Thermal Comfort

Building Physics, Lo-Lo CDT

Thursday 6th October 2011

Thermal Comfort

What is Thermal Comfort?

Definition of Thermal Comfort

“That condition of mind which expresses satisfaction with the thermal environment”

(ISO 7730)

Thermal Comfort Variables

(7-Point ASHRAE Scale)

hot

warm

slightly warm

neutral (comfortable)

slightly cool

cool

cold

3

2

1

0

-1

-2

-3-3 -2 -1 0 1 2 3

Thermal Sensation

0

20

40

60

80

100

Per

cen

tag

e D

issa

tisf

ied

[%

]

Thermal Sensation Percentage of Dissatisfied

Thermal Comfort

TS may also be the mean vote for a group of individuals

Necessary Conditions for Thermal Comfort

1. Heat balance of the human body

2. Comfortable psychological conditions

Heat Balance of the Human Body

Metabolic heatproduction

Components of the Environmental Heat Loss and the Human Heat Balance

Ta , rh

rh

Factors Affecting Human Heat Balance

• Environmental parameters– Air temperature (Ta)– Mean radiant temperature of surfaces (Tr)– Air velocity (va)– Air relative humidity (rh)

• Personal factors– Physical activity (M)– Clothing insulation (Icl)

Personal Factors: Clothing

Clo: a unit used to express the thermal insulation provided by garments and clothing ensembles: 1 clo = 0.155 m2 °C/W

Personal Factors: ClothingEnsemble

Icl (clo)

Briefs; knit, short-sleeve sport shirt; walking shorts and belt; calf-length socks; hard-soled shoes

0.4

Briefs; broadcloth, long-sleeve shirt; long fitted trousers and belt; calf-length socks; hard-soled shoes

0.6

Panties; half-slip; broadcloth, long-sleeve blouse; single-breasted suit jacket; A-line, knee-length skirt; pantyhose; thongs/sandals

1.0

Briefs; t-shirt; broadcloth, long-sleeve shirt; long-sleeved, round-neck sweater; thick, straight, long, loose trousers and belt; calf-length socks; hard-soled shoes

1.0

Flannel, long-sleeve, long nightgown; thick, long-sleeve, wrap, long robe; slippers

1.7

Personal Factors: Metabolic RateMet: a unit used to describe the energy generated inside the body due to metabolic activity. It is defined as 58.2W/m2 which is the energy produced per unit surface area of an average person, seated at rest: 1 met = 58.2 W/m2

Metabolic Rates for Typical TasksActivity

Metabolic rate (Met)

Reclining 0.8

Seated, quietly 1.0

Sedentary activity (office, dwelling, lab, school) 1.2

Standing, relaxed 1.2

Light activity, standing (shopping, lab, light industry) 1.6

Medium activity, standing (shop assistant, domestic work, machine work)

2.0

High activity (heavy machine work, garage work) 3.0

© ASHRAE

Effect of Activity Level

Physiological Conditions for Thermal Comfort

Esk = 0.00305 [5733-6.99(M-W)-Pa] + 0.42 [(M-W) - 58.2]

Sweat rate

Predicted Mean Vote• The mean vote of a group of individuals can

be predicted.• The Fanger model of human thermal comfort

enables this. • The model is really only applicable for a body

in thermal equilibrium with it’s surroundings.• PMV is calculated on a 7-point scale from -3

to +3 – so it is directly comparable with TS.

Comfort Equation (Fanger)

Metabolic heat generation [W/m2]

External work [W/m2]

Dry heat losses [W/m2]:- Convection: C = hc (Ts-Ta)- Radiation: R = hr (Ts-Tr)

Skin evaporation [W/m2]:- Moisture diffusion- Evaporation of sweating

Respiration [W/m2]:- Latent heat loss: Lr- Convective heat loss:

CrLr+Cr=1.72.10-5M(5876-Pa) + 0.0014M(34-Ta)Ts = Body’s mean surface temperature

Tr = Mean (radiant) temperature of surrounding surfaces

Resultant Temperature - Tres

h = hc + hr

Dry heat loss = Surface convection + Thermal radiation

hc Ta + hr Tr

hc + hr Tres =

In indoor spaces: hc ~ hr :

Calculating PMV

• When a body is out of equilibrium then thermal strain is experienced, as measured by L:

• The value of L can be used to calculate PMV as follows:

The complete equation set

Comfort Zone / Envelope

(7-Point ASHRAE Scale)

hot

warm

slightly warm

neutral (comfortable)

slightly cool

cool

cold

3

2

1

0

-1

-2

-3-3 -2 -1 0 1 2 3

Thermal Sensation

0

20

40

60

80

100

Per

cen

tag

e D

issa

tisf

ied

[%

]

Thermal Sensation Percentage of Dissatisfied

Comfort ‘zone’or ‘envelope’

Comfort Envelope (ASHRAE Std 55R)

o o

CIBSE: Recommended Design values for Tres (now Top is used)

Building Space Tres / °C

Hotel Bedrooms (luxury) Public rooms Entrance halls and foyers

24°C 21°C 18°C

Libraries Reading rooms (height >4m) Store rooms

20°C 15°C

Offices General

20°C

Schools and Colleges Classrooms

18°C

Swimming Baths Changing rooms Pool hall

22°C 26°C

Warehouses Working and packing spaces Storage space

16°C 13°C

Tres has been replaced in UK thermal comfort work by the operative temperature (Top).

ASHRAE (USA): acceptance of adaptation

• Applicable to buildings that are: naturally ventilated buildings, not mechanically cooled, have operative windows.

BS EN15251: UK and Europe

• Applicable to free-floating buildings PPD=20%

Operative temperature

Weighted running mean ambient temperature

Cat IIICat II

Cat I

BS EN15251: UK and Europe

Thermal comfort: an

important factor in satisfaction

and productivity assessment.

ISO 10551 measurement questionnaire

Q.1: How are you feeling at this precise moment?

Q.2: Do you find this . . . ?

Q.3: Please state how you would prefer to be now?

Q4. How do you judge this environment (local climate) on a personal level?

Clearly acceptable

Just acceptable

Just unacceptable

Clearly unacceptable

Q5. Please state your personal tolerance of this environment. In your opinion. Is it . . .?

POE: Comfort and productivity

• Thermal comfort ranked most important for productivity in all building types.

• Perceived productivity increases with better thermal comfort in all building types.

Source: Leaman and Bordass (2007) Are users more tolerant of green buildings, BR&I, Vol. 35, no. 6, pp 662-673.

Summary• Thermal comfort is important for well-being

and productivity• Comfort depends on environmental factors

(Ta, Tr, Va, Rh)• Comfort depends on personal factors (Met,

Clo)• Guidelines for indoor comfort are now based

on Top (which replaces Tres) or comfort envelopes