Real Estate Inspection Services
Based upon posts at www.NACHI.org
Just interesting that radiant floor heat is actually based on convection .
(the term radiant fools you.)
Despite its name, radiant floor heating depends heavily on convection, the natural circulation of heat within a room as air warmed by the floor rises
While the term "radiant heat" may seem a deceptive phrase, home inspectors (especially infrared thermal imaging thermographers) should understand the theory of heat transfer.
Heat transfer- heat transfer is the flow of energy from a body of higher energy to one of a lower energy. So, heat travels from a warm body to a cold body, transferring its heat energy to the cooler body (whether it be a solid, liquid or gas).
The first law of thermodynamics- this law basically states that energy is not created or destroyed, it is transformed. When discussing radiant heat we produce heat (measured in BTUs) at a rate of about 3.4 BTUs/watt. Regardless of what happens to the heat energy produced by the heater as it passes to adjacent cooler body's, the amount of the heat produced always remains and never disappears into thin air.
Everything must equal 1.0 (or 100%)- when we turn on a heater (which is a device that has the primary purpose of intentionally producing heat) for the intention of heating your skin, many transformations occur along the way and the "efficiency rating" of that type of heater is based upon the percentage of heat it produces to the amount of energy it can apply to your skin and maintain or raise its temperature.
Type of heating device- there are many types of devices to produce heat to control our environment. They are classified according to how they transfer heat (most efficiently) from the source to the subject being heated. In this discussion we are focusing specifically on "radiant heat".
Direct and Indirect heat transfer- direct heat transfer is basically a source of heat where you can see the heating device. Indirect heat transfer is when heat transfers from the source to you and you cannot directly see the source. I.e. a wood stove with a glass door is a direct heating source which radiates energy from the flame to your body. It also conducts heat to the air creating a convection. Your furnace in the basement is an indirect heating source because it relies on forced convection of heated air from a source you cannot see (there are many different types of heat transfer occurring along the way).
A radiant heater is a device that primarily transfers heat through radiation.
If you touch a radiant heater it will burn you through conduction, but that is not its designed source of heat transfer. If you put your hand above a radiant heater you will feel air movement from convection, but that is not its design source of heat transfer. When you stand in front of a radiant heater you feel the heat on your skin and if someone walks between you and the heater you will immediately feel cool because you are now relying on indirect heating (which the radiant heater is not efficient at producing).
The percentage of these three means of heat transfer equals 100%.
The largest amount of heat transfer occurs through radiation even though lesser percentages of conduction and convection occur simultaneously.
Why are radiant heaters so inefficient?
For one, they are controlled by a thermostat that measures air temperature not radiant output.
Radiant heaters warm the contents of the room just as they warm your body. The rate at which the room contents heats the room is based upon its ability to absorb radiant energy and heat the air by convection. Each time that you change the method of heat transfer an efficiency loss occurs. Though the energy has not been lost, it has been removed from the equation of how well the source of energy heats our skin. If you touch the radiant heater it will burn you, but as we all know if we are in the room, out of sight of the radiant heater the air temperature is not sufficient to keep us warm in many cases.
Radiant heaters in the ceiling or floor become inefficient because they are using indirect heat transfer. It's not aesthetically pleasing to have glowing heating elements hanging from our ceilings.
Radiant heaters have a reflective surface above to redirect the heating elements below. When the reflective surface becomes dirty with time it begins to conduct. A reflective material is highly conductive and the conductive loss can be significant as it deteriorates from dirt and corrosion.
So ultimately, to efficiently use a radiant heater in our homes we would want everything in the rooms to be highly reflective and not absorb and subsequently lose energy (directing it away from from the purpose of warming our bodies). This will prevent absorption losses and will more evenly warm your body from all directions. Not a likely solution!
Just because radiant heaters do not work well in the residential application for controlling the environment in our homes does not mean that they are no longer radiant heaters. Radiating heat is still the primary method of heat transfer.
Through proper application and design, radiant heaters can be extremely efficient. Their most prominent effective use is when it is undesirable or impractical to heat the air (such as outdoor patio heaters).