What's a Load Calculation?

What's a Load Calculation?

Don’t make any upgrades to your heating and cooling system without getting a professional load calculation first. Calculating your home’s heating and cooling “load” means determining the precise BTU capacity required to keep the house comfortable in winter and summer. Without this vital piece of information, you can’t select a furnace or air conditioner that’s the right size for the job. An oversized unit — one with too much BTU capacity for the house — is very energy-inefficient and will result in high operating costs. An undersized unit, on the other hand, won’t keep the house comfortably warm or cool.

Calculating the load involves a room-by-room survey of your house to gather data about certain characteristics that affect heat gain and heat loss. Every home is different when it comes to heat gain and loss. An accurate load calculation requires information specific to your home — general "guesstimates" or rule-of-thumb averages won’t do.

During the inspection a qualified HVAC technician will record factors such as:

  • Age of the house, square footage and number of rooms.
  • Orientation to sun exposure.
  • Amount and type of insulation.
  • Condition of weatherstripping, caulking and other factors relating to air leakage.
  • Number and size of windows.
  • Type of light fixtures and number of large appliances.
  • Number and age of occupants.

The data is typically entered into software called Manual J, the standard program for computing heating and cooling loads in the HVAC industry. The software calculates all factors and also incorporates information about the local climate to generate a precise heating and cooling load, expressed in BTUs. With that information, your HVAC contractor can accurately select a furnace or air conditioner with the correct capacity to accommodate that load calculation. For a furnace, "capacity" refers to the amount of BTUs of heat energy the unit generates in an hour. Air conditioner capacity, conversely, refers the amount of BTUs the unit extracts from indoor air in the same time period.

For more information or to schedule a professional load calculation, contact the experts at Valley Heating, Cooling, Electrical.


Our goal is to help educate our customers in the South Bay Peninsula and Santa Cruz areas of California about energy and home comfort issues (specific to HVAC systems).