ASHRAE Journal - February 2015 - 34

TECHNICAL FEATURE | Fundamentals at Work

qup =112.6 Btu/h

Structure S4
q=0

q=0

Material

Temperature (°F)
78.8

Floor Covering
λ = 0.08 Btu·in/h·ft2·°F
s = 0.6 in.

77

Screed
λ = 0.40 Btu·in/h·ft2·°F
s = 2.4 in.

73.4

75.2

71.6

Thermal Insulation
λ = 0.01 Btu·in/h·ft2·°F
s = 1.2 in.

69.8
68

Concrete
λ = 0.70 Btu·in/h·ft2·°F
s = 7.1 in.

66.2
64.4
qdown =22.3 Btu/h

FIGURE 7 Temperature distribution and cooling effect up and down for a floor system calculated by FEM software for a floor cooling system with 19°C (66.2°F) water tem-

peratures and 26°C (78.8°F) room temperature.

regardless of the embedded radiant system type.
Therefore, it is possible to establish basic formulas or
characteristic curves of heating and cooling for the all
heating and cooling surfaces independent of the embedded system types. The heat transfer between the surface
and the space do, however, depend on the different surface heat transfer coefficients (Figure 2).
The heat transfer between the water and surface is different for each system configuration. Therefore, the estimation of heating/cooling capacity of systems is very important
for the proper system design. Two calculation methods
7
included in ISO 11855-2 are simplified calculation methods depending on the type of system, and finite element
method (FEM) or finite difference method (FDM). Given
system types are Types A and C, Type B, Types D and G.
The simplified calculation methods are specific for the
given system types within boundary conditions. Based
on the calculated average surface temperature at given
heat transfer medium temperature and space temperature, it is possible to determine the steady state heating
and cooling capacity. In case a simplified calculation
method is not applicable for the considered system,
either two- or three-dimensional finite element or finite
difference method, or laboratory testing may be applied.
The temperature distribution in floor cooling system,
calculated using FEM software, is shown in Figure 7.
Heat exchange coefficient is the parameter that
determines the amount of heat transferred between
34

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surface and the space in relation with the system type.
Acceptable surface temperature is determined based on
comfort considerations and the risk of condensation.
7
Heating/cooling capacity of the systems is:
Floor heating and ceiling cooling, q = 8.92 (to − tS,m )1.1;
Wall heating and wall cooling, q = 8 (|to − tS,m |);
Ceiling heating, q = 6 (|to − tS,m |);
Floor cooling, q = 7 (|to − tS,m |).
Where
to (°C) is the operative temperature in the space
tS,m (°C) is the average surface temperature
The ceiling has the capacity up to 100 W/m² (31.7
Btu/h·ft²) for sensible cooling and 40 to 50 W/m² (12.7
to 15.9 Btu/h·ft²) for heating. The floor has the capacity up to 100 W/m² (31.7 Btu/h·ft²) for heating and
40 W/m² (12.7 Btu/h·ft²) for sensible cooling. When
direct sunlight strikes on the floor, the sensible cooling
capacity of the floor may be more than 100 W/m² (31.7
Btu/h·ft²). This is why floor cooling is often adopted in
spaces with large window area like airports, atria and
lobby halls.
For the thermally active building systems (TABS), the
steady-state heating/cooling capacity calculation is not
sufficient, and analysis with a dynamic computational
program that can predict the dynamic behavior and
performance of the system together with the building
ASHRAE Journal - February 2015

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