Is a Manual J Load Really Necessary on Residential Retrofits?

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Is a Manual J Load Really Necessary on Residential Retrofits?

Is a Manual J Load Really Necessary on Residential Retrofits?

Why is it essential to provide a load calculation before retrofitting a new heating and cooling system in an older house? To make sure the right size systems get installed, use Manual S. To design the proper distribution system, use Manual D. Neither of these, though, can be performed without having done a Manual J calculation first.

Why does everything have to be so progressive? An industry full of real-world, detached experts tells residential installers that we should perform load calculations anytime we change a system. Sure, I can understand new construction mandates. Everything is drawn out and perfectly planned. But ask anyone who installs retrofit HVAC systems in 20- to 50-year-old homes and we’ll tell you nothing ever goes as smoothly as the salesperson told the customer it would.

Let’s discuss a few things:

  • I have been in the industry since 2010. Been through a few load calc classes and I still can’t get it right!
  • The 1% summer design temperature was 8% in 2022. Customers don’t care about a 30-year average.
  • Who has a design specialist at the average “mom-and-pop shop”?
  • The five questions to ask to bridge the gap between design software and the way that house will perform after the new system goes in.
  • Like this article, I have limited time to get in, present, and get out.

It’s Not Just Me

Three times, I attended a course from our local utility company on how to perform load calcs with popular design software. Over two days, we learned to draw the house, input R-values, U-values, insulation levels, shading, and other building envelope details needed to perform a Manual J.

I’ve run a successful, double-digit net profitable business that has grown year-over-year by 15% to 20% each year since it started. So I’m pretty savvy at learning, understanding, adapting, and executing tasks. But for the life of me, I cannot figure out how the numbers they have me enter in class compute to a 2.5-ton system for a 2,000-sqare-foot home.

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