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The best example of radiant heating is the effect of the sun on your car. The energy from the sun travels through the vacuum of space as radiant energy (there is no convection or conduction heating in a vacuum). The suns radiant energy travels thought the windows of your car. Some of the energy is absorbed by the cars interior and some is bounced back out the windows. Radiant heat is linear and requires line of sight. Radiant energy is sensitive to the color of any object. Black object absorb the most energy while white or silver (mirror) absorb very little energy. A radiant heater works just like the sun, it produces invisible light waves that carry the energy through space until they hit and object that absorbs. When the wave hit your cloths or skin, the energy is absorbed. Radiant heaters include outdoor patio heaters and any indoor heaters that glow orange. Radiant heaters vary in efficiency. Many older home have fireplaces designed to heat with radiant energy. Tall shallow fireplaces radiate heat while the consume the warm air in the room to support the combustion. These fireplaces usually cool the majority of the house by pulling in cold outside air while heating the area right next to the fire. This is very inefficient heating. In outdoor areas like a patio where the wind may be blowing cold air, radiant heating it the only reasonable way to distribute heat.
Convection heating is based on the motion of air to move the heat from one area to another. Forced-air central-heating systems use convection for distribution of the heat that is generated in the furnace. In gas combustion system, the hair from the house contact a heat exchanger where is rapidly heats up. This warm air is then blown using a fan to other parts of the house. Some small room heater may use a combination of radiant and convection. By blowing air over a glowing heating element, some of the heat is transferred by waves and some by the motion of the air. The process of convection heating is based on the temperature difference between the air and surface plus the velocity of the air. A convection oven uses fast moving air to put more heat into the object being cooked. This accelerates the heating process. A hair dryer use convection (fast moving hot air) to evaporate the water in your hair. Convection heating also has efficiency limitation. Some forced air system have ductwork exposed to outside temperatures. Therefore the system wastes energy not only heating the ductwork but the heat is lost anywhere there is a leak. This is not as bad for system where all the ductwork is inside the controlled envelope since the heat is not lost, just mis-placed.
Convection heating is based on transferring energy through the rubbing of molecules. When you touch hot water and it burns you, this is due to conduction of the energy from the water to your skin. Conduction heating is what makes the heat exchanger in your HVAC work. It is the physical contact of air on either side of the heat exchanger that transfers the energy. Since conduction requires contact with the heat source is only practical for spot heating like heating blankets, some stovetops and your hot water heater.
Most homes use a combination of all 3 types of heat transfer although radiant and convection are the dominant ways of transferring heat to your body.
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