Online educational resource on achieving indoor environmental quality with radiant based HVAC systems
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20 points every architect, engineer, contractor and their clients should know about heat transfer - sample slides. For additional support visit our visitor services page.

Our integrated design program has over 2100 slides illustrating architectural, interior design and HVAC engineering principles which contribute to indoor environmental quality and energy allocation for conditioning the occupants and building.

The following course materials on radiant theory are samples from the heat transfer lecture and based on a Steven Covey principle of "Begin with the End in Mind". They are a very small but important sample of the Covey principle and are provided here to give you an idea of what kind of materials we'll be discussing during the program.

The course is also registered with AIA and participants can earn up to 21 Learning Units.

For more sample slides visit our list of training modules.

Note: links below will take you to additional electromagnetic and solar simulators and videos on heat, light and astronomy which is excellent background study for understanding radiant energy. We encourage you to follow these links to develop a broader understanding of radiant heat transfer theory.

Figure 1: Without a doubt, the power of solar energy carried by electromagnetic waves (50% in electric fields /50% magnetic fields) is influential enough that it will cause people to move towards it (L) or away from it (R). Electromagnetism, like gravity, is one of four fundamental forces of nature...and it behaves in such a way that in its presence or absence in certain wavelengths you can feel warmer in cooler air or cooler in warmer air. It was during the 18th century that Frederick W. Herschel discovered this scientifically when he noted different colours of light appeared to have different temperatures and discovered that "invisible light" existed beyond the visible spectrum - this invisible light is known today as infrared radiation (colloquially 'radiant heat') and the natural force for which we can effectively and efficiently heat and cool occupants in spaces. Life as we know it - is structured around the daytime of radiant energy for heating and night time of radiant energy for cooling...yes - the radiant force will move you. Photo credit: SAGE Electrochromics

Figure 2: Solar energy described as shortwave electromagnetic energy, in part created the street image above. Notice the snow accumulation on the left side of the street and notice the lack of snow on the right side...the air temperature is the same as is the average intensity of sunlight (≈1000 W/m2) but why the difference in snow? The answer is in the seasons, direction of travel of the street and orientation of objects...can you tell which way you are looking and explain why there is more snow accumulation on the left side than the right? What can we learn from this when designing and site orientating buildings.

Figure 3: Unlike air whose buoyancy changes as its density changes resulting in drafts / convective currents (cool air falling/warm air rising), radiant can travel in any direction though space regardless of the space temperature; as demonstrated by the photons of energy emitted by the sun, radiant transfer is very effective at moving energy very long distances and at the speed of light. When photons from an incident ray within a certain range of frequencies hit atoms and molecules in an absorbing mass, they begin to vibrate. If the molecules of the mass vibrate enough the mass can change state, i.e. "melt" or "evaporate". As noted in NASA literature, "The more an object's molecules move and vibrate, the hotter it becomes. This heat is then emitted from the object as thermal energy. Some objects, such as darker colored objects, absorb more incident light energy than others. For example, black pavement absorbs most visible and UV energy and reflects very little, while a light-colored concrete sidewalk reflects more energy than it absorbs. Thus, the black pavement is hotter than the sidewalk on a hot summer day. Photons bounce around during this absorption process and lose bits of energy to numerous molecules along the way. This thermal energy then radiates in the form of longer wavelength infrared energy", (ref./credit: NASA/Mission Science).

So now what do you think scientists and engineers think when they hear people from the construction industry use phrases such as, "heat rises" or "radiant is slow"? I know that if this were true we would have to put heat lamps on the floor and hope that the earth was above the sun every day at sunrise (grin)...check out this animation from our friends at Equinox Graphics and Science Photo Library.

Figure 4: Just like in any topic of study, you need to learn the words and the words associated with radiant are listed above. We'll briefly discuss them here but we go into these descriptors in detail during the three day program. In the meantime play with this simulator to watch a nail go through various visible states as it is heated, melts and subsequently cooled.

Figure 5: Here is some stuff you can use to impress people at weddings and birthday my better half likes to sarcastically point out, "Robert you know stuff most people don't care about", "yes dear but mankind's survival depends upon this knowledge"...she to our guests, "who cares?"...oh well - just another day in paradise...ok so what's important here is as mentioned in Figure 3, that photon's of energy in the form of electromagnetic waves will get converted to heat energy as it is absorbed by objects like people and buildings; but we can manage this energy to our benefit with shading and coatings in windows and color on walls and roofs. One take away from this discussion is their is no heat in radiant...yes we know the term "radiant heat" is used all the time but in fact the heat only manifest itself when the radiant energy is absorbed where it can be measured or felt as a heat gain (energized molecules); or when it is emitted (released) where it can measured or felt as heat loss (de-energized molecules). See our pages on Radiant Mythology.

