This column features helpful information, innovative equipment, systems and applications utilities around the nation can use to save energy and improve service.

Radiant heaters work by warming people and surfaces directly as opposed to conventional heating systems, which warm the air.  Some radiant systems have relatively "low mass" (i.e., low thermal mass), such as lightweight electric panels or modern hydronic panels.  Others have high thermal mass, such as radiant floors and concealed ceiling panels.  Radiant heat may be either surface mounted or embedded in the building structure. 

Savings over conventional heating

Low-mass, surface-mounted radiant heating panels can often save significant energy compared to conventional heating systems such as electric baseboard heat and heat pumps, and compared to other radiant heating types.  Savings are achieved in several ways:

With radiant heat, people are typically comfortable at much lower air temperatures.

• Radiant heat can be easily zoned so to heat only to those areas requiring heat, while heat to other areas may be turned off. 
  
  
• With low-mass panels, occupants feel the heat soon after it is turned on, so they may be more likely to set the thermostat back during unoccupied periods. In contrast, high-mass radiant systems are often left on to avoid discomfort during an extended warm-up period. 
  
  
• Fan energy may also be reduced.  While hydronic panels consume pumping energy, modern water panels circulate lower volumes than older radiators required.
  
  
• Surface-mounted systems have much lower losses due to conduction through the building envelope than embedded radiant systems.

While energy savings may be achieved in many situations, the best savings occur in rooms with high ceilings and/or that are drafty or poorly insulated, have doors that are frequently open, that require high ventilation to maintain indoor air quality or other circumstances that increase the air volume a conventional system would have to heat.

Hydronic vs. electric systems

Surface-mounted panels may be electrically heated or may be hydronic, i.e. heated by circulating warm water.  Hydronic surface-mounted panels have lower mass than older cast iron radiators—due to their lighter construction and smaller water volume—and so now have shorter time lags.  An advantage of hydronic panels is that the fuel source is not limited, since electricity, natural gas and other combustible fuels, or solar thermal energy may be used to heat the water. 

Electric panels can offer greater flexibility in meeting heating needs than hydronic systems, since their surface temperature is not limited by the boiling point of water.  Hydronic panels may have surface temperatures up to about 180 degrees Fahrenheit while, according to one manufacturer, the surface temperature of their electric panel may range from 100 degrees or less, up to about 260 degrees. 

In addition, lightweight electric panels may be installed relatively easily in either new construction or as retrofits.  (Be sure to check with the manufacturer for specific requirements of their products.)  Electric panels also do not have costs associated with the water circulation system and boiler.   

System Design

Placement of radiant heaters is very important.  To be most effective, the panels must be in "view" of the occupants; that is, there should be no objects between the person and the panel.  Ceiling-mounted panels will generally have a better view of the entire room compared to mounting on a wall or embedding in the floor.  If a person is frequently seated at a table or desk, however, a ceiling panel will not "see" the person's legs or feet, possibly resulting in discomfort.  Installing smaller radiant panels under desks as a complement to ceiling panels may solve this problem.

From an aesthetic point of view, surface-mounted radiant panels are available that have been designed to be unobtrusive in a residential or office setting.   Some panels can be painted, either by the owner or the manufacturer, to match their mounting surface.  Others are available in a variety of colors and finishes.
 
Electric radiant panel products include Enerjoy and Qmark.  Hot-water panels include Runtal Radiators and the TWA Panel System.  When talking with manufacturer's representatives, you should discuss control options (e.g. programmable and on-off control) and panel mounting, and ask questions concerning system design and operation.

Expected energy savings

In a case study conducted for the U.S. Department of Energy, the low-mass electric radiant ceiling panels examined resulted in energy savings of 53 percent compared to electric-resistance baseboard heat and 33 percent compared to a heat pump (USDOE 1994). 

Another case study of 18 residential units found the electric, "fast-acting, surface radiant system" yielded energy savings of 20 to 80 percent compared to all other systems examined, including heat pumps, electric and natural gas furnaces, concealed radiant panels and baseboard electric systems (Watson 1998). These case studies involved a small number of homes, so they may not represent typical savings, but do indicate the potential for significant savings. 

Radiant heat has been widely used in industrial settings where savings from 30 percent to 60 percent are not uncommon, depending on the base case heating system and the characteristics and use of the space.  Different types of radiant heaters, such as gas-fired tubes, are often used in industrial settings that may not be appropriate for use in residences or offices.

References

Enerjoy Case Study - An Evaluation of Thermal Comfort and Energy Consumption for the Enerjoy Radiant Panel Heating System (.pdf)
May 1994. U.S. Department of Energy and the National Association of Home Builders Research Center

"Case Study: Seven-System Analysis of Thermal Comfort and Energy Use for a Fast-Acting Radiant Heating System," ASHRAE Transactions, 1998. Watson, Richard, Kirby Chapman and Jeanne DeGreef