ASHRAE Journal - February 2015 - 32

TECHNICAL FEATURE | Fundamentals at Work

Type A and C

Type B

2
5
3

Type G

Type D

1 = Floor Covering
3 = Thermal Insulation

Flooring Material

Joist

2 = Weight Bearing and Thermal Diffusion Layer (Cement Screed, Anhydrite Screed, Asphalt Screed or Wood)
4 = Structural Base
5 = Heat Diffusion Device

FIGURE 6 Embedded radiant system types.7

may easily get contact with the surface or whether occupants are more sensitive persons such as children or the
elderly. Wall cooling is limited by the risk of condensation and the development of a downdraft of cold air.
A vertical air temperature difference between head
and feet of less than 3K (5.4°F) is recommended. Most
heating and cooling systems will, in modern buildings,
normally have vertical air temperature differences
within this limit. In high ceiling spaces it is, for energy
reasons, important to avoid large vertical temperature
differences. This is why floor heating is especially recommended here (atrium, foyer, industrial space, etc.)
People are very sensitive to radiant temperature
asymmetry from a cold window and a warm ceiling.
Occupants may feel discomfort caused by a temperature
asymmetry of 5K (9°F) for warm ceiling, and a temperature asymmetry of 10K (18°F) for walls or windows (Figure
4, Page 30). The critical factor at cold surfaces (windows,
walls) is, however, the risk of downdraft that may cause
discomfort.
The radiant heating and cooling system operates with
less dust transportation, as it is not a convective system,
and does not require the cleaning of heat emitters or
filters. With the radiant floor heating systems, carpets
are not necessary. Thus, the possible allergen sources of
emitting pollutants and a sink source can be eliminated.
The higher mean radiant temperature in radiantly
heated space means that the air temperature can be
kept lower than in convectively heated space. This has
the advantage that the relative humidity in winter may
be a little higher. Studies show that lower air temperature and lower air humidity have a significant effect
3
on perceived air quality (Figure 5 ). Due to the higher
heating surface temperatures, there is less chance
32

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FEBRUARY 2015

for condensation and mold growth. The relationship
between air temperature and humidity is one of important comfort issues in radiantly cooled spaces. Where
the humidity is not controlled by the air system, as in
naturally ventilated spaces, radiant cooling capacity will
be limited to avoid the forming of condensation on the
radiant surface (see section on control in Part 2 of this
article in next month's Journal).
With air heating or cooling system more air has to
be circulated than the amount needed for providing
acceptable air quality. This may increase the noise level
in a space and also increase the risk for complaints
related to draft. When a part of sensible heating and/
or cooling load is taken care of by a water-based radiant system, the ventilation system may have reduced
duct size and lower air velocity because it will only
treat the air renewal for required IAQ and, if needed,
dehumidification.
In buildings with thermally active building systems
(TABS) you will normally prefer to have free access to
the concrete surface to increase the heat transfer with
the room. This may require special solutions for the
acoustics. Acoustic panels on the ceilings and suspended
ceiling panels will reduce heat transfer. It will be more
16
efficient to hang down vertical acoustical panels. The
application of the raised floor or the thermal/acoustic
insulation in floor will decrease the upper heat flow
from the TABS, which normally is much less than the
heat exchange from the ceiling.

Load Calculations and Heating/Cooling Capacity
At a given average surface temperature and indoor
temperature (operative temperature, to), a surface
will deliver the same amount of heat flux to a space

ASHRAE Journal - February 2015

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