Air Conditioner Heat Load Calculations

What Is Heat Load?

• Heat load, also known as heat gain, is the sum of all heat sources inside a building. Solar radiation, absorbed through the roof, walls and windows, is the biggest source. Warm air infiltrating through cracks or open doors, as well as interior sources, including humans, also play a role. Total heat is expressed in BTUs (British Thermal Units) or Kw (Kilowatts). An air conditioner is effective only if its output is greater than the heat gain of the building or room. Heat load calculations are crucial to determining the size of the air conditioner needed as well as the ductwork required to effectively deliver the cooled air.

IECC Requirements

• The 2009 International Energy Conservation Code, which ensures the design of energy-efficient buildings, requires that sizing calculations be performed on homes when retrofitting heating and air conditioning mechanical systems. The ACCA (Air Conditioning Contractors of America) Manual J calculation is the most widely accepted method. Software programs that simplify the procedure as well as online calculators on a variety of HVAC websites are available to help simplify the process. Heat loads are calculated may be performed on a whole-house basis or by individually measuring each room.

Factors

• Key factors that affect heat load calculations include: local climate; size, shape and orientation of the building; insulation types; number of windows as well as location and type; air infiltration rates; number of occupants and their comfort preferences, and number and types of lights and appliances that emit heat and their energy efficiencies. The difference between the average outside and inside temperature is called the design temperature. Manual J determines design temperature for a given area from average hourly and seasonal measurements taken over a 10-year period, and includes the local "grains of moisture" level, which measures humidity.

Calculations

• ACCA’s Manual J lists the factors by which BTU is determined. The first calculation is determining the space to be cooled and multiplying that by 31.25. Step two measures heat gain through windows, with different factors for northern and southern exposures as well as shading. Next is the number of occupants, multiplied by 600. For heat-generating equipment, BTU is measured by multiplying the number of watts in each by 3.4. The calculation for heat emitted by lighting is determined by multiplying total wattage by 4.25. Heat load is obtained by adding together the five previous BTU calculations. Contractors determine if an air conditioner will sufficiently meet a building’s needs by dividing the heat load by the cooling capacity of the system, which is also measured in BTUs.