Radiant — The hottest topic in cooling

 

BY GARY FRIES,

contributing writer

 

It’s amazing when you sit back and take a look at how radiant systems have evolved in North America over the past three decades. Radiant floors started with a humble beginning in small garages and bathrooms. Now the technology has grown into mainstream specifications for residential, commercial and industrial projects.

 

The latest step in the evolutionary process is radiant cooling. Radiant cooling started to show up on the radar screen in Europe in the early to mid-1990s. By early 2000, North American engineers were becoming interested and slowly beginning to use radiant cooling in their designs as well. Today, radiant cooling is the hottest topic of discussion among commercial design engineers.

 

What is radiant cooling?

 

Radiant cooling systems circulate chilled (commonly 55°F to 58°F) water or glycol-aqueous fluid through tubing embedded in the floors and walls or through panels positioned on the walls or ceilings. This chilled fluid draws the 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.

 

Can radiant cooling supply all the cooling needs of a building?

 

Radiant cooling systems are not designed to be standalone systems. Rather, they work in conjunction with an HVAC system. One of the advantages of using radiant cooling is the efficiencies it provides by allowing the designer to downsize the HVAC system.

 

Cooling loads consist of three components: sensible, latent 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?

 

The humidity in a building is controlled through the HVAC system. Humidity is in the latent portion of the cooling load. It is commonly referred to as the wet bulb load or gain. Radiant cooling systems cannot address the latent load.

 

The ventilation system addresses not only the latent load, but also the balance of the 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. In most cases, an RH of 45% is sufficient to prevent condensation on the cooling surface. In a radiant cooling system, the controls for the radiant system should monitor the dew point in the building and adjust fluid temperatures to avoid condensation issues.

 

What kind of floor coverings can be used with radiant cooling?

 

Highly conductive solid surfaces like tile, concrete or slate are recommended for use with radiant cooling systems. Using thick carpet and pad, or similar soft-rug-type surfaces is not recommended.

 

Can radiant cooling be used only in concrete installations?

 

There are two primary construction types for radiant cooling systems: high-mass and low-mass. Both are hydronic-based and circulate chilled fluid to draw the excess heat from the structure.

 

High-mass constructions incorporate embedded tubing within the concrete floor or wall of the structure. 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 slowly absorb energy throughout the next day until it is cooled once again that following night.

 

With an active strategy, the panels are in active play when there is a call for cooling in the structure.

 

How much energy can I anticipate the radiant cooling system to support?

 

The maximum capacity of a radiant floor cooling system is approximately 14 Btu per hour per square foot when using a design set-point temperature of 78°F. In floor areas with absorption of short-wave radiation (i.e., solar, lights, etc.), the cooling capacity can be as high as approximately 25 to 32 Btu/h/ft2. The capacity can vary depending on the floor construction employed and the system fluid temperatures used at the time

 

Gary Fries is the associate product manager of Commercial Heating and Cooling for Uponor. He holds over two decades of experience in the design and installation of radiant systems, and is a professional member of the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE). He can be reached at Gary.Fries@uponor-usa.com.