Figure 6: One of our favourite animated tools for demonstrating the sun-earth relationship comes from the "Motions of the Sun Simulator" developed by the University Nebraska-Lincoln. It's very good at illustrating solar loads and shading. We use this simulator in our course to explain the importance of positioning buildings correctly on the development site, and use of architectural and interior and exterior design features and landscaping in controlling solar heat gains. Image credit: University Nebraska-Lincoln.

Figure 7: As noted in Figure 5, electromagnetic energy waves are described by their wavelengths...we can see and feel some of this energy but only in specific ranges as noted above and as compared to other ranges below. One key message here...the maximum recommended operating temperatures of a radiant floor heating systems (less than 85°F/35°C) is for all practical purposes at the same wavelength of the human body (see human body and skin temperatures) which is in a completely different range than the shortwave energy of the sun. What does this imply? Contrary to popular myth, it means that floor heating can not heat the body the way the sun does...what is in fact occurring is the floor is bringing the interior surfaces up to a temperature which is closer to that of human body which reduces the temperature difference between the body and the space which reduces the heat loss from your skin: translation - it is the heat you are retaining more than it is the heat you are absorbing which provides the perception of thermal comfort. Look at it this way...if the floor could heat you the same way as the sun, get the heck out of your house because it's likely on fire. Image credit: Dr. R. McCluney, Florida Solar Energy Center

Figure 8: In architectural and indoor climate engineering we are interested in electromagnetic waves between 0.1 μm and 30.0 μm. In the shorter ranges we are concerned about UV destruction of certain building materials but are also aware that the UV-c spectrum between 0.245 μm and 0.285 μm has a germicidal effect. Lighting and interior designers are concerned about the range between 0.4 μm and 0.8 μm and the entire range is of concern for HVAC designers as both the solar short wave radiation and terrestrial long wave between 3.0 μm to 30 μm become converted to heat upon absorption. We can control absorption by choosing materials characteristics such as colour and texture.

Figure 9: The human eyes are fascinating sensory organs in that they are sensitive to the spectrum know as visible light...though we don't cover lighting in our three day course we do want to point out that light is electromagnetic energy that can be seen just as infrared is electromagnetic energy that can be felt as heat...and if you really want to go further on into other interesting aspects of radiant energy we suggest you take a quick look at our page on, Radiation Pressure of Light and Phosphorescence. Photo Credit: Mark Garlick/ Science Photo Library

Figure 10:  As noted in Figure 8, indoor climate engineers are concerned with the entering short wave solar energy and the reflection and emittance of that energy as long wave energy. In the shortwave form, this energy will/can be absorbed, reflected or transmitted (through transparent surfaces) (or as conducted heat energy in opaque surfaces); electromagnetic energy absorbed excites the molecules of the absorbing surface and thus the conversion from electromagnetic energy to heat energy takes place. This heat energy is then (re) emitted as long wave energy into the space and it can also be transmitted via conduction to the cooler side of the absorbing mass.  We can use window coatings and color to regulate the amount of shortwave energy allowed on or into the space and use radiant cooling to remove absorbed energy. Photo credit: SAGE Electrochromics

Figure 11:  Comparison of solar spectrum wavelength and room surface temperature spectrum wave length...note the transmittance control layer which can be selected to filter out some but not all wavelengths depending on the need for light and heat intensity. Image credit: Dr. R. McCluney, Florida Solar Energy Center

Figure 12:  The radiant intensity on a building can be a help or hindrance depending on how it is controlled. Here the solar energy is being used passively to heat the interior floor which would then radiate into the rest of the room raising the mean radiant temperature. In theory this sounds nice but as many have learned over the years, a passive home = an active homeowner as the occupant is forever trying to either manual retain or discharge heat to maintain a thermal comfort zone. Both the Florida Solar Energy Center and NRC/IRC in Canada have published excellent papers describing the proper choice of window systems (aka fenestration) for hot and cold climates respectively. One take away from this slide is unlike spaces heated with forced air, the air in a space heated with radiant from the sun or floor heating system is warm only because the surfaces are warm. Since these surfaces are warm you retain more of your own internal heat (as opposed to giving it away via radiation to the cold surfaces) and this contributes positively to the sensation of thermal comfort. Image credit: C. Raines, Square One

Figure 13:  The four key descriptors when studying the transfer of energy via radiation. These are known as "thermal optical" properties and every surface in a building can be described by these terms.

Figure 14:  The electromagnetic energy emitted from one surface upon another is called the incident wave and this wave can in a combination of effects be reflected, absorbed and transmitted. How much of each function occurs is dependant on the receiving surfaces characteristics and at what wave length the incident ray is at; texture (roughness/smoothness), color and degrees of transparency or translucency are also considered as are the other characteristics noted below in Figure 15.

