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HVAC Industry Terms Explained: Manual J Load Calculation

Manual J load calculationA Manual J load calculation ensures that the capacity of your new central A/C or furnace correctly matches the unique requirements of your home. “Capacity” refers to the BTU specification of a specific air conditioner or furnace. The cooling and heating requirements of your home, on the other and, are known as its “load.”  This information is vital to make sure the system installed in your home performs at maximum efficiency and provides optimum cooling and heating.  Accurately matching an A/C or furnace to the home’s cooling or heating load is accomplished by a Manual J load calculation performed by a qualified HVAC technician.

The Manual J survey identifies specific thermal characteristics of your house. This data is entered into load calculation software published by the Air Conditioning Contractors of America. The computer calculates all factors and generates a cooling and heating load figure expressed in BTUs. With this critical info, your HVAC contractor can suggest units with the appropriate BTU capacity to match your home’s cooling or heating load.

Here are some factors included in the Manual J load calculation:

  • Local climate data.
  • Square footage of the house.
  • Orientation to the sun and hours of daily sun exposure.
  • Number of windows in the house and the type of glass.
  • Ceiling height in rooms.
  • Type of roofing material.
  • Crawl space or slab?
  • Attic dimensions.
  • Estimated air infiltration due to leakage and other factors.
  • R-value and type of installed insulation.
  • Number and type of light bulbs.
  • Major appliances in use.
  • Number of occupants and age of each.

Rough estimates aren’t good enough for an accurate load calculation. An A/C or furnace with too much capacity for the house will run short cycles, making temperatures unstable and the indoor environment overly humid. When  HVAC capacity is insufficient, conversely, the unit runs nearly non-stop, raising monthly operating costs and incurring excessive wear and tear on the system.

To make sure your new HVAC system is a perfect match for the requirements of your home, contact Hansberger Refrigeration and Electric Company for more info about a Manual J load calculation.