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Cooling/Heating Loads – Builder

Ask yourself the following questions:

  • How much money per ton am I overpaying for my equipment?
  • How much time, money and customer satisfaction am I losing due to incorrectly sized or improperly matched equipment?

Studies show that HVAC systems are oversized by an average of 30 to 50 percent.(1) At Enerlogic, we provide accurate, independent, third-party HVAC design services that ensure you are not overpaying for oversized HVAC systems. You can take comfort knowing that your HVAC systems are properly sized, matched with the correct equipment and laid out accurately to provide optimal comfort and energy efficiency for your clients.


Determining Cooling and Heating Loads

The Air Conditioning Contractors of America (ACCA) Manual J (8th Edition) – Residential Load Calculation for heating and cooling load calculations is the first of four steps in correctly designing an HVAC system. The HVAC design process should include the following steps:

  1. ACCA Manual J (Residential Load Calculation)
  2. ACCA Manual S (Residential Equipment Selection)
  3. ACCA Manual D (Residential Duct System Design)
  4. ACCA Manual T (Air Distribution)

The ACCA developed these steps to ensure that HVAC contractors install correctly sized equipment and duct systems. However, this is not always the case.

Why, you may ask?

  • Many contractors simply do not perform the load calculations for every home where they install equipment. They use a “rule of thumb” approach or, to be more accurate, they guess(2).
  • When they do complete a Manual J, they often do so incorrectly by measuring block load vs. room-to-room load, guessing the data to input into the software or using defaults they do not understand(2).


The two methods for calculating a Manual J load are whole-house and room-by-room load.

Whole-House (Block) Load

The block load method provides the heating and cooling load for the home as a whole. It is typically used for a quick estimate of the building load or if there is no need to design or modify an existing duct system, which is not realistic in most cases. Whole-house load calculations are commonly used to determine the HVAC equipment size that is required when replacing an existing HVAC system. Unfortunately, this type of calculation inadequately determines new homes’ equipment selections, resulting in poor system performance.

Room-by-Room Load

The room-by-room method determines the heating/cooling load and airflow delivery necessary to properly heat and cool each room. To determine the proper individual duct sizes per room and the overall size and layout of the duct system, this information is absolutely critical. Otherwise, you will hear homeowner complaints regarding comfort in various rooms or zones of the home.

Additionally, if a room-by-room load is completed incorrectly, the rooms will be improperly sized and/or balanced (airflow), leaving your home buyer uncomfortable and unhappy. If the Manual J is completed correctly but the ACCA Manual S used for correctly matching the equipment is not conducted, airflow will be inadequate, resulting in the same type of issues.

The calculation should be completed on a room-by-room basis for the whole house, which allows you to determine the correct amount of heat that needs to be added or removed (BTUh) and conditioned air delivered (CFM) to each room to ensure occupant comfort.

Building and room loads are further broken down into two load sub-types:

  • Sensible (related to actual temperature) – heating and cooling
  • Latent (related to humidity) – cooling only


We’ve all heard the saying, “It’s not the heat; it’s the humidity.”

A cooling system that is oversized lowers the temperature very quickly, preventing an adequate run-time for the removal of sufficient moisture or humidity from the air. This results in occupants’ feeling “cool but clammy.” They then turn down the thermostat to make the air conditioner run longer, increasing their utility bill. In some cases, the lower temperature, combined with insufficient moisture removal, may result in condensation in high moisture areas and, subsequently, bacterial growth and other types of moisture-related issues.

An ACCA Manual J analysis requires accurate detailing of the following characteristics:

  • Geographic location (outdoor design temperature/moisture concentration)
  • Elevation (sea level or mountains)
  • Orientation (geographical compass point)
  • Building envelope:
    • All surface areas (walls, ceilings, floors, windows and doors)
    • Thermal characteristics (heat transfer capability)
      • Insulation v R-value
      • Window – SHGC and U-factors
      • Door – U-factor
    • Infiltration rate (tightness or leakiness)
  • Duct system:
    • Location (conditioned or unconditioned space)
    • Tightness or leakiness
  • Internal gains:
    • Sensible (lighting, appliances and people)
    • Latent (cooking, washing, showering and people)

A proper analysis requires a knowledgeable and experienced professional who can correctly determine and input the information into the approved software. Guessing or using assumptions is no longer an option for accurate heating and cooling load simulation.

If you would like to see an example of a Block or Room by Room load report, use the Contact Form to request a copy either of these.

(1) Technology Fact Sheet, “Right-Size Heating and Cooling Equipment”, Office of Building Technology, State and Community Programs (BTS) – Energy Efficiency and Renewable Energy – U.S. DEPARTMENT OF ENERGY

(2) “How Contractors Really Size Air Conditioning Systems.”, Vieira, Robin K., Danny S Parker, Jon F. Klongerbo, Jeffrey K. Sonne, Jo Ellen Cummings, Florida Solar Energy Center (FSEC)

Our staff at Enerlogic would be happy to assist you with all of your ACCA Manual J load calculation questions and provide both design and installation verification services in a timely manner.

At Enerlogic we would be happy to answer all of your HVAC System design questions and provide you with design services in a timely manner.

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