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Radiant Cooling - Hydronic

Keeping homes cool with chilled water rather than chilled air.

Diagram of suspended ceiling panel radiant cooling system.

Radiant cooling systems rely on chilled water pipes to distribute cooling throughout a building rather than a conventional system that uses chilled air and ductwork. Radiant cooling systems rely mainly on the direct cooling of occupants by radiative heat transfer (heat transfer through space like the sun’s warming effect on an object in its direct path) because the pipes, which are commonly run through ceilings, maintain the surface at temperatures of about 65°F. Through radiative heat transfer, people in the room will emit heat that is absorbed by the radiant cooling surface. To manage indoor humidity levels and air quality, a separate ventilation system to supply fresh air is needed. Due to the ease of controlling water flow, independent control of areas of the home is relatively simple.

There are at least three methods of delivering chilled water in radiant cooling systems. The panel system is the most common of these. Aluminum panels that carry tubing can be surface mounted or embedded on floors, walls, or ceilings. Another, the capillary tube system, consists of a mat of small, closely spaced tubes that are embedded in plastic, gypsum, or plaster on walls and ceilings. Or, similar to hydronic heating systems, a concrete core with embedded tubes can provide the conduit and thermal storage capacity for a cooling system. In each of these systems, the water is mixed with glycol and cooled by an air-to-water heat pump, a cooling tower, a ground-source heat pump, or even well water. Because the radiative surface is typically a whole floor or ceiling surface, the water can be as warm as 65ºF and still provide comfort.

Energy Efficiency

Water movement via pressurized pipe uses less energy than blower fan delivery of air. Energy losses in ductwork are eliminated.

Kind of difficult

With professionally configured and installed units, a radiant cooling system can be used in any climate. However, because of the risk of condensation in very humid areas, proper design and installation is critical. These systems are most economical in desert climates such as Arizona where the humidity is less low. 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 upon start-up. Once room surfaces reach a desired temperature, however, they tend to maintain that temperature with relatively little extra energy.

System cost will vary with climate conditions. In climates that require dehumidification, the controls and equipment necessary to effectively cool with hydronics will be more costly than a conventional central forced air system.

The preferred installation is ceiling mounted, as this reduces air stratification and facilitates collection of condensation. Radiant cooling should provide a comfortable indoor environment using less energy than conventional forced air systems.

A new and seldom-used cooling method, hydronic radiant cooling is not prescribed in the codes like radiant heating.

Not Applicable

The most efficient installation will be ceiling mounted. The piping network is tied into a heat source such as a heat pump. Coordination among mechanical and finish trades is more complex for a radiant cooling installation than with forced air cooling. Panel and capillary systems can be compromised by trades people or homeowners penetrating the hydronic supply with fasteners.

Not Applicable

Due to water’s superior ability to store and transport energy, thermal energy can be transported in water through pipes with little pump power, saving approximately 70 to 80% of the energy used by fans to deliver conditioned air in a building. This alone can reduce the peak-power usage of the air conditioning system by 30 to 45%. Cooling can be more comfortable and zoned cooling is relatively simple with hydronic systems.

In many climates, dehumidification is an important aspect of air conditioning. Radiant cooling systems require exacting design and, possibly, added equipment and controls to assure that indoor environments are comfortable and free from excess moisture that can result in surface condensation.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.