HVAC Load Calculation

Load calculations are performed to determine what causes heat in your home, where it originates from and why, in order to get the correct amount of cold air to the appropriate places. This calculation is made with a whole home approach, and takes insulation, tightness, roofing, materials, windows, and other factors into consideration. System capacity (size) should ideally be a product of these factors. For these reasons, using general rules of thumb to size HVAC systems can result in inaccurate sizing.

Three Major System Design Factors

All three of the following design guides are important in order to have a system that can create and distributes the required amount of cold air to the proper places efficiently. Air isn’t able to flow properly to the right rooms without appropriately sized and configured ductwork.

  • Manual J is the foundation. It identifies cooling and heating need determined by construction and geographical location at certain engineering constants (laboratory conditions).
  • Manual RS determines appropriately specified equipment for a specific environment’s needs (when real world is different from the laboratory constants).
  • Manual D deals specifically with distribution. Air conditioning capacity is dependent on CFM (airflow) to not only distribute the cold air, but to create the cold air in the first place. For example, you could have 5 tons of cold air for a house needing 5 tons of cold air, but without the right ductwork and airflow, there may only be 3 tons of cold air getting to the rooms within the home.

Since code now requires all Texas HVAC contractors to use these design guides and provide Manual J heat load calculation reports on installations, we are seeing many contractors plugging information into Manual J software to calculate tonnage without understanding the implications of the information they entered or the results they are given. Often times Manual J is the only thing used to determine size, but Manual RS and Manual D aren’t taken into consideration as well. The size of a system goes well beyond tonnage. Simply inputting data into software without considering the other two design guides often results in inefficient incorrectly sized systems that don’t comfortably cool homes, cause higher light bills, and break down more often.

Common Problems Caused by Incorrect Application

  • Short cycling – Large units cooling smaller homes only run for a short time before reaching the set temperature before shutting off. Units that frequently turn off and on tend to have a shorter lifespan.
  • A cooling unit that is too large for a home can result in humidified air, because the unit only runs for a short amount of time before shutting off. This can leave the air in your home humid and sticky, and may not feel as cool as it would with a unit that runs for a longer period of time.
  • Equipment that is undersized based on underestimated loads is generally unable to keep up with the home’s heating & cooling needs. The system may then be required to work overtime, resulting in inefficiency.

Expert Evaluation

Our licensed design professionals utilize ACCA-approved software to determine Manual J® full residential load calculations for each individual room of your home. Our staff is trained to evaluate structural load to calculate cooling and heating equipment size as applied to your specific home and conditions to achieve the most energy efficient central cooling system possible.

Houston’s year-around hot and humid climate require the highest performing HVAC units possible to keep homes cool & comfortable and prevent breakdowns. Elements including ductwork, structure & windows, lighting, and much more play a role in determining accurate load calculations. Let the experts of Aardvark A/C & Heating make your home comfortable & efficient!

Request a technician or call us at 281-955-1163