Hydraulic Radiant Cooling
William Ryan Parrish
BCN 6586
Spring 2006
Today’s cooling systems consume a significant amount of energy in forced all-air
systems. Radiant cooling has recently resurfaced as a potential remedy to this problem.
Since its inception over 100 years ago lack of research and public acceptance has led to
its low marketability outside of Europe and Canada. With energy issues gaining
popularity once again this method of cooling has grown over the past 30 years. This
paper gives an overview of the system today with primary concern lent to energy
advantages and condensation disadvantages.
To start with there are three types of heat transfer: conduction, convection, and
radiation. Conduction is the transfer of energy through two or more materials that are in
direct contact. Convection is the transfer of heat from a warmer body to a cooler body by
an intermediate fluid, such as air. Radiation is the emission of energy as electromagnetic
waves or as moving particles. The most common source of radiant transfers can be found
in heating where the heat source radiates its heat from one object to another without
heating the air in between. Thermal radiation is transferred due to the difference in
temperature to all bodies “seen” by the source that are at a lower temperature (TWA
Panel Systems, n.d.).
Radiant cooling works in the same manner as heating, but instead of the cooling
panels emitting electromagnetic energy they absorb the radiating heat from the room’s
inhabitants. Many times the cooling system is located in ceiling panels. The difference
between the temperatures in the panels and the room causes the energy transfer thus
cooling the room. Conventional methods work with the principle of convection where air
is blown over coils and cooled before it is distributed into a space. With radiant cooling
both convection and radiation are used in combination. Radiation can represent as much
as fifty five percent of the transfer while convection accounts for the rest. The
convection occurs due to the cooling of the air in contact with the panels. After the air is
cooled it begins its natural pattern of circulation as warmer air rises in the controlled
space (Hydraulic Radiant Cooling, n.d.).
Hydraulic radiant cooling systems have many proven benefits, but with recent
focus on energy conservation, energy savings have been of primary concern. First, water
is a better cooling transport agent than air. Air requires nearly 1000 times more volume
to carry the same amount of cool. The energy needed to transport this volume of water is
30% less than an equal forced air model. This is accomplished by reducing the motor HP
by 75%. This alone reduces the peak-power of the air-conditioning system by about 30
to 45% (PATH, Hydraulic Radiant Cooling, n.d.). With less HP the inhabitant notices
the additional benefit of noise reduction. The capillary tubes are virtually silent. Another
energy savings occurs as a result of the compression principles of the liquid filled system.
This is due to the physical properties of liquids verses gases. Air can be compressed
substantially more than liquid thus making its circulation more energy intensive. It takes
more energy to push air, a compressible gas, than water, a non-compressible liquid. A
unique feature to this cooling model is found in its ability to self regulate based on room
load. When there are no radiating bodies in the room there is no energy to transfer thus
energy saved.
Conventional HVAC systems require additional floor/ceiling space for ductwork.
Radiant panels, which can be added to existing structures or new construction, are flat
and can accommodate a wide variety of designs. These circulatory tubes can be applied
to concrete, plaster or drywall, either on the exterior or built into the wall/floor. Indoor
environmental quality (IEQ) is also improved. For one reason is that the space is cooled
evenly with the creation of even temperatures. Radiant cooling does not operate on the
standard notion that heat rises; heat does not rise, hot air rises. Better air quality is
achieved because 100% fresh air (with no draft) can be used eliminating contamination
that results from re-circulated air. Bringing in fresh air with no draft is important when
considering historical buildings, hospitals, and laboratories. Optimum conditions, such as
dry and arid environments, are best suited for allowing 100% fresh air changes. Hot and
humid environments require sealed building envelopes to avoid moisture intrusion
(Hydraulic Radiant Cooling, n.d.).
Humidity is an important factor in the decision whether to use radiant cooling
within a design and has been the greatest reason for its lack of use in these types of
environments. Over the past 30 years technology has improved the situation with more
advanced control systems. Condensation occurs when temperature reaches the dew
point. With proper control this can be avoided. When sensors read that the temperature
is reaching this limit, sensors modulate the water accordingly. Water temperature is not
changed; it is the amount of water in the system that is altered according to the desired
temperature. The system’s limiting factor lies in the temperature of the water and the
space’s dew point. Though controls may be able to avoid condensation, the temperature
has a limit that cannot be crossed without further modifying the indoor environment.
