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Load Calculations: Imperative Tools for Correctly Sizing HVAC Systems

When you’re replacing the HVAC system for your home, the most important step that your contractor will take is the cooling and/or heating load calculation. Your home’s conditioning load has a number of components that relate to its energy efficiency, your lifestyle, and your home’s size, design and orientation to the sun. The software applications contractors use are Manuals J and D. Manual J calculates your home’s heating and cooling loads, in order to choose the right-sized equipment, while Manual D calculates ductwork size and design using information from the load calculation.

Manual J takes these factors about your home into account:

  • Cubic footage
  • Number of windows, their efficiency and placement
  • Insulation levels in the attic and walls
  • Air infiltration rates
  • Layout of the home
  • Orientation of your home to the sun
  • Heat-producing appliances indoors, including the type of lighting you use
  • Number and age of home’s occupants
  • Preferred indoor temperatures
  • Landscaping factors.

If the contractor doesn’t take these elements into consideration, your system likely will be the wrong size. A system that’s too large will short-cycle, which means it runs for short periods, repeatedly. These frequent start-ups increase your energy bills, leave more humidity behind when cooling, and decrease indoor air quality. If the system is too small, it won’t be able to effectively cool or heat during extreme weather.

The second aspect of the load calculation uses the results from Manual J to accurately size the ductwork, using Manual D. The contractor should carefully inspect the ducts for air leaks and adequate insulation. The HVAC equipment relies on proper airflow from the air handler through the ducts to reach peak performance. When ducts are too small, they will be noisy. If they’re too large, the flow may be sluggish, increasing the running time of your equipment.

Conducting the load calculation before selecting new cooling and heating equipment helps you lower the size you need by fixing air leaks and installing more attic insulation, driving down the cost of the equipment and its operating costs over the system’s life.

If you’d like more information about a load calculation for your Orlando area home, please contact us at Rinaldi’s Energy Solutions.

Our goal is to help educate our customers in Orlando, Florida and surrounding areas about energy and home comfort issues (specific to HVAC systems).  For more information about load calculations and other HVAC topics, download our free Home Comfort Resource guide.

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