Radiant in-space heaters address an aspect of thermal comfort that no other systems do: radiant heat transfer. Like the sun’s rays or a warm fireplace, radiant heat warms directly, and the occupant or object then reradiates heat to the surrounding air. The use of radiant heating can lead to energy savings because occupants usually feel comfortable at lower temperatures in spaces that are radiantly heated. In homes, occupants are often comfortable with air temperatures several degrees Fahrenheit (F) lower than would otherwise be possible. In large warehouses, occupants working in small radiantly heated areas can be kept comfortable even though overall building temperatures may be as much as 20°F lower. In addition, some radiant heating systems make it possible for individual rooms, areas, or zones to be separately controlled, allowing comfort to be maintained at a lower average building temperature. The use of radiant heating can also reduce or eliminate the inefficiencies that result from the excessive air changes that can occur with poorly balanced forced-air heating systems. However, because radiant systems require a direct line between the heater and the person or object being heated, care must be taken to determine the best applications and locations for radiant heaters.
What are the options?
Radiant heaters are available in both electric and gas-fired versions (Table 1). The most common types of systems are as follows.
Electric panel radiant heating. Electric radiant heating panels are designed to be mounted directly on the surface of a ceiling or wall, but they can also be made as replacements for drop-in ceiling panels. All the electrical connections for these drop-in replacements are on the back of the panels and need only be connected to electricity above the ceiling. This makes it possible to quickly relocate the panels as office functions change or as home furnishings are rearranged. Control systems can allow radiant heaters to be used in conjunction with occupancy sensors and timers to minimize operation during times when the space is unoccupied.
Electric radiant heating panels are available in a variety of sizes, typically ranging from about 2 feet by 2 feet up to 4 feet by 8 feet. Heating capacity is proportional to the panel area, with power densities varying from about 50 to 125 watts per square foot (170 to 425 Btu per hour per square foot), depending on the manufacturer’s specifications. The low-intensity panels (50 watts per square foot) are self-supporting, requiring only a metal frame to maintain their integrity. Higher-intensity units weigh more and need a metal back as well as a metal frame to maintain the shape of the panel. Panels usually include rigid fiberglass insulation on the back, a heating element, and a textured surface.
Some panels are designed as “cove heaters,” which can be mounted near the intersection of the wall and ceiling or over a chilly window or sliding-glass door. Individual cove-heater units are usually available in lengths ranging from 3 to 11 feet, with heating capacities of about 150 watts per foot (500 Btu per hour per foot). Other designs include wall-mounted “plug-in picture” heaters and under-desk heaters, both of which are based on the same principles of radiant heating (Figure 1).
Electric cable radiant heating. Often used when cables can be placed under concrete floors or buried in the plaster of walls or ceilings, these systems offer good thermal storage capabilities that tend to reduce temperature swings. However, those same capabilities also slow response time and reduce setback opportunities. Cable systems use a wire with a resistance considerably higher than that of standard copper wire. When current passes through it, the wire becomes warm and conducts heat to the floor or ceiling material, which then radiates the energy into the space that is to be heated. Areas above the ceiling or below the floor (depending on where the system is installed) must be well insulated to reduce energy losses.
Hydronic systems These systems have a major advantage over electric heating systems—they can provide cooling as well as radiant heating. They make use of metal pipes or plastic tubing that can be buried in floors or ceilings and then covered with concrete or plaster. It is also possible to configure wall panels to use this approach. In addition, metal-fin systems are available. However, when hydronic systems are used for cooling as well as heating, care must be taken to account for humidity control. If surface temperatures fall below the dewpoint of the air, condensation can occur, resulting in dripping water, slippery surfaces, or mold.
Gas-fired surface heaters and reflector-focused heaters Commonly used to heat industrial facilities and to provide heat and drying for manufacturing processes, gas-fired surface heaters combust fuel on or near the surface of ceramic or stainless-steel materials without using an open flame. The surface of the heated material radiates that energy to the other surfaces and objects that need to be warmed. Reflector-focused heaters combust fuel and the hot combusted gases flow through a series of exhaust tubes suspended above the area that is to be heated. The tubes are usually shrouded with a reflector that directs the radiated energy to the desired area.
