air or space and subsequent absorption by another body. When you stand in front of a cold window, you radiate heat to the window and you feel cold, even though the room is warm. If all bodies emit radiant energy continuously, why don’t they eventually radiate away all their energy and cool down to absolute zero? The answer is they would if their energy wasn’t being replaced in some way. All the bodies around them are also radiating energy. Some of this radiated energy is intercepted and absorbed. The temperature of the body determines the rate at which it radiates energy. The rate at which it absorbs energy depends upon the temperature of its surroundings. When a body is hotter than its surroundings, its rate of emission is greater than its rate of absorption. There is a net loss of energy. When a body is colder than its surroundings, the rate of absorption is greater than the rate of emission and the body’s temperature rises. Slowing Heat Transfer Insulators are used to slow conductive heat transfer. Pan handles are made of plastics or wood because they are good insulators, thus keeping the handle cool. To slow convective heat transfer (movement of hot air or water), the movement must be physically blocked. Windows in your house prevent the warm inside air from mixing with the cold outside air. 6 National Oilheat Research Alliance Measuring Heat In the United States, heat is measured with the British Thermal Unit (Btu). A Btu is the quantity of heat needed to raise the temperature of one pound of water (about one pint) one degree Fahrenheit. A thermometer does not measure heat. It measures temperature. Solid, Gas, or Liquid? Latent Heat Water is an important ingredient to many heating systems. Water (H2O) can exist in any of three states: liquid, solid or gas. When water is solid, it is called ice, and when it is a gas, it is called steam. Moving from a solid to a liquid and a liquid to a gas are called phase or state changes. Changing phases requires a tremendous amount of energy compared to only changing the temperature. When water changes phases, it absorbs or releases heat. To raise one pint of 32°F water (water can exist as a solid or a liquid at 32°F) to 212°F, 180 Btus of heat must be applied to the water. That would yield one pint of water at 212°F—but not steam (water can also exist as a liquid or a gas at 212°F). To get that pint of 212°F water to change state and become 212°F steam, an additional 970.3 Btus of heat must be applied. This 970.3 Btus of energy now in the steam is called the latent heat of vaporization. It takes only 180 Btus to get a pint of water Figure 4 Latent Heat
NORA Advanced Efficiency
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