1. Chapter 3A: THERMAL COMFORT
Agami Reddy (rev- Dec 2017)
Indoor environment quality
Basics of comfort: Definition and categories of factors
Thermal balance of human body
Metabolic rates: met
Environmental indices: Direct and Indirect
Mean radiant and operative temperatures
Clothing insulation: clo
ASHRAE Thermal sensation scale model: PMV and PPD
ASHRAE comfort chart- graphical method
Adjustments to ASHRAE comfort chart
Adaptive comfort model
Other considerations
HCB-3 Chap 3A: Thermal Comfort

2. Indoor Environment Quality
What is Indoor Environment Quality?
Thermal Comfort
Indoor Air Quality
Other Environment Issues
Visual, Acoustic, Access to daylight,…
Why do we need to care?
90 % of our lives are spent indoors
70 % of US work force (90 million persons)
Comfort -> Productivity
Comfort -> Health
HCB-3 Chap 3A: Thermal Comfort

3. HCB-3 Chap 3A: Thermal Comfort
From Kolderup, 2009
HCB-3 Chap 3A: Thermal Comfort

4. HCB-3 Chap 3A: Thermal Comfort
Factors influencing human comfort
HCB-3 Chap 3A: Thermal Comfort

5. HCB-3 Chap 3A: Thermal Comfort
Objective criteria
From Kolderup, 2009
HCB-3 Chap 3A: Thermal Comfort

6. Basics of Thermal Comfort
- Definition: ASHRAE
Thermal comfort is that condition of mind that expresses satisfaction with the thermal environment
Task of AC system is to maintain a thermally comfortable environment by simultaneous control of temperature, humidity, cleanliness and air circulation.
HCB-3 Chap 3A: Thermal Comfort

7. Thermal Balance of Human Body
Thermal comfort is maintained by heat/mass transfer
Human body generates heat (about 100 W under sedentary conditions with body area = 1.5 to 2 m2)
For comfort to be maintained:
heat generated = heat loss
Fundamental trends:
Heat flows from body to ambient air (generally)
Heat flow rate is proportional to temperature difference which is affected by clothing
More layers of clothing=more insulation= less heat loss
More activity = More heat generated
HCB-3 Chap 3A: Thermal Comfort

8. Metabolic Rates: unit of “Met”
1 M = 1 met = 58.2 W/m2 =18.4 Btu/h.ft2
HCB-3 Chap 3A: Thermal Comfort

9. HCB-3 Chap 3A: Thermal Comfort
Heat Balance Network
Heat flow can be represented
by a thermal network diagram: . .
(3.1) . .
Fig. 3.2 Thermal network model of sensible
and latent heat flows from a human body
98.6 F = 37 C
Skin temperature conducive to comfort (Eq. 3.14): . .
where M and W are in Watts
HCB-3 Chap 3A: Thermal Comfort

10. Importance of different heat transfer mechanisms at diff temperatures
Convection
Conduction
Radiation
Evaporation
Fig. 3.3 Variation of heat generated and individual heat losses
from a human at rest (RH fixed at 45%)
HCB-3 Chap 3A: Thermal Comfort

11. Environmental Indices for Measuring Comfort- Direct Indices
(a) Dry-bulb temperature: single most important index, especially influential when RH is in the range 40-60%
(b) Moisture: three measures
- Dew point temperature: good single measure but is of limited usefulness for comfort
- Wet bulb temperature: useful for describing comfort conditions in regions of high temperature and where dry-bulb has less significance
- Relative humidity: has no real meaning in terms of comfort unless accompanying dry-bulb temp. is also known,
(very low or very high values associated with discomfort)
(c) Air movement: most difficult of the direct indices to describe. It affects only convective heat exchange from body.
HCB-3 Chap 3A: Thermal Comfort

12. Environmental Indices for Measuring Comfort- Derived Indices
(a) Mean radiant temperature (MRT): surface temperature of an imaginary black body (or enclosure) to which a person, also assumed to be a black body, exchanges the same amount of heat by radiation as in the actual environment
(b) Operative temperature (OT): uniform temperature of a radiantly black enclosure in which an occupant exchanges the same amount of heat by radiation plus convection as in the actual non-uniform indoor environment. Numerically, it is close to the average of indoor dry-bulb and mean radiant temperatures
(c) Effective temperature (ET): the operative temperature of an enclosure at 50% RH that would cause the same sensible plus latent heat exchange from a person as would the actual environment
(combines temperature and humidity in one index)
HCB-3 Chap 3A: Thermal Comfort

13. HCB-3 Chap 3A: Thermal Comfort
(a) Mean radiant temperature (MRT): surface temperature of an imaginary black body (or enclosure) to which a person, also assumed to be a black body, exchanges the same amount of heat by radiation as in the actual environment
Basic index to describe
radiative conditions in space
(cold walls, sunlight walls)
Recall concept
of view factor
Simplified
methods
From Bobenhausen, 1994
HCB-3 Chap 3A: Thermal Comfort

