Home Heating and Cooling Loads, Deconstructed

Posted on: July 28, 2015

The heating and cooling loads your furnace and air conditioner carry represent the amount of heating or cooling the house demands to maintain a comfortable temperature, usually in the mid-70 degrees. This is expressed in British thermal units (BTUs) of energy. Heating and cooling loads differ for every home depending on the size, design and construction of the house, as well as its overall state of energy efficiency based on variables like air-tightness and the amount of insulation.

Design Load

The design load is the standard figure that expresses how much heating or cooling is required to maintain a specified indoor temperature at an average outdoor temperature. In a given locale, for example, the design cooling load refers to the number of BTUs of extracted heat energy necessary to keep the indoors at 75 degrees while the outdoor temperature is 90 degrees. During winter, the respective heating load specs might be 70 degrees and 25 degrees.

Extreme Load

To compute worst-case demands, extreme loads assume severe outdoor conditions generally expected to occur only about one percent of the time. Substantially higher than the design load,  the extreme load mainly represents a “what if?” calculation for severe heat waves or cold spells. Because some safety margin is built into the design load calculation to accommodate uncommon weather events, however, sizing new HVAC equipment to the design load is standard in most climate zones.

Part Load

Much of the time, your HVAC system carries neither the design load nor an extreme load. Part load refers to temporary time spans when cooling or heating requirements are minimal. A good example of a part load are the cool morning hours preceding a hot summer day or a winter afternoon warmed by the sun before a frigid, clear night. Transitional seasons of fall and spring often include entire days of part load conditions.

For more information on heating and cooling loads, check out Air Experts’ HVAC solutions, or call 919-887-2132.

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