ASHRAE Journal - December 2013 - 23

Technical FeaTure
Fea

a

18

radiant
air

18

radiant
air

15
Cooling rate (Btu/h· Ft 2 )

Cooling rate (Btu/h· Ft 2 )

15

B

12
9
6

12
9
6

3

3

0

0
5

10

hour

15

20

5

10

hour

15

20

Figure 2 Comparison of design day cooling rate profiles between air system and floor taBS (Figure 2a) and ceiling taBS (Figure 2B).

rate, representing the hourly sensible cooling load, reaches
a higher peak value for both the floor TABS (85% higher)
and ceiling TABS (49% higher). The higher peak cooling
load for the floor TABS is indicative of the acknowledged
increased cooling capacity of radiant floor cooling when
directly illuminated by solar radiation.3,11,12 Note that the
air system results, although similar, are slightly different
due to the location of the thermal mass for the two cases.
Summarizing other simulation results when solar
was not present, peak cooling rate differences between
chilled radiant ceiling systems of all three types and air
systems ranged from 12% to 35% higher for perimeter
zones and 7% to 27% higher for interior zones.10 Although
the results from this limited and simplified simulation
study indicate that peak cooling loads for radiant systems are higher than those for air systems, it is important to recognize that control and operational differences between the two systems may lead to lower cooling
loads, energy use and costs for radiant systems. For
example, a common control strategy for the thermally
massive TABS is to use nighttime pre-cooling to reduce
or eliminate active cooling during daytime hours.
To explain why radiant system peak zone cooling rates
are higher than those for the equivalent air systems, Figure
3 compares the heat transfer fundamentals for the two
systems for a typical case from the above simulation study.
The simulated case shown represents an interior zone
with adiabatic walls, floor, and ceiling, so that the only
cooling loads are the result of interior heat sources. The
figure shows how convective and radiative heat gains are
converted into zone cooling load for the air system (on the
left) and a radiant cooling panel system (on the right).
In this example, the total internal heat gain (4.8
Btu/h·ft2 [15 W/m2]) during occupied hours, 6 a.m. to 6

p.m.) was divided into convective heat gain (1.9 Btu/h·ft2
[6 W/m2]) and radiative heat gain (2.9 Btu/h·ft2 [9 W/m2]),
representing a typical 60% radiation factor. For the air
system, 100% of the convective heat gain instantaneously
becomes cooling load, while a large portion of radiative
gains are absorbed by zone thermal mass and released
after a time delay as convective load. The fact that building mass delays and dampens the instantaneous heat gain
is well recognized by cooling load calculation methods.
For the radiant system, not all convective gains instantaneously become cooling load. During occupied hours, part
of the convective heat gain contributes to a higher zone air
temperature which is reached to balance the cooler ceiling
surface temperature, thereby maintaining an equivalent
operative temperature in the zone. Because of the higher
zone air temperature, a small part of the convective heat
gain is absorbed by non-activated building mass and
removed by the radiant surface via longwave radiation.
As shown, a significantly larger portion of the radiative heat gain converts directly to cooling load during the
occupied period due to the presence of the actively cooled
surface. The bottom plots add up the two cooling load
components, and the solid black lines in the bottom plots
represent hourly cooling loads, which reach their peak
value at the end of the occupied period for both systems.
These predicted cooling load profiles display the total
amount of heat being removed by each system to maintain the same operative temperature profile. Note that for
this example, the peak cooling rate for the radiant system
is predicted to be 13% higher than that for the air system.
ExpErimEntal Study

An experimental study was undertaken to verify the
observation that sensible zone cooling loads for radiant
D ecem ber 2013

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ASHRAE Journal - December 2013

Table of Contents for the Digital Edition of ASHRAE Journal - December 2013
