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What Is a Manual J?

If you’re looking at replacing an existing HVAC system, or installing a system in a new home your HVAC professional should perform a Manual J calculation on the home, and if they aren’t – you could be the one who pays the price!

Manual J is the calculation used to make sure you’re getting the right size HVAC system for your home. It’s a very complicated calculation and we don’t expect you to know how to do one, but you should definitely expect your HVAC contractor to know how to do it. Running a manual J for your home is extremely important so that you can install the right HVAC system, otherwise, your system may not run efficiently or have trouble heating or cooling your home.

Rule-of-thumb measurements or basing the size of your new system on what you already have is not best practice.

Simply stated a Manual J load calculation factors in the different characteristics of your home (such as square footage, position of your walls, building materials, number of windows, etc.) to determine how much heating or cooling capacity your home needs.

Here are some other factors that can make a big difference in your load calculation:

  • What's beneath the house? Is it a crawlspace, basement, or slab - and how well is it insulated?

  • The attic - Is it insulated? Is it air sealed?

  • The type of roof you have, whether it's made of asphalt shingles, metal, or something else

Once measurements are taken the information is loaded into an ACCA approved software application to calculate the heat load. The final results of your Manual J load calculation is peace of mind knowing that you’re getting the right size HVAC system. You’ll know exactly what size heating or air conditioning system is right for your home.

Some homeowners will be surprised to find that the Manual J suggests a smaller system than they have now, other may find that their unit it undersized to properly accommodate the home. Whatever the result, you’ll know you’re making a smart buy in purchasing a system accurately sized and designed to fit your home and family’s personal needs.

Contact us today for a FREE in-home design consultation to learn more about your homes heating & cooling needs.

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