14. Instrument to Measure MRT
Vernon’s globe thermometer
(Hollow sphere 6” diameter
painted black with a
thermocouple at the center)
One measures globe temperature Tg, ambient temperature Ta
and the air velocity v (which determines hcon - Table 3.2)
from which Tmrt can be deduced (globe emissivity close to 1) from:
HCB-3 Chap 3A: Thermal Comfort

15. HCB-3 Chap 3A: Thermal Comfort

16. (b) Operative Temperature
HCB-3 Chap 3A: Thermal Comfort

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18. HCB-3 Chap 3A: Thermal Comfort
If we had used the simple arithmetic average: Top =( )/2=19.25 oC
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19. HCB-3 Chap 3A: Thermal Comfort
Clothing Insulation
The insulation value of clothing is measured in unit: clo
(1.0 clo is equivalent to the typical American Man’s Business suit in 1941)
1 clo = 0.88 ft2.h.oF/Btu (0.155 m2.K/W)
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20. HCB-3 Chap 3A: Thermal Comfort
Example 3.2: Sensible Heat Loss
Consider the same conditions as in Example 3.1. The person is active (met level of 2.6) wearing trousers and a long-sleeve shirt. Calculate the total (convective plus radiative) sensible heat loss.
Given: Top = 18.9°C (66°F)
Assumption: Steady-state condition;
skin area Ask = 1.8 m2 (19.6 ft2) (DuBois skin surface area)
and work rate = 0; M = 2.6
Lookup value: From Table 3.3, the total thermal resistance
IT = 1.21 clo and Also, Acl/Ask = 1.20.
HCB-3 Chap 3A: Thermal Comfort

21. HCB-3 Chap 3A: Thermal Comfort.9
2.16 x (31.54 – 18.9) / = W
(similar analysis to be done for humidity effects- section 3.2.4)
HCB-3 Chap 3A: Thermal Comfort

22. Perception of Comfort ASHRAE Thermal Sensation Scale
This is an alternative approach to the thermal environmental analysis approach discussed previously
The ASHRAE Thermal Sensation Approach is an empirical approach which captures occupant psychological aspects, i.e., subjective differences between individuals.
Developed from extensive test chamber studies with humans
Recall
Thermal comfort is characterized by:
air temperature, MRT, air humidity and air velocity
with M and Clo kept constant
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23. HCB-3 Chap 3A: Thermal Comfort
A thermal sensation index, called the predicted mean vote (PMV), has been proposed to represent occupant
acceptability of the indoor environment.
- It can be calculated through a complex mathematical correlation
- The PMV index is used to quantify the degree of discomfort and ranges from +3 to −3, with zero indicating neutral or comfort condition.
HCB-3 Chap 3A: Thermal Comfort

24. Predicted Mean Vote (PMV) Model
(3.19)
(see Table 3.4)
HCB-3 Chap 3A: Thermal Comfort

25. HCB-3 Chap 3A: Thermal Comfort
Coefficients for use in Eq.(3.19) to compute PMV
The PMV has some experimental uncertainty.
-0.5 < PMV < 0.5
HCB-3 Chap 3A: Thermal Comfort

26. HCB-3 Chap 3A: Thermal Comfort
Eq.(3.19)
HCB-3 Chap 3A: Thermal Comfort

27. Percentage of People Dissatisfied (PPD)
Due to individual preferences, one would expect a distribution of votes for the PMV. An index meant to characterize this variability is PPD.
It has been found that PPD is empirically correlated to PMV.
Acceptable comfort range in view of exp. uncertainty
With experimental uncertainty in PMV being 0.5, even when PMV =0, one can expect PPD =10%,
i.e, upto 10% of the occupants may be uncomfortable
PPD=10%
Figure 3.4 PPD as a function of PMV
Method widely used in studies investigating tradeoffs between energy use and human comfort
HCB-3 Chap 3A: Thermal Comfort

28. ASHRAE Comfort Chart Valid for: Sedentary or slightly active person
Summer: light slacks & Short sleeve
shirt (0.5 clo)
Winter: Heavy slacks & long sleeve
& sweater or jacket (1.0 clo)
Air motion: < 30 ft/min in winter
< 50 ft/min in summer
No direct solar or other radiation
When application conditions are not standard, use eq. (3.21) given in a later slide
For example:
For each 0.1 clo increase, decrease the comfort zone borders by 1 °F and visa- versa
At least 80% of occupants
will be comfortable
-Why two regions?
- Why tilt in comfort regions?
HCB-3 Chap 3A: Thermal Comfort

29. HCB-3 Chap 3A: Thermal Comfort
(ASHRAE comfort chart)
Fig. 3.5 Acceptable ranges of operative temperature and humidity
for sedentary activity with typical summer and winter clothing
HCB-3 Chap 3A: Thermal Comfort

