Choosing a furnace depends on the heat energy needed to warm your home. A house in a cold area would require a larger furnace than one in a warmer area. Typically, a larger furnace has a higher heating capacity, expressed in BTU/h or British Thermal Units per hour. It takes 1 BTU to increase the temperature of one pound of water by 1°F. Therefore, heating more water, or a larger room area, would then require more BTUs.
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