This modification, such as supplemental ventilation or dehumidification requires
increasing energy inputs. Hydraulic radiant cooling can be implemented in any type of
building, but is better suited for some. Radiant cooling requires careful control of its
environment. Residential housing is one type of built environment that may not benefit
as much as more controlled building types. This is due to air infiltration caused by leaks,
left open doors and windows, and the unpredictable nature of home occupancy. More
controllable indoor environments in hot and arid zones stand to benefit the most from this
form of cooling.
This method of indoor cooling has been recognized by LEED (Leadership in
Energy and Environmental Design). Credits that can be achieved are found under the
categories of water efficiency, energy and atmosphere, materials and resources, indoor
environmental quality, materials and resources, and LEED innovation credits (The Green
System, n.d.).
Hydraulic radiant cooling is best suited for dry climates and climates that do not
require drastic cooling, such as where it is currently popular, in Europe and Canada.
Costs currently range from $4.00 USD to $6.00 USD per square foot, at the time of this
paper (Solar Energy and Earth Energy, n.d.).
In summary hydraulic radiant cooling systems functions much like radiant
heating. The main exception is that the inhabitant of the space is the radiating heat
source and the cooling system acts as a sink, drawing the heat energy out of the room and
into the circulating liquid. After the liquid absorbs the heat it is transported back to a
chiller with the help of a small pump where the temperature is lowered once again.
Capillary tubes made of either plastic, rubber, or copper are used to transfer the water
throughout the system. Benefits for this type of cooling are numerous. Energy efficiency
is the most beneficial in relation to today’s market. Controllability is a primary factor in
the success of the radiant cooling system. Since fluctuating humidity and dew points can
lead to many problems, such as condensation and resulting mold, careful consideration
needs to be paid to the design of each system within differing climates and conditions.
Some of the disadvantages include cooling that is limited to the space’s surface area,
indoor rain, higher initial costs, and lag time in controlling the system (especially when
used in slabs or materials of high density). Though this system is gaining new respect
and seems to be starting its progression into the U. S. markets, it is more advantageous in
specific temperate zones. The ideal environment or these cooling systems are dry semiarid areas that have low fluctuating temperatures throughout the year. Though these
systems can be implemented in hot, humid, tropical regions, there needs to be careful
attention given to sophisticated control and sensor systems, and even then the benefits
may not out weigh the disadvantages when compared to today’s standard HVAC forced
air systems. Strategies such as turning off the system during periods when the building is
not occupied and additional ventilation can provide added effectiveness, but further
studies need to be made to determine whether these additional energy inputs offset the
savings. In this paper primary concern has been paid to energy consumption and costs.
There are many other advantages to this system that benefit the environment and when
compared to forced air systems the radiant cooling emerges as the best suited for today’s
situation. With further studies and research this form of cooling may become a
substantial force within the U.S. market.
Hydraulic Radiant Cooling (n.d.). Retrieved January 25, 2005, from
http://www.socalgas.com/construction/builders/Builders%20Resource%20Guide/Hydroni
c%20Radiant%20Cooling.htm
PATH, Hydraulic Radiant Cooling (n.d.). Retrieved January 26, 2005, from
http://www.toolbase.org/techinv/techDetails.aspx?technologyID=188
Radiant Cooling or Floor Slabs (n.d.). Retrieved January 24, 2005, from
http://www.greenengineer.com/ideas/radiant.htm
Radiant System as compared to Forced Air System (n.d.). Retrieved January 24, 2005,
from http://www.radiantcooling.org/radiant.htm
Solar Energy and Earth Energy (n.d.). Retrieved January 24, 2005, from
http://www.kalbay.com/radiantheating.html
The Green System (n.d.). Retrieved January 24, 2005, from
http://www.jebcom.com/Radiant-Cooling.html
TWA Panel Systems (n.d.). Retrieved January 24, 2005, from
http://www.twapanels.ca/faq.html