How to make the best choice
Consider the type of application Low-intensity radiant heaters are typically used where total heating of a large area is required. High-intensity radiant heaters can be used both for spot or localized heating and for heating large areas. Small retail and office buildings can be successfully heated with low-intensity radiant ceiling panels or gas-fired tube heaters. Retrofitting steam heat systems to low-intensity tube heating has been shown to be an effective way to reduce energy use in older commercial buildings.
Conventional heaters are often ineffective in large areas such as warehouses and loading docks, where maintaining air temperatures of 60° to 70°F is costly and inefficient. In such situations, if reflector-focused gas or electric radiant heaters are mounted above the areas where workers need warmth, the air temperature of the building as a whole can often be reduced to as low as 40° to 50°F.
Think about maintenance requirements Maintenance and associated costs are virtually nonexistent with electric radiant heating panels. These units have no pumps, blowers, compressors, or burners to fail, and there are no filters to change. They operate with hardly any attention throughout their useful life. And if a panel does fail, it can quickly and easily be replaced, often with an interruption of an hour or less.
Account for eliminating or reducing the size of ducts In new, heating-dominated commercial buildings, radiant heating may eliminate the need for air ducts or allow ducts to be downsized, which, in turn, may make it possible to reduce floor-to-floor height. Where building heights are restricted by code, the use of this technology may even make room for an additional floor—and more rentable space—as compared with traditional construction and other types of HVAC systems.
Remember the limitations The effectiveness of radiant heating decreases in the presence of substantial winds or drafts. To minimize discomfort from the movement of cold air, pressurize the building or take steps to block winds. Radiant systems also require a direct line between the heater and the person or object being heated. Care must be taken not to place furniture where it might interfere with the heater’s ability to warm the room’s occupants. Also, note that radiant systems can’t be controlled by conventional thermostats, because comfort doesn’t correspond to air temperature alone; thermostats that sense both temperature and radiation should be used.
If you answer yes to any of the following questions, radiant heating could be beneficial in the application you have in mind:
Are occupants uncomfortable on the coldest days? As outdoor temperatures drop, the radiant temperature of walls and windows also decreases, which adds to the sensation of being cold even when the thermostat is set at what would normally be a comfortable temperature. Radiant heating panels can increase the temperature of inside surfaces of the building envelope, making occupants feel more comfortable at lower thermostat settings.
Is the area frequently unoccupied? Radiant heaters are particularly effective in areas that are frequently unoccupied—because there is little delay in providing heat, there’s no reason to keep the heat on in unoccupied rooms. They also work best in areas with open architecture, without lots of doors or other obstructions.
Is the primary heat source for the building electric baseboard heating? Baseboard heaters warm the air, which then warms occupants and furnishings, whereas radiant technologies directly warm occupants and furnishings, which then reradiate heat to the surrounding air. With resistance heating, higher space temperatures are required to achieve the same level of comfort that can be achieved at lower temperatures with radiant technologies. When used as a supplement to, or replacement for, electric resistance heaters, radiant heating allows for lower thermostat settings that translate directly into reduced peak demand for a given building.
What’s on the horizon?
One reason that radiant heating systems aren’t more widely used in commercial settings is that their performance has not been as thoroughly investigated and documented as the performance of other heating technologies. A number of questions bear further investigation: How much lower can a thermostat be set in a radiantly heated space and still deliver comfort comparable to what’s provided by a heat pump, furnace, or baseboard system? What effects do setback temperatures and radiant panel response times have on occupant comfort and the likelihood of occupants using the setback? How do radiant heating systems compare to the alternatives in new and retrofit applications in terms of energy use and operating costs? Research sponsored by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) is providing answers to these questions.
Who are the manufacturers?
Thirty or more companies manufacture radiant heating equipment. What follows is a short list of a few of the leading suppliers.
- Advanced Radiant Systems
- Combustion Research Corp.
- Detroit Radiant Products
- Marley Engineered Products
- SSHC’ Enerjoy
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