
280 p5=308.7-.028*mw+p2*(tra/100)^4
290 xn=tcla/100
300 xf=xn
310 n=0'number of iterations
320 eps=.00015'stop iteration when met
330 xf=(xf+xn)/2'natural convection conductance
340 hcn=2.38*abs(100*xf-taa)^.25
350 if hcf>hcn then hc=hcf else hc=hcn
360 xn=(p5+p4*hc-p2*xf^4)/(100+p3*hc)
370 n=n+1
380 if n>150 goto 500
390 if abs(xn-xf)>eps goto 330
400 tcl=100*xn-273'clothing surface temp (c)
410 hl1=.00305*(5733-6.99*mw-pa)'heat loss diff through skin
420 if mw>58.15 then hl2=.42*(mw-58.15) else hl2=0'heat loss by sweating
430 hl3=.000017*m*(5867-pa)'latent respiration heat loss
440 hl4=.0014*m*(34-ta)'dry respiration heat loss
450 hl5=3.96*fcl*(xn^4-(tra/100)^4)'heat loss by radiation
460 hl6=fcl*hc*(tcl-ta)'heat loss by convection
470 ts=.303*exp(-.036*m)+.028'thermal sensation transfer coefficient
480 pmv=ts*(mw-hl1-hl2-hl3-hl4-hl5-hl6)'predicted mean vote
490 goto 510
500 pmv=99999!:ppd=100
510 print tc,wa,vel,clo,pmv
520 next case

dry bulb      humidity      air vel.      clothing     predicted mean
temp (f)      ratio         (m/s)         (clo)        vote (pmv)

68.8          .0062         .05           1            -.4997552
83.9          .0062         .5            .5            .4852427
67.1          .0121         .05           1            -.491671
82.9          .0121         .5            .5            .4890098

nrel says average daily highs and lows are 48.8 and 66.1 f with w = 0.0062
pounds of water per pound of dry air in december and 67.3 and 77.8 with
w = 0.0121 in august. an airtight well-insulated house with thermal mass
and internal heat gain and a smart whole-house fan controller would barely
need heating or cooling all year. architects call this "thermal sailing."

with inexpensive solar heating and whole-house fan heating and cooling,
it would be more efficient to make the house air temperature closer to
the upper comfort limit on an average winter day and closer to the lower
comfort limit on an average summer day, in order to store lots of thermal
energy in the mass of a house. a mean radiant (wall) temp that's less than
the house air temp in winter and greater in summer would allow raising
the upper winter air comfort temp limit and lowering the lower summer air
comfort temp limit. occupants might also vary activity levels and wear 
clothing with more or less resistance, eg a sweater as well as a long-
sleeve shirt in wintertime. 


innova airtech instruments has an excellent comfort web site:

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