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Load Calculations: The Most Important First Step Before an HVAC System Upgrade

Whether you’re making an HVAC upgrade or installing the first system in your new home, it’s important to include load calculations on a pre-installation checklist. These calculations are what determine the size of a new furnace and air conditioner or heat pump. The consequences of skipping formal load calculations and sizing new equipment incorrectly are severe.

To avoid these consequences, hire a professional to do load calculations. The technician will take many factors into account, such as:

  • Climate
  • Square footage
  • Home orientation
  • Airtightness
  • Insulation levels
  • U-value of the windows
  • Window size and location
  • Occupancy level
  • Internal heat gain

Knowledgeable contractors use Manual J to size the heating and cooling equipment itself alongside Manual D which sizes and helps determine the design of the ductwork.

Here’s what you need to know to ensure load calculations are performed correctly by your technician.

  • Don’t rely on rules of thumb. Some HVAC contractors skip formal calculations, believing that knowing the square footage and climate are enough to size HVAC equipment accurately. Some contractors even forgo these rules of thumb and choose new equipment that simply matches the size of the old equipment. However, since code requirements have tightened in the past decade or two, homes are tighter than ever, and “x-tons per square feet” is sure to result in an oversized system.
  • Get all the information upfront. Gather all plans and specifications for the home. If the numbers needed to perform the calculations in Manual J are missing, track them down or do tests to acquire them. It’s important to obtain correct information upfront for load calculations.
  • When it doubt, test. If you’re not sure of the accuracy of estimated insulation R-values or other figures to do with airtightness, request a home energy audit from your HVAC contractor. The audit involves a blower door test and infrared cameras to spot leaky areas. You can fix these leaks to improve home efficiency and possibly qualify for a smaller, more affordable HVAC system.
  • Utilize available tools. The technician should keep a hard copy of Manual J for frequent reference while performing load calculations. When it comes time to design the ductwork, the same should go for Manual D.
  • Trust the numbers. All parts within your home are interconnected, which is why so many factors go into load calculations. Having said that, some technicians think they’re doing homeowners a favor when they purposely round up. This deliberate miscalculation could cause your system to be drastically oversized. Technicians need to trust their calculations and size the system based on what they find for the greatest efficiency and affordability possible.
  • Bigger is not better. A little extra horsepower in an oversized system merely serves as insurance on extreme days, right? Wrong. An inefficient system operates with short cycles that lend themselves to the following problems:

Noisy operation
Uncomfortable, clammy interior
Overly humid environment, which is a perfect breeding ground for mold and mildew
Premature system failure caused by greater wear and tear
Higher operational costs
Frequent thermostat adjustments

Don’t Lose Sight of the Three Primary Design Factors

While conducting the calculations necessary to determine the heating and cooling load, a technician must keep the following three factors in mind:

  • Home location and orientation – These play a tremendous role in the calculations. The same house plan in the same city could have different heating and cooling loads based on directional orientation, elevation and solar gain through windows.
  • The home’s thermal enclosure – This, which includes insulation levels, airtightness, window U-values and shading, is essential to consider as well. Keep in mind that in more enclosed homes, solar gain through the windows imparts a greater relative impact on load calculations.
  • Internal loads – These factors, from the number of people living in your home to the electronics, lighting and heat-generating appliances, are a key part of load calculations. In addition, to help the HVAC system perform as efficiently as possible, the ductwork should be installed within the home’s outer envelope so it can avoid passing through the unconditioned attic, garage or crawlspace.

With energy costs continually fluctuating in an upward direction, there’s no reason to skip load calculations. Taking the time to formally calculate equipment size is sure to improve comfort and lower energy bills from day one.

For more information on load calculations and other heating and cooling topics, please contact Comfort24-7. We proudly serve Chicago residents, as well as homeowners throughout northwest Indiana and southwest Michigan.

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