ASHRAE Journal - February 2015 - 30

TECHNICAL FEATURE | Fundamentals at Work

Y
80%
60%
40%

0°F

3.6°F

7.2°F

10.8°F

14.4°F

18°F

0°F

Y
80%
60%
40%

20%

27°F

1

10%
8%
6%
4%

2%

18°F

36°F

45°F

54°F

63°F

2

20%

10%
8%
6%
4%

9°F

2%
X

1%
X = Air Temperature Difference Between Head and Feet

3

4

1%

Y = Dissatisfied

X
X = Radiant Temperature Asymmetry
2 = Cool Wall

FIGURE 3 Local thermal discomfort caused by vertical air temperature difference.6

Y = Dissatisfied
3 = Cool Ceiling

1 = Warm Ceiling
4 = Warm Wall

FIGURE 4 Local thermal discomfort caused by radiant temperature asymmetry.6

Where
ta = air temperature °F (°C)
tr = mean radiant temperature °F (°C)
The occupants can maintain the same comfort level
with a lower air temperature in a radiantly heated
space, and the same comfort level with a higher air
temperature in a radiantly cooled space in comparison
to convective heating and cooling systems. Therefore,
reduction of the energy loss due to ventilation and infiltration is possible while maintaining the same comfort
level compared with conventional heating and cooling
systems.
As the reference temperature for the transmission heat
loss is closer to the operative temperature than to the air
temperature, there will not be any significant difference
of transmission heat loss between radiantly heated or
cooled spaces.
Interestingly enough, the difference between air- and
mean radiant temperature is normally smaller in radiantly heated or cooled spaces. This is due to the fact
that in winter the windows will have a lower surface
temperature than the air temperature, which is compensated by a higher surface temperature of the radiant
30

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-1

80

-0.3
-0.2
50.4°F

60

0.0

40

41.4°F

30

40
50
60
Relative Humidity (%)

0.1

32.4°F

20
0
20

-0.1

Acceptability

to = 0.5(ta + tr)

100

Dissatisfied (%)

the simple average of air and mean radiant temperature
in spaces with low air velocities (<0.2 m/s [39 fpm]), or
with a small difference between mean radiant temperature and air temperature (<4K, 7°F).
The operative temperature (to) is in spaces with low air
velocities determined from the following expression:

0.2
0.3
70

80

1.0

FIGURE 5 Human satisfaction with the IAQ depending on relative humidity and air

temperature.3

system and vice-versa in summer. With air systems the
colder window temperatures in winter will be compensated by a higher air temperature, which will result
in an air temperature higher than the mean radiant
temperature.
For rooms with sedentary and/or standing occupants,
the maximum permissible floor temperature for heating
is 29°C (84°F), and the minimum floor temperature for
cooling is 19°C (66°F). For spaces with occupants in bare
feet (bathrooms, swimming pools, dressing rooms, etc.),
the optimal floor temperature for comfort also depends
on the floor covering material.
For wall heating, a maximum surface temperature
range of 35°C to 45°C (95°F to 113°F) is recommended.
The maximum may depend on whether the occupants
ASHRAE Journal - February 2015

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