The HVAC Factor: Radiant Cooling
By Dan Sullivan, LEED®AP, CEM
Originally published in the June 2010 issue of Today’s Facility Manager
Radiant heating systems have evolved significantly in North America over the past three decades. Radiant heat floors started out with humble beginnings, being installed in small garages and bathrooms. Now the technology has grown into mainstream specifications for residential, commercial, and industrial projects.
The latest step in this evolutionary process is radiant cooling, which appeared on the radar in Europe in the early to mid-1990s. By early 2000, North American engineers were slowly beginning to use this technology in their designs as well. Today, radiant cooling is a hot topic of discussion among commercial design engineers and architects, as these systems are increasingly being used in LEED (Leadership in Energy & Environmental Design) projects to save energy and deliver superior thermal comfort to building occupants.
For example, the new National Renewable Energy Laboratories Research Support Facilities in Golden, CO is a LEED Platinum project and one of the first large-scale, net-zero buildings in North America. It features radiant slab heating and cooling, plus other integrated mechanical design and architectural approaches, such as (UFAD) Under Floor Air Distribution Systems ventilation and natural daylighting.
Another LEED Platinum facility is the newly rebuilt California Academy of Sciences, which opened in October 2008. One aspect of the 410,000 square foot facility is a radiant floor heating/cooling system that provides comfort to 38,000 square feet of the main exhibition level (pictured above). This radiant system consists of 100,000 linear feet of tubing connected to six 20,000 BTU/hour boilers and a trio of four ton chillers.
To better understand radiant cooling—what it is and how it works—an overview of the technology is presented here.
What is radiant cooling? Radiant cooling systems circulate chilled (commonly 55°F to 58°F) water or a glycol-aqueous fluid through PEX (crosslinked polyethylene) tubing embedded in floors, walls, and ceilings or panels placed on walls or ceilings. This chilled fluid draws excess heat from the structure and sends it to a chiller or ground source system for exchange. Its operation is similar to radiant heat, only in reverse.
Some common misconceptions about heat transfer are that heat rises and that floors can be used only for heating. The fact is that hot air rises—not heat. According to the second law of thermodynamics, heat passes from a hot surface to a cold surface. Consequently, the floor can also be used for radiant cooling by absorbing heat from the surrounding surfaces in the structure.
Can radiant cooling supply all cooling needs of a building? These systems are not designed to be standalone. Rather, they work in conjunction with an HVAC system. Among the advantages of radiant cooling are the efficiencies it provides by allowing designers to downsize the HVAC package in a building.
It is important to note that cooling loads consist of three components: sensible (dry heat), latent (moisture), and solar. The radiant cooling system can only affect the sensible and solar portions of the cooling load; the HVAC system must address the latent portion.
How is the humidity in the building controlled? Isn’t there a problem with condensation on the cooled surfaces? As noted above, the humidity is in the latent portion of the cooling load. Commonly referred to as the wet bulb load or gain, humidity in a building is controlled through the HVAC system in one of two ways:
- through mechanical refrigeration: sub-cooling and then reheating the air; or
- through a desiccant absorption system that absorbs water from the air and then must be periodically regenerated, similar to the main operating principles of a water softener system.
Beyond the latent load, the ventilation system also addresses the balance of the remaining sensible load, if any exists. It also controls the level of humidity within the air system and meets the requirements of the indoor air quality (IAQ) standards for fresh air.
Relative humidity (Rh) and temperature determine the dew point within the space. To avoid condensation issues in a building, the controls in a radiant cooling system should monitor the dew point and adjust fluid temperatures.
What kind of floor coverings can be used with radiant cooling? Highly conductive solid surfaces like tile, concrete, or slate are recommended. Thick carpet and pad or similar soft rug surfaces are not recommended and should be avoided. Light carpeting may be used, although it will add an insulating effect that can reduce the cooling capacity.
Can radiant cooling be used only in concrete installations? There are two primary construction types—high mass and low mass. Both are hydronic based (that is, they use water as the heat-exchange medium) and circulate chilled fluid to draw excess heat from the structure.
High mass constructions incorporate embedded PEX tubing within the concrete floor or wall. This method is common in hybrid systems that both cool and heat a structure. It is also an excellent application for passive control strategies.
The low mass technique uses surface mounted ceiling or wall panels that react fairly quickly to temperature change, with little or no residual energy left in the panel. These panels are ideal for retrofit applications, because low mass systems must be used with active control strategies to gain maximum benefit.
What are passive and active control strategies? With a passive strategy, the mass in the structure is cooled during the evening and night, usually taking advantage of off-peak energy. The mass is then allowed to absorb energy slowly throughout the next day until cooled again the following night.
With an active strategy, the panels are in active play when there is a call for cooling in the structure. In this scenario, pumps will continuously circulate chilled water through the panels. The water temperature is actively controlled to the appropriate supply water temperature set point via a mixing valve or an alternative equivalent mixing design.
How much energy can one anticipate the radiant cooling system to support? The maximum capacity of a radiant floor cooling system is approximately 14 Btu/hours per square foot with a design set point of 78°F. In floor areas with absorption of shortwave radiation (e.g., solar, lights), the cooling capacity can be as high as approximately 25 Btu/hours per square foot. The capacity can vary, depending on the floor construction employed and the system fluid temperatures used at the time.
Are radiant systems easy to commission and maintain? A radiant system that is properly designed and constructed will require maintenance similar to other types of hydronic systems. Radiant systems can be balanced using a balancing valve for each zone that is configured. This is highly recommended, so each zone can be adjusted for changes in load.
Unlike air systems, radiant systems do not require filtration or ductwork that might leak. In addition, radiant systems are not affected by any static pressure variations that can cause an air system to become unbalanced over time. This means that, once the system is commissioned, its performance will remain consistent over time.
Sullivan (Daniel.Sullivan@uponor.com) is senior product manager of commercial heating and cooling for Uponor, located in Apple Valley, MN. With more than eight years of product management experience in HVAC and radiant systems design, he is LEED® AP certified, a Certified Energy Manager (CEM) with the Association of Energy Engineers, and a professional member of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).
What experience do you have with radiant systems? What maintenance highlights or challenges have you encountered? To discuss your experiences with other facility managers, visit FacilityBlog at facilityexecutive.com/facilityblog.