Figure 15:  An overview of surface characteristic considerations are presented above. As we discuss in our radiant floor covering lecture, choices in flooring have an impact not only on indoor air quality but also on the operating temperatures of the system and thus the efficiency of the cooling and heating plant. Credit: Pedersen, C.O., Fishers, D., Lindstrom, P.C., Impact of surface characteristics on radiant panel output, ASHRAE RP-876, 1996

Figure 16:  One of the biggest misunderstandings amongst novice designers has to do with heat transfer within the mass supporting a radiating surface and the energy released from a radiating surface - they most definitely are not the same thing. When we discuss the conductivity or resistivity of a material we are only interested in the flow of energy in the form of heat 'through' the mass; but when we talk about the release of this energy 'from' the surface we are only concerned about its emissivity or its ability to emit radiant energy. What does this mean? It means different materials of construction and interior surface finishes might have higher conductivities relative to other materials and layers; in the case of radiant heating and cooling system this would impact the choice in tube depth, spacing (and if required heat transfer fins) which influences the fluid temperature; but at the emitting surface - if the surface temperature is equal for vinyl flooring, paint, carpet, wood or tile etc., they will all emit similar amounts of electromagnetic waves of energy because they all have similar emissivities as shown in the table above. Make note of the low E discussion below in Figure 17 as well as the differences between emittances and absorptances for different materials of construction.

Message: surfaces of similar temperature and similar emittances will release similar amounts of radiant energy (see Figure 18) regardless of their thermal conductivity or resistivity.

Credit: Pedersen, C.O., Fishers, D., Lindstrom, P.C., Impact of surface characteristics on radiant panel output, ASHRAE RP-876, 1996

Figure 17:  Most people are familiar with the term low E windows. Low E equals low emittance and materials that have a low emittance make poor radiators likewise those that have high emittances make good radiators. For radiant based heat/cool systems we want surface materials which are both good emitters and absorbers. Did you note how materials that have low emittances are also known for their high reflectivity?  The last thing you want for a radiant cooling system is a surface with a high reflectivity and low absorptivity - that's why it's important to work with interior designers when doing radiant systems. Credit: Emittances and Absorptance for Some Surfaces, Section 3.9, Heat Transfer, ASHRAE Fundamentals Handbook, 2001, reprinted with permission from ASHRAE

Figures 18: This formula describes the net radiated power between two surfaces based on their emissivity, area and temperature differential. This can be used to describe for example, the relationship between a warm human body and a cold wall; or describe the relationship between the interior surface of a window and an occupant. Incidentally the emissivity of human skin is approximately 0.98. Note: this is a simplified expression as a more accurate model is needed for multiple emissivities and areas as well as the geometric position of the surfaces in relationship to each other. You can see a sample of such an analysis at our pages on mean radiant temperature (MRT).

Figure 19: One of our favourite papers on radiant cooling and heating and interior finishes comes from Pedersen, C.O., Fishers, D., Lindstrom, P.C., with their paper titled, "Impact of surface characteristics on radiant panel output" the outcome from ASHRAE Research project 876. We highly recommend it to all students interested in radiant transfer.

Figure 20:  So much of our natural world is dependant on our understanding of radiant transfer and yet it is the least understood function amongst architects, engineers and contractors alike. Everyday mankind gets up and walks on the biggest radiant absorber and radiator known in our world - it's called earth. Our life as we know it would be destroyed if we didn't receive the electromagnetic waves of energy from the sun and likewise we would all die if we didn't release the heat back into space via electromagnetic waves. The air on earth is warm because of radiant energy likewise it is cool in the absence of radiant energy. It's why here at we say that radiant based HVAC is the natural choice in conditioning occupants and spaces.

So there you have it, a few sample slides from our radiant theory lecturer...just a hors d'oeu·vre from our library of over 2100 slides addressing a small but important element of integrated design and radiant based HVAC systems. In the program we will get into this and a whole lot more? How much more? Well just follow the links to the other parts of our website and you’ll get a feel for the scope of materials that we’ll be covering.

See you soon.

Robert Bean, R.E.T., P.L.(Eng.)
Registered Engineering Technologist - Building construction (ASET #8167)
Professional Licensee (Engineering) - HVAC (APEGA #105894)
Building Sciences / Industry Development
ASHRAE Committees: T.C.61. (CM), T.C.6.5 (VM), T.C. 7.04 (VM), SSPC 55 (VM)
ASHRAE SSPC 55 - User Manual Task Leader

Note: The author participates on several ASHRAE and other industry related committees but be advised the materials and comments presented do not necessarily represent the views of these societies, only the president of the society or nominated representative may speak on behalf of the organization.

For further studies on this topic visit:

NASA Mission Science (Electromagnetic energy)
University Nebraska-Lincoln (Astronomy Education )
Scienceworld Wolfram
Engineering Fundamentals (efunda)

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