How to do heat loss calculations for HVAC systems

Written by erin watson-price
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How to do heat loss calculations for HVAC systems
A home's heat loss increases with a greater number of windows and doors. (Jupiterimages/Photos.com/Getty Images)

Homeowners and HVAC technicians use heat loss calculations to determine the heating system size necessary to maintain the most comfortable indoor environment for the least energy usage. In order to calculate the heat loss or gain for a particular house, the Air Conditioning Contractors of America (ACCA) published the Manual J "Residential Load Calculation." This complicated mathematical process takes into account every possible factor affecting the heating or cooling envelope in a house. While an HVAC technician should perform a Manual J analysis for an accurate heat loss calculation, determine an estimated heat loss for the home by following a few mathematical calculations.

Skill level:
Moderately Challenging

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Things you need

  • Calculator
  • Pad
  • Pencil
  • House plans
  • Tape measure

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Instructions

  1. 1

    Calculate the design temperature (DT) for the home using the formula DT = IT - OT. The design temperature takes into account your preference for indoor temperature (IT) and the average outdoor temperature (OT) for your region. For example, if IT = 75 and OT = 65, the DT for the home is 10.

  2. 2

    Obtain a copy of your house plans with clearly marked dimensions for walls, windows, doors and ceilings. Use a tape measure to determine any dimensions not provided on the house plans.

  3. 3

    Calculate the surface area (SA) for individual doors and windows, as well as the ceiling. The formula for surface area is SA = height x width. It may be helpful to label each door and window on the house plans and create a separate list. Write the surface area of each window next to the label for quick reference in later calculations.

  4. 4

    Calculate the surface area (SA) of an outside wall of the home. When calculating the surface area of an outside wall, subtract the surface area of any doors or windows in that wall. The formula for the surface area of an outside wall is SA(wall) = (height x width) -- SA(door) -- SA(window). If there is more than one window or door, subtract the surface area of each window or door located on that wall. Repeat the surface area calculation for each outside wall of the home.

  5. 5

    Calculate the U-value (UV) for the walls, doors, windows and ceiling of the home. The U-value is the typical heat loss for a particular surface in the home. The formula for U-value is UV = 1 / R-value, where R-value is the insulating potential of a surface. If you do not know the R-value for the windows, doors or insulation in your home, see Resources for a list of typical R-values.

  6. 6

    Calculate surface heat loss (SHL) using the formula SHL = U-value x SA x DT. Calculate the surface heat loss for each wall, window, door and ceiling.

  7. 7

    Add together the surface heat loss for each wall, window, door and ceiling. This equals the total heat loss for the home.

Tips and warnings

  • This calculation is only a rough estimation. For an accurate heat loss calculation, contact your HVAC technician.

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