So you can see that heat gain is a little more complex. Notice that items 1 and 2 are directly related to the temperature of the outside air, just like their counterparts in winter heat loss calculations, but items 3, 4, 5 and 6 occur no matter what the outside temperature is. To make things a little more complex, heat gain calculations take moisture into account as part of the Design Heat Gain. Fortunately, a computer program like HVAC-Calc handles this complexity for you. Sensible Gain and Latent Gain The heat gain associated with the temperature of the air is called the Sensible Heat Gain. The heat gain associated with the water in the air that leaks in due to infiltration and the water that evaporates from peoples skin as well as the moisture in their breath is called the Latent Heat Gain. If you add up the Sensible Gain and the Latent Gain you get the Total Heat Gain. There is a Total Heat Gain at every outside design condition however the one of interest is the Total Design Heat Gain at the outside Summer Design Conditions.

The Summer Design Conditions consist of more than just the outside temperature. They consist of the Summer Design Temperature (only 2� % of time warmer than this) and Summer Moisture Content (measured in grains of water per pound of air, typical Houston 113, New York 98), Daily Temperature Range (High, Medium or Low). The daily range is a measurement of how the temperature varies during the day. A high daily range means temperatures start cool in the morning, hot in midday and cool down at night. A high daily range will result in a lower heat gain than a low daily range where it starts out hot and stays hot all day. Fortunately, with a computer program such as HVAC-Calc, the Summer Design Conditions and Winter Design Conditions for hundreds of cities are built in to the program. You select them once and then forget it. There is also an additional unit of measurement that is used to describe the cooling capacity of air conditioners and that is the "Ton". One Ton is 12,000 BTU per hour (BTUH).