ASHRAE Journal - August 2020 - 61

COLUMN SOLVING PROBLEMS

In high-ceiling spaces such as storage buildings and
hangars, floor heating provides uniform temperature
distribution and provides heat where people are, he said.
Floor cooling is especially attractive in spaces with significant solar radiation (direct or diffuse) like atriums,
airports and entrance halls. TABS are attractive in commercial multistory buildings, like offices and museums,
where cooling is most important, he said. If cooling is a
significant part of the load, like in an office, a radiant ceiling panel system can be attractive, according to Olesen.

Energy Savings with Radiant Systems
A common claim is that because of the radiant effect,
it is possible to obtain the same comfort at a lower room
temperature and correspondingly save energy for heating. This is only partly true.
The room temperature for comfort is specified as operative temperature in ANSI/ASHRAE Standard 55, Thermal
Environmental Conditions for Human Occupancy, which is an
average of air and mean radiant temperature. However,
the operative temperature for comfort is independent
of the system type, so a radiant system and an air system
must be controlled at the same operative temperature. It
is possible to obtain the same operative temperature at a
lower air temperature for radiant systems, which could
then reduce the energy loss for ventilation.
Similar arguments exist for the summer cooling season. The transmission losses are, however, more related
to the operative temperature in the space.
The significant energy benefits are because of the
energy distribution by water instead of air and having
a water temperature close to room temperature, which
increases energy efficiency of supply and increases the
possibility of using renewable sources.

Acoustics
For TABS and radiant ceiling panel systems you often
have a hard ceiling surface that may result in some
acoustic problems. Other surfaces in a space can be used
to improve acoustics like furniture, walls, floors, etc.
Studies also show that including free-hanging acoustical
batts (vertical or horizontal) will take care of the acoustics without reducing the heat exchange with the ceiling
surface significantly. On the other hand, with radiant
systems you do not need to circulate so much air, only
what is needed for ventilation, which reduces the risk of
noise from the air system.

Control of Different Types of Radiant Systems
For operation purposes, distinguishing between the
types of radiant systems is critical, said Olesen.
Embedded water-based systems, especially TABS, have
a significant amount of thermal mass in the systems.
However, as a radiant heating and cooling system uses
water temperatures close to the required room temperature, the system has a very significant "self-control," he
said.
The radiant surface reacts immediately when a room
temperature changes, whether the change is because
of a change in occupancy or solar heat gain. Olesen said
this is true for a TABS system operating with the surface
temperature of the ceiling slab being 1 K to 3 K (1.8°F to
5.4°F) to lower than room temperature. If there is a 1 K
(1.8°F) increase in room temperature, there will be an
increase in heat transfer to the slab (cooling) of 30% to
40%.
Olesen also recommended that engineers divide the
controls in zones in larger buildings, such as northsouth facing zones.
Embedded systems can control for changes in internal
loads and keep the room temperature within the comfort range. However, if a space needs to be controlled at
different room temperature levels during a day, these
systems are too slow because of the thermal mass.
Olesen said radiant ceiling panel systems that hang
from the ceiling are controlled like other heating and
cooling systems, even if the operators want to change
temperature levels throughout the day. For cooling, it is
important that the control makes sure no condensation
occurs on the cooled surface.
"This can be avoided by limiting the supply water
temperature according to the dew point in the space,"
he said. "The surface temperature will then always be a
couple of degrees higher than the dew point."
Olesen said it is also possible to avoid condensation
with an air system that uses dehumidification control.

Load Limitations
Some radiant systems' heating and cooling capacities
might not always be able to cover the total loads and
need to be combined with a second system such as an air
system, said Olesen. Not only can this pairing cover the
total load, air systems are needed to provide ventilation
for good indoor air quality. Radiant systems work very
well with chilled beams in high-load spaces. The two
AU G UST 2020

ashrae.org

ASHRAE JOURNAL

https://www.ashrae.org/

ASHRAE Journal - August 2020

Table of Contents for the Digital Edition of ASHRAE Journal - August 2020