Heat capacity in HVAC is the amount of heat energy required to raise the temperature of a given mass of air by one degree. It is measured in British Thermal Units (BTUs) and is an important factor in determining the size of an HVAC system. Heat capacity is affected by the air’s humidity, pressure, and temperature, as well as the type of material used in the system. The higher the heat capacity, the more energy is required to raise the temperature of the air. Heat capacity is also used to calculate the efficiency of an HVAC system, as it helps to determine how much energy is needed to maintain a desired temperature.