ASHRAE Journal - February 2015 - 29

TECHNICAL FEATURE

Floor

Ceiling

Wall

TABS

FIGURE 1 Examples of water based radiant systems.3

The radiant system is defined as a system where at
least 50% of the heat transfer takes place by radiation.
Figure 2 shows the total heat transfer coefficients between
a heated-cooled surface and a room. The radiant heat
transfer is, in all cases, 5.5 W/m²·K (0.97 Btu/h·ft²·°F).
The convective heat transfer then varies between 0.5
and 5.5 W/m²·K (0.09 and 0.97 Btu/h·ft²·°F), depending on the surface type and on heating or cooling mode.
This shows that the radiant heat transfer varies between
50% and 90% of the total heat transfer. The heat transfer coefficient for cold ceiling and warm floor will vary
between 9 and 11 W/m²·K (1.59 and 1.94 Btu/h·ft²·°F),
depending on the temperature difference between surface and room.
The radiant heat transfer does not directly affect the
room air temperature. The long wave radiation heats or
cools the surrounding surfaces, which then indirectly
heats or cools the room air.

Standard for Radiant Heating and Cooling Systems
As the heat transfer process between water and room
is quite different from conventional air systems, an
international standard on radiant heating and cooling
systems has been developed based on system design and
existing standards from different countries and was
published in 2012.
ISO 11855, Building Environment Design-Design,
Dimensioning, Installation and Control of the Embedded Radiant
6-11
Heating And Cooling Systems, consists of six parts:
Part 1: Definition, symbols, and comfort criteria;
Part 2: Determination of the design and heating and
cooling capacity;
Part 3: Design and dimensioning;
Part 4: Dimensioning and calculation of the dynamic
heating and cooling capacity of thermo active building
systems;

Btu/h·ft2·°F
2.03
1.85
1.67
1.50
1.32
1.14
0.97
Heating

1.94

1.94

1.41
1.41

1.06
1.23
Cooling

Floor

Ceiling

Wall

FIGURE 2 Heat transfer coefficients between heated/cooled surface and room.4,5

Part 5: Installation; and
Part 6: Control.

Comfort
Occupants' thermal comfort is the primary objective in radiantly heated or cooled space. To provide an
acceptable thermal environment for the occupants, the
requirements for general thermal comfort shall be taken
into account by using the index of predicted mean vote
(PMV) or operative temperature, to, and local thermal
comfort, e.g., surface temperature, vertical air temperature differences, radiant temperature asymmetry, draft,
etc.
For radiant or convective systems the comfort requirements are the same when expressed by the PMV-PPD
index (-0.5 < PMV <+0.5) or expressed as an operative
temperature range corresponding to: 20°C to 24°C (68°F
to 75.2°F) for heating season and 23°C to 26°C (73.4°F
to 78.8°F) for cooling season in spaces with sedentary
12,13
activity.
14,15
The operative temperature is the combined influence of air temperature and mean radiant temperature.
The operative temperature can be approximated with
FEBRUARY 2015

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ASHRAE Journal - February 2015

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