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Why You Need HVAC Manual J Load Calculations

Would you buy a pair of pants twice your size so you have that extra capacity just in case you might need it? No? Did you know that most new homes have a similarly ridiculous case of over sