Hydronic Radiant Cooling
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Summary
Comfort relies as much
on radiant heat transfer as it does on air temperature. Conventional air
conditioning units are only designed to control air temperature. Because
radiant energy travels through space without cooling the air itself but rather
objects, hydronic radiant cooling systems seperate ventilation from thermal
conditioning, providing fresh air and conditioning independent from each other.
Radiant cooling is a sophisticated approach to comfortable, high-efficiency
cooling.
Details
The human body
transfers heat in three different ways: radiation, convection, and evaporation.
Thermal comfort is when these three factors are in thermal neutrality. Ideally
50% of body heat is cooled by radiation, 30% by convection and 20% by
evaporation (sweat and breathing through the mouth).With an ideally designed
conditioning system,
the occupant would not know if it is cooling or heating.
Radiant cooling falls
in the "air-and-water" category where the outside air ventilation
system provides only the necessary fresh air while the hydronic thermal
distribution system with a radiant heat exchanger provides the cooling.
Therefore radiant cooling or heating systems address both, the convection and
radiation factor of body heat whereas conventional air conditioning mainly
address the convection factor by
high air circulation.
There are currently
three different hydronic radiant cooling systems available. The panel system is
the most common radiant system. The panels, usually aluminum, can be surface
mounted or embedded on floors, walls or ceilings. The capillary tube system
characteristics are small closely spaced tubes that are embedded in plastic,
gypsum, or mounted on ceiling panels. The many tubes allow for better heat
absorbing distribution and a thinner ceiling. And finally, the concrete core
conditioning systems with tubes embedded in a concrete slab allows for peak
load shifting because of its thermal storage capacity. This could be used also
as a wall mass. In each of these systems, the water is mixed in a glycol
solution and cooled by an air-to-water heat pump, a cooling tower, a
ground-source heat pump, or even well water. Since the radiative surface is
typically a whole floor or ceiling surface, the water can be as warm as 65º.
Installation
The preferred
installation is ceiling mounted, as this reduces air stratification,
facilitates collection of condensate, and involves less structure. The piping
network is tied into a heat transfer source such as a heat pump. The transfer
medium can also cool the coil for an air handler to provide air movement.
Benefits/Costs
Due to the high heat
capacity and density of water, thermal energy can be transported in water in
pipes with little pump power, saving approximately 70 to 80% of fan power
normally used to condition a building. This alone reduces the peak-power of the
air-conditioning system by about 30 to 45%. There is no space conditioning
equipment necessary on exterior walls, and the panel system can use the automatic
sprinkler system piping in some multi-family dwellings. Radiant cooling panels
with
minimum fresh air
supply is a draft and free system that also eliminates fan-coil and induction
unit noise. Cooling is more evenly distributed, and drafts can be eliminated.
There is less exfiltration of cooled air when doors or windows are opened.
Prices vary
extensively depending on climate, existing conditions and system. According to
distributors, amortization within 5-7 years is common.
Limitations
With professionally
configured and installed units the system can be used in any climate, however
because of the risk of condensation in very humid areas proper design and
installation may be even more critical. These systems are most economical in
areas like Arizona where the humidity is less than 15%. A forced air system,
such as an air handler or dedicated fresh air system, may need to be
incorporated to remove humidity and pollen. Some systems have a relatively slow
system response. Once room surfaces reach a desired temperature, however, they
tend to maintain that temperature with relatively little extra energy.
Code/Regulatory
A new and seldom-used
technology, radiant cooling is not regulated in the codes like radiant heating.
Availability
Since radiant cooling systems
are constructed of components from the radiant heating and air conditioning
industries, components are widely available, but engineers experienced in
residential radiant cooling design are not.
Contact(s)
If you have any
specific questions about this technology and/or its applications please contact
the following resources.
Radiant Panel
Association
P.O. Box 327
6205 S. 1800 W.
Hyrum UT 84319
970-613-0100
Hayward Gieraud
Northwest Energy
7500 212th St. SW Ste.
115
Edmonds WA 98026
425-778-7277
EHT, Inc.
714-766-9197
Burnham Corporation
717-397-4701
Fact Sheet Source
The material provided
in this section is used with permission by the National Association of Home
Builders Research Center and the Partnership for Advancing Technology in
Housing (PATH).
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