Radiant Cooling - Cold Enough For The 21st Century?
What is radiant cooling? It works on the same principals as radiant heating — the primary principal being comfort. The Radiant Panel Association defines a truly comfortable environment as one “designed to draw heat away from our bodies at precisely the correct rate.” When we lose heat too fast, we’re cold; too slow, we’re hot. We are always producing heat. Even at sleep we generate an average of 400 btu’s per hour. About half of that heat is lost to cooler objects in the room. When we stand we produce about 500 btu’s per hour and running results in 1,800. With radiant heating we lose less heat because the objects in the room are the same temperature as our bodies — therefore we’re comfortable. Radiant cooling allows our bodies to lose more heat, much in the same way we do when we walk down the frozen food section in a grocery store. By providing a cool surface, all other surfaces, including our bodies, will give up heat to the panel.
Radiant energy travels in any direction and does not convert to heat until it strikes an object. In radiant cooling, our bodies are the primary heat panel. Therefore the objective of a system is to keep room surfaces cooler than body temperature. When this is achieved, a person will not be aware that the environment is being heated or cooled — the measure of true comfort. ASHRAE lists the following as some of the principal advantages of radiant systems used to achieve this comfort:
- Comfort levels are better because radiant loads are treated directly and air motion in the space is at normal ventilation levels.
- Space-conditioning equipment is not needed at the outside walls, simplifying the wall, floor and structural systems.
- No space is required within the air-conditioned room for mechanical equipment.
- Cooling and heating can be simultaneous, without central zoning or seasonal changeover, when four-pipe systems are used.
- System provides a draft-free environment.
- The panel system can use the automatic sprinkler system piping — see NFPA Standard 13, Chapter 3, Section 3.6.
Panel Options: Ceiling, wall, and floor panels can all be used for cooling. Thus answering that question every radiant heat specialist faces — “But what about air conditioning?” The question now is whether or not they can do it in an efficient and cost effective manner.
Studies have shown that hydronic radiant cooling systems can remove a given amount of thermal energy using less than 5 percent of the fan energy that would otherwise be necessary. It also takes up about 20 percent of the space requirements of forced air systems. But there are problems requiring additional equipment that can send cost and energy requirements soaring.
“Ceiling panels work much better than floor panels for radiant cooling,” says Larry Drake, executive director of the Radiant Panel Association. “The capacity of the floor for cooling is about half of what it is for heating because cold air falls and with the falling, connective patterns occur that help increase cooling capacity.” A radiant cooling ceiling panel absorbs thermal heat and works at a higher operating temperature while the occupants perceive a lower temperature. But for residential applications, ceiling panels, which are often large and require expensive controls, are impractical.
The primary problem with using ceiling, wall or floor radiant panels for residential cooling is condensation on the panel in humid climates. “There are two things you must remember in radiant cooling,” says Drake. “First, the system must be set at a steady state. You can’t turn the system off during the day then expect it to be immediately cool when you click on the thermostat. Secondly, you must make sure that the controls you chose can handle the humidity so the panel does not hit the dew point, causing condensation.”
The panel surface temperature must always be kept above the dew point, a problem in warm climates such as Clay Thornton’s, (Thornton Plumbing & Heating, Sandy, UT). “Radiant cooling is a good idea and certainly I’d like to do more of it, but we found out when it gets hot outside, the radiant just didn’t quite do the job,” said Thornton. “You can only cool those floors down so much without getting condensation and the only way you can get the humidity down is through an air handler of some sort.” The addition of a dehumidification element of course drives up the price of the system — a definite deterrent to consumers who can often have air conditioning thrown in for free from the furnace installer.
A Promising Ad-On: One cost-effective solution available to installers working in the West is the Indirect/Direct Evaporative Cooler from Davis Energy Group, Inc., Davis, CA. “The IDEC system allows installers to be directly competitive with conventional air conditioning systems,” says Morgan Muir, president of HSC Hydronic Specialties Co., Berkeley, CA, the largest supplier of hydronics in the West. Developed through a research contract from the California Energy Commission’s Energy Technologies Advancement Program, the IDEC system was designed to provide a low-cost alternative to conventional vapor-compression cooling technologies for residential and small commercial applications.
The prototype is a two-stage evaporative cooler that uses indirect and direct heat exchange stages to cool air with less added moisture compared to direct evaporative coolers. The indirect section cools air in a heat exchanger, which transfers heat without adding moisture to the supply airstream. The direct section cools supply air by direct contact with water, either on extended wetted-surface material or with sprays. Compatible with hydronic radiant floor heating, IDEC adds less moisture to delivered air than direct evaporative coolers, thereby eliminating humidity-related problems that can destroy wood floors and walls. The unit runs about $2,500 for a 1,500 to 3,500 square foot home — but for that price you don’t have to deal with refrigerants, condensing units or ducting.
“IDEC is designed to work in areas with relative humidity below 60 percent, so we’re not talking about St. Louis or Baltimore here,” says Muir. “It all depends on what you are trying to accomplish, in what climate zone and what application. If you are not in the West and you’re set on radiant cooling, there are different things you can do like install miniduct air handling systems and straight air handlers, but that will again, add cost.”
Cold Hard Cost: Even though interest in radiant cooling is building, it has far to go before it can even face the types of problems radiant heating is dealing with today. Radiant heating has been proven to be more comfortable and cost-efficient than forced air systems, and now faces a public relations battle. Radiant cooling, on the other hand, demands a combination of systems, often resulting in increased cost and lower efficiency. But there is hope. With developments like IDEC, the creation of materials like desiccants (materials which can absorb a great deal of moisture), and the exploration of the residential market by manufacturers, radiant cooling could be cool enough for the 21st century.
For more information on radiant cooling contact the Radiant Panel Association at 801/245-0128.