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Manual J Calculations: What and Why?

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If you’re installing a heating or cooling system in a single family home, you need to have a Manual J ‘right sizing’ calculation performed. In order to find out exactly how much heating and cooling any home needs (called the ‘Loads’), it’s critical to run an analysis of the air leakage, insulation, windows, and appliances- the instructions for running this analysis are laid out in a 500-page manual, called the ACCA Manual J. What can happen if we DON’T run the Manual J?

1. You buy too much equipment.

Imagine installing Arnold Swartzenegger in your house to give your kids access to the cookie jar on the shelf- “One cookie per day is your allowance,” you say.
Arnold lifts your children up much too fast and much too high- “NO, put me down!” your children cry as their heads bump the ceiling. Then he drops them much too fast, and they ask to be lifted to the cookie jar again, and the cycle repeats. Pretty soon you have throw-up on the kitchen floor even though zero cookies have actually been eaten.
This analogy is strange and disgusting, but it is 100% accurate. Your house only needs a certain amount of heating and cooling, and if you give it too much, the equipment starts up and shuts off a lot- it’s called ‘Short Cycling’, and the result is indoor air that’s cold and clammy, plus worn-down equipment.

2. The furnace and air conditioner isn’t properly sized for the ductwork.

This can result in temperature variations between rooms and even whistling noise in the ducts.

3. Not only are you uncomfortable and unhappy with the two other side effects above, you also paid more for the initial installation, and you’ll continue to pay more for equipment in the future, since it’s bigger!

When having a Manual J calculation done , you should either have a home performance firm like Green Dream Group (specifically a HERS Rater) do the analysis along with the blower door and duct tightness testing, or have the report reviewed by a HERS Rater. There are pitfalls and common mistakes made on these calculations, including incorrect assumptions about outdoor temperatures (called ‘Design Temperatures’), air leakage rates, and even floorspace calculations.

Manual J isn’t perfect, though- in a Building Performance Podcast interview with a world expert on moisture control, a few major downfalls of the calculation were pointed out to me. You can hear that Building Performance Podcast here .

All in all, it’s imperative to look before leaping into a $4,000 HVAC system, to think before acting. Have the HVAC analysis done and you’ll be certain there aren’t hidden side effects that are going to be plaguing your home for years to come.

You can learn more about home performance from John, and the rest of the team at the Green Dream Groups Building Performance Workshop at

John Bergman
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Posted In: Building Performance, Residential Buildings, Technical Tips

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