Heat and Comfort Basics 3D
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Heat losses occur when heat leaves a structure, and heat gains occur when heat enters a structure. When there are significant heat losses, a furnace or heat pump adds BTUs of heat to compensate for those losses. When there are significant heat gains, an A/C system or heat pump removes BTUs to balance out the gains. BTUs (British thermal units) are units of heat equivalent to the amount of energy it takes to raise the temperature of 1 pound of water by 1 degree Fahrenheit.
Heat transfer occurs in three different ways: conduction, convection, and radiation. Conduction occurs when two substances of different temperatures make direct contact with each other; the hotter object will transfer its heat to the cooler object until both objects are at the same temperature (equilibrium). We use insulation to oppose conduction and reduce the rate of heat transfer.
Convection occurs when molecules of fluids (vapors and liquids) move and bring their heat with them. Our homes experience temperature changes due to convection when we have gaps or cracks in the structure or leave windows or doors open; we refer to air movement via these sources as infiltration and exfiltration.
Radiation occurs when objects give off or absorb heat via electromagnetic waves. When the sun shines on surfaces in the home through glass windows, the room gets warmer because the heat from the sun’s electromagnetic waves passes through the glass and warms the surfaces in the room. Our bodies also give off heat via radiation, which is why you feel cooler when you stand near a cold wall; your body gives off heat to the cooler surface of the wall.
Heat can be sensible or latent. Sensible heat is heat that we can measure with a thermometer, and latent heat cannot be measured because it refers to the heat required to complete a phase change (the temperature does not change). It takes 1 BTU to raise the temperature of a pound of water by 1 degree Fahrenheit, but it takes about 970 BTUs to change a pound of 212-degree liquid water to 212-degree water vapor. There is a lot more energy involved in phase changes than mere temperature changes; the latent heat required to change solid ice to liquid water or vice versa is the latent heat of fusion (144 BTUs), and the latent heat required to change liquid water to water vapor or vice versa is the latent heat of vaporization (~970 BTUs). Larger heat sources (including flames or electric heat) transfer more heat than smaller ones, meaning that they transfer more BTUs and can make phase changes happen more quickly.
Latent heat is important for HVAC applications because most HVAC systems in temperate or humid climates also remove moisture from the air. Many people will notice that cooler air sinks and warmer air rises. Cooler air is denser than warm air, which perpetuates the common but slightly misleading idea that “heat rises;” heat itself doesn’t rise or fall. Warm air will rise and go into the return, where it will pass over the evaporator coil. The refrigerant in the evaporator coil can absorb a lot of heat because it is boiling and requires a large number of BTUs to complete its phase change to a vapor. As heat transfer happens, some of the moisture in the air will also condense on the coil; the coil must be cold enough to be below the dew point for this to happen.
We can calculate how much heat enters and leaves a home by using ACCA Manual J. This manual allows us to use local climate conditions and consider the structure to design an HVAC system tailored to a home’s BTU gains and losses.
However, the real conditions may vary due to human activities, especially because our bodies add heat to structures via conduction (touching surfaces), convection (movement), and radiation. Humans also add latent heat when they exhale. Heat gains added by humans or animals in a structure are known as internal gains.
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