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NORA Advanced Efficiency

Air temperature has a big impact on comfort. The higher the temperature of the room, the slower heat will be lost from the body. The colder the temperature, the more quickly heat will be lost. This is a basic law of heat transfer. People are most comfortable in the winter when the heat transfer from their bodies to the surroundings is minimized. People also lose heat from direct contact by sitting or lying on a cold surface. The heat leaves the body and goes to the colder surface, making the person feel cold. Drafts blowing cool air over skin also carry away heat. Skin temperature is about 91°F. If the air blowing over the skin is colder than that, the person feels a chill. Heat radiates from the body to colder surfaces nearby, and people also lose heat from evaporation. As perspiration evaporates, it cools the skin. The dryer the air, the faster evaporation takes place and the cooler people feel. Heat Transfer Heat is energy in transit. Put another way, it is energy transfer due to temperature difference. Heat always travels from something hot to something cold. Heat given up by one Heat and Air Travel The basic rules of how heat and air move are: • Heat flows from hot to cold. • High pressure flows to low pressure • Airflow carries heat through the walls and roof. • Airflow carries water vapor into the building envelope. • Hot air rises through cold air. 4 National Oilheat Research Alliance object must equal the heat gained by the other. For one object to warm another, actual contact is not needed. Heat can travel by conduction, convection and radiation and it continues to travel until the temperatures are the same. Air Pressure Air pressure is created by the weight of the atmosphere. There are over seven miles of air above the earth. The weight of this air is called atmospheric or barometric pressure. It is a little over 14 pounds per square inch at sea level. The higher the elevation above sea level, the lower the pressure. Barometric pressure is not constant. It changes with the weather, altitude and the dynamics of hot air rising through colder air. Fans also mechanically change air pressure. High-pressure air moves toward low pressure. Outdoors, this airflow is called wind. Indoors, it is called draft. Conduction Conduction transfers heat by molecular vibrations and the movement of electrons. If one end of a silver teaspoon is placed in a cup of hot coffee, it does not take long for the handle of the spoon to get hot, too. Heat is transferred by conduction from one end Figure 1 Conduction of the spoon to the other. Figure 1. Neither the spoon nor the coffee move, only the heat moves. Conduction is the flow of energy through a material. It is also the flow of heat from one thing to the other if they are in direct contact. Metals such as silver, copper, steel and cast


NORA Advanced Efficiency
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