Energy Source Builder

Research Supports Benefits of Radiant Ceiling Panels

Researchers at the NAHB Research Center found that heating a home with ceiling-mounted radiant panels produced energy savings of 33 percent compared to a heat pump and 52 percent compared to baseboard heaters. The research project, completed in May 1994, was sponsored by the U.S. Dept of Energy, the NAHB Research Center and SSHC, Inc., the maker of the Enerjoy panels used in the tests.

Enerjoy HeatModule
The heaters used in this study were surface-mounted, low mass panels.

A field test was conducted in a house equipped with the electric panels and a two-zone heat pump system. Two employees of NAHB Research Center lived in the house. The two heating systems were operated at different times over the course of the same heating season. Data for electric baseboards came from a previous test in the same house.

Thermal Comfort

Thermal comfort depends on more than air temperature. The temperature of surrounding surfaces also comes into play. You may feel chilled sitting near a cold window even if the air temperature is in the comfort range.

Averaging the temperatures of the air and surrounding surfaces gives the "operative temperature," which is a better indicator of comfort than air temperature. During cold weather, people may feel uncomfortable even when the thermometer says 70 degrees F. When that happens, they often turn up the thermostat setting, and there it stays. On the other hand, warm surfaces can maintain comfort even when air temperature is lower.

Radiant Savings

Radiant heaters are unique because they can maintain comfort at a lower air temperature. The Enerjoy panels operate at 150-170 degrees F. Heat radiates from the panels to objects and people in the room. Eventually, heat migrates from objects to room air, and the air temperature stabilizes.

The Enerjoy panels differ from other types of radiant heaters in several ways. First, they mount to the ceiling surface, not behind or inside gypsum board. Second, the lightweight construction has little thermal mass that must come up to temperature. Third, the textured surface adheres directly to the heating element. These characteristics make the Enerjoy panels fast acting. The panel itself reaches operating temperature in only three to five minutes.

Marley Electric Heating (803-479-4006) makes the Aztec surface-mounted panel. This product is similar but is constructed a bit differently. Because the panels respond quickly, people can turn the heat on and off as they would the lights.

No Forced-Air Losses

Other benefits of radiant panels have more to do with avoiding problems inherent with forced-air systems:

 

Ducts - Heat loss from ducts, even insulated ones, reduces the efficiency of forced- air systems. Duct losses are eliminated in a radiant system.

 

Air leakage - A blower door test showed that sealing off heating registers and returns cut air leakage in the test house by 12.5 percent. Additional leakage is created by the internal air pressure generated by forced-air systems. Radiant panels don't suffer these problems.

 

Parasitic losses - Furnace blowers can use considerable energy that doesn't get delivered as heat.

 

Zoning - Energy can be saved by maintaining comfort temperatures only in the rooms occupied by people. Called zoning, this idea can be used with most types of heating systems. The controls needed to zone a forced-air system make it very expensive. However, zoning electric heaters (baseboards, wall heaters and panels) is inexpensive. Zoning allows each room to be adjusted to the level of physical activity. A person on an exercise bike in one room will want a different setting than a person reading in another room.

The quick recovery feature of Enerjoy panels promotes zoning, because many people don't mind turning back the thermostat, if they know it will recovery quickly. To reinforce the idea, SSHC recommends placing the thermostat next to the light switch.

Comfort Comparison

Thermal comfort is very subjective. The two occupants of the test house showed considerably different responses. Despite a few complaints of discomfort, both occupants said the level of thermal comfort was comparable between the heat pump and radiant panels.

Radiant panels distribute heat differently than forced-air systems. Most heat flows directly beneath the panel and falls off gradually with greater distance. The operative temperature drops about 5 degrees F over the first six feet.

People who expect even temperatures through the house might consider this a disadvantage. Occupants of the test house liked the idea. It allowed each person to find a comfortable spot in the same room, even though they had markedly different temperature preferences.

Operative temperature is highest directly below the radiant panel, but declines gradually with distance. Air temperature remains constant.

One discomfort complaint occurred when two people were sitting at the dining table directly beneath the panel when it cycled off. A review of the data showed that even though the air temperature did not change, the operative temperature dropped four degrees over about 20 minutes. Occupants not located directly beneath the panels reported no discomfort. The potential for discomfort can be reduced with careful panel placement. In this test, the panels were installed in an existing house. Several panels could not be placed according to the manufacturers requirements because of ceiling fans, sprinkler heads and other obstructions.

Recovery Time

In theory, the temperature in each room was reduced from 68 degrees F to 60 degrees F, although the occupants admitted it was difficult to remember to set back the thermostat when leaving the room. After setting the thermostat forward, the panel would raise the temperature directly beneath it to comfort level in 10-15 minutes. Room air stabilized at the set point after about 45 minutes. Both occupants thought 10-15 minutes was a reasonable recovery time.

Installed Cost

Installation of the Enerjoy system typically costs the contractor between $1.25 and $1.50 per sq. ft. of conditioned space. For a 2,000 sq. ft. house that would be $2,500 to $3,000. That's considerably less than the cost for a forced-air system. The lower first cost can be a major advantage to a builder.

The panels are lightweight and operate on 120v or 240v circuits.

Capacity

Utilities will like the fact that the installed capacity is about 60 percent of the building's design heat load. That was half the connected load of the test house's heat pump. For a utility that means less system capacity to maintain and lower peak demand. The lower installed heating capacity didn't pose a major problem. The radiant panels kept up, even when the outside temperature dropped below the design temperature of 13 degrees F for morethan 24 hours.

Cooling

The major limitation of radiant heat is that it can't cool the house. Installing a ducted central air conditioner would undermine many of the advantages of using the radiant panels. One alternative is a ductless air conditioner, often called a "mini-split," that provides cooling to a small area. Like the typical split-system air conditioner, a mini-split has an indoor unit and an outdoor unit. Refrigerant carries heat between the units. Mini-splits blow cool air directly into the room instead of through ducts. They range in capacity from 1/4 ton to 4 tons, although the larger units are intended for commercial buildings.

Preference

In the NAHB study, both occupants preferred the radiant system over the heat pump, even before they knew of the money it saved. Their preference was based less on thermal comfort than other features:

• Because the occupants had different temperature preferences, they liked to control rooms independently. Even in the same room, they could position themselves for greatest comfort.
• The panels operated quietly and without air movement.
• Sinus comfort was higher with the radiant panels.

This research helps quantify the benefits of radiant heating. And, it gives builders confidence that radiant heating can be a comfortable and efficient alternative for well-insulated houses.

For more information about radiant heating or Enerjoy panels, contact SSHC, Inc.