30. ASHRAE Standard Comfort Conditions
Comfort Standard applies to sedentary conditions:
Temperature - Dry Bulb Temperature (70 – 750 F)
Humidity – Relative Humidity (Up to 60%)
Air Motion – air velocity
0 – 50 ft/min: Still
50 – 250 ft/min : Noticeable
> 250 ft/min Drafty
Activity plays a role (metabolic or met level)
M=1 (sedentary) to M=5 (heavy activity)
Clothing plays a role (Table of clo): 0.5 (summer) to 1.0 (winter)
ASHRAE Standard : Thermal Environmental Conditions for Human Occupancy
HCB-3 Chap 3A: Thermal Comfort

31. Operative temperature
Note: Instead of using direct indices, comfort stated in terms of
Operative temperature
HCB-3 Chap 3A: Thermal Comfort

32. Corrections to Standard Conditions
For different clo values and met values between 1.2 and 3,
ASHRAE 55 (2013) recommends that the comfort operative
temperature be determined from:
Eq. 3.21
Where
clo is the clothing insulation
M is the metabolic value
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33. HCB-3 Chap 3A: Thermal Comfort
Example 3.6: Operative Temperature for Other Conditions
Consider a gymnasium where the metabolic rate of the occupants is 3.0 and the clothing level is 0.3. If the occupants are to experience the same level of comfort as when sedentary, what should be the
operative temperature in this space?
Given: M = 3.0, clo = 0.5
Find: Topt,active
Solution
From Equation 3.21:
Note that the gymnasium need not be kept at this low temperature since typically people do not stay longer than about an hour and do not exercise continuously, while the ASHRAE comfort chart applies to steady-state
conditions (occupancy of 3 h)
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34. HCB-3 Chap 3A: Thermal Comfort
Corrections
to ASHRAE
Comfort
Chart
Conditions:
(a) Change in
Clothing
Figure 3.6 Clothing insulation necessary to be within ASHRAE 80% acceptability limits
The effect of clothing insulation on the ASHRAE comfort recommendations.
Note the rather wide uncertainty bands (about +-2 oF) reflective of the large uncertainties inherent in the comfort relations.
For more than an hour, the minimum operative temperature should not be below 18°C (65°F).
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35. HCB-3 Chap 3A: Thermal Comfort
(b) Change in
Activity level
(ASHRAE Fundamentals, 2013)
Figure 3.7 Recommended operative temperatures for active people
Note inverse relationship between operative temperature and activity level
HCB-3 Chap 3A: Thermal Comfort

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(c) Change in
Airspeed
Fig. 3.8 Airspeed required to increase the air temperature above the summer comfort zone
Air movement plays a role because the convective heat transfer from the body depends on air velocity.
An excess may be perceived as draft, and too little as stuffiness.
In hot weather, upper range of temperatures could be extended if the airspeed is increased (upper limit 160 ft/min (0.8 m/s)- loose paper can be blown away
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38. HCB-3 Chap 3A: Thermal Comfort
Other Considerations: Local temperature variations
Additional criteria for discomfort:
local draught, high turbulence, high radiant temperature asymmetry and unacceptably high vertical air temperature difference
ASHRAE allows an additional 10% PPD for such considerations –
So upto 20% people can be dissatisfied
Fig PPD of seated occupants as a function of air temperature difference
between the head and ankles
(ASHRAE Fundamentals, 2013)
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39. HCB-3 Chap 3A: Thermal Comfort
Adaptive Comfort Model for Naturally Ventilated Buildings
Fig. 3.11
(ASHRAE Fundamentals, 2013)
(3.22 SI)
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40. HCB-3 Chap 3A: Thermal Comfort
Relationship between Loss of Productivity and PPD
Fig Illustration of how loss of indoor office occupant productivity tracks PPD.
Field study results (Roelofsen, 2001)
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41. HCB-3 Chap 3A: Thermal Comfort
This is important for O&M staff
Fig Variation of rate of unsolicited thermal complaints with mean indoor environment temperature (based on filed study of six commercial buildings in 3 cities)
(ASHRAE Fundamentals, 2013)
HCB-3 Chap 3A: Thermal Comfort

42. HCB-3 Chap 3A: Thermal Comfort
Outcomes
Understand the basics of human comfort & health and the various factors which affect them
Familiarity with the metabolic rates, unit of "met", unit of "Clo“
Be able to analyze simple cases to predict response of the human body to different environments using the thermal network model
Familiarity with the various environmental indices for measuring comfort: direct and indirect indices
Understanding of the concepts of mean radiant and operative temperatures
Familiarity with the ASHRAE thermal sensation scale and the concepts of PMV and PPD
Be able to use the standard ASHRAE chart to determine acceptable range of comfort temperature and relative humidity
Be able to use correlations and associated charts to analyze non-standard indoor conditions
Understanding of the applicability of the adaptive comfort model
Familiarity with how occupant productivity and complaints rate are affected by indoor conditions
HCB-3 Chap 3A: Thermal Comfort