Radiant HVAC The How, Where & Why
Radiant heating is about as natural of a heat source as there is. Radiation is a form of heat we experience daily when we step outside and feel the heat of the sun. The sun’s radiant heat feels great to many on a winter day on the cold ski slopes in Colorado. Others enjoy the sun’s radiant heat in the summer as they relax and get their tans on beaches throughout the country. The sun’s radiant heat is a renewable energy source that is being used more and more in the building industry with both photovoltaic and hydronic solar collectors.
Fluid flow and heat transfer are the fundamental engineering principles that are used by engineers to design products and systems that provide comfort in buildings. Whether the medium is refrigerant, water, or air there is heat transfer involved. There are three types of heat transfer used in building systems: conduction, convection, and radiation. Radiation may be one of the most underutilized forms of heat transfer in buildings, and it ought to be considered more often as we seek better ways to save energy and provide on-demand zoned heating.
In today’s building designs, there are more and more application possibilities for radiant heating (and cooling) than before. With architectural constraints of ceiling space and tall perimeter glass window wall systems, there is a need for engineers to look at radiant systems from a practical standpoint as a system alternative in prudent engineering analysis. Radiant heating (and cooling), when properly designed, can be the perfect system in conjunction with dedicated outside air systems to meet the ever-demanding criteria in codes and standards for energy, ventilation, and comfort.
Radiant heating can be used for slab heating in residential, commercial, or industrial facilities. In addition, radiant heating can be used for ice and snow melting outside of buildings.
These systems can also offer an advantage with a fully integrated building design and construction strategy by reducing smaller space requirements when the heating and cooling is provided by panel systems as opposed to other forms of terminal heating and cooling devices. In addition, radiant system components are being designed with architectural appeal in appearance.
Many engineers have never considered using radiant heating (let alone cooling) in HVAC system designs simply because they are used to using convective type heating and cooling systems and haven’t taken time to research the products and potential advantages. Some engineers are skeptical that radiant heating will work in severe climates. Still others are concerned with how to control the space air temperature when the heat source is from radiant products that heat surfaces and not air.
Taking a simplistic explanation from Wikipedia where all things are defined: “Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. When the temperature of the body is greater than absolute zero, interatomic collisions cause the kinetic energy of the atoms or molecules to change. This results in charge-acceleration and/or dipole oscillation which produces electromagnetic radiation, and the wide spectrum of radiation reflects the wide spectrum of energies and accelerations that occur even at a single temperature.”
Thermal radiation is different from thermal convection and thermal conduction. A person near a bonfire or fire in their home fireplace feels radiant heating from the fire even when the surrounding air is cold. It wasn’t too many years ago when homes in the farmlands of America were heated with wood burning stoves, and the heat radiating from the stoves was one form of heat that heated the people sitting by the stove. Even today there are homes that are heated with freestanding wood stoves that provide this type of heat with newer technology products than years ago.
Radiant heating heats people and objects in a space through radiant heat with the heat emitted from a warm element, such as a floor, wall, or overhead panel. The space air temperature for radiant heated spaces may be lower than for a space heated with air to achieve the same level of body comfort. One of the key advantages of radiant heating systems is decreased circulation of air inside the room and the corresponding spreading of airborne particles and the energy to circulate the larger amounts of air.
Radiant heating systems and products are typically one of three types:
• Underfloor heating system
• Wall heating systems
• Radiant ceiling panels
There are many installations and applications in all types of climates that will dispel these fears and concerns with utilizing radiant heating (and cooling). Even for engineers in the “Show Me” state of Missouri, where winters can get pretty brutal, there are applications for radiant heating. In fact, one of the largest and tallest buildings in downtown St. Louis (The Met Square Building) has perimeter radiant heating that has been functioning successfully for over 25 years.
With appropriate solar heat gain anticipating control algorithms for demand control of the radiant panels on the various exposures, energy savings can be achieved along with greater comfort when properly coordinated with architectural glazing specifications. The integration of engineering and architectural principles using radiant heating has been in existence long before organizations like the USGBC came along and the LEED rating system became an industry advocate of such discipline integration.
One advantage of radiant heating systems using ceiling panels, wall panels, or floors is that there are no moving parts in the terminal devices. Although this is a challenging economic analysis to do, it only makes sense that maintenance dollars can be saved when radiant panels are used in lieu of terminal units that have fans, dampers, motors, and refrigeration valves in them over the life of a building. The zone valves can also all be installed in one location with zone piping distribution routed to the corresponding zones, keeping the maintenance of the zone valves in a central location outside of the occupied spaces.
With radiant heating, either hydronic or electric, the controls can be kept simple while also providing great zoning control for safe energy at minimal cost premium with proper system layouts and control strategies. There are several sources in the industry for piping and automatic balancing and control valves that can provide products that have been used in radiant applications.
As with any system, there needs to be awareness of installation considerations with radiant heating. When tubes are installed in flooring, the temperatures of the fluid need to be absorbed by the adjacent materials, whether concrete or wood. When pre-manufactured radiant panel products are mounted in ceilings and walls, the substrate surfaces and fastening devices must be able to deal with any heat that is conducted to adjacent materials. It is wise to do the appropriate homework to study the installation, operation, and maintenance manuals of the products being specified and ensure that the entire contract documents provide appropriate information for bidding the project accordingly. Performance specifications may be worth considering since the various manufacturers of the products will likely not all have the same mounting techniques.
Although radiant cooling seems to be a harder concept to grasp and comprehend by some people, it is very effective in the right applications and can solve problems that can occur with air systems when a high heat load in a space requires a large amount of air. Radiant cooling provides the opportunity to offset internal heat gains from equipment without the concern of air turbulence in a space where sensitive operations dictate low space air velocities.
Gas Fired Radiant Heaters – Industrial Applications
Another form of radiant heating that is used in commercial and industrial facilities is gas fired tube heaters. These are high-intensity heaters that are mounted above areas where some form of human or product heating is needed although the air temperature does not need to be directly controlled.
When radiant tube heating is used, there is often a wall thermostat controlling the radiant heating. The direct heat from the radiant heater does not hit the thermostat, as this would not be effective. The objects and space surfaces, wall and floors, are heated by radiant heat. Then these surfaces in turn give off heat through the space air convective currents that heat the space air, which then affects the thermostat. This is an indirect control of the radiant heater, but it is a common approach in applications.
Another more direct type of control would be to use a surface temperature sensor. It would control the gas radiant heater based on the need for a specific surface temperature that would be dictated by a process or some desire to keep products at or above a specific temperature.
In addition to gas-fired radiant tube heaters, there are high-intensity electric heaters that are used for both inside spot heating and outside spot heating. Spot heating is a good application for radiant heating in industrial buildings; however, whole-space heating (where warm space air temperatures are also desired) can generally be done more efficiently with systems like direct-fired 100% outside air systems that provide space pressurization to offset air infiltration.
When it comes to heating, keep in mind that there are three types of heat transfer. Radiant heating products and systems are viable options for residential, commercial, and industrial applications. Take time to research some of the current options both online and in your local markets, where product sales representatives will be more than happy to help provide education and case studies where radiant heating has been installed successfully.
The next time a client asks for system alternatives, it would be wise to keep in mind that radiant heating (and cooling) can be a viable option worth putting on the matrix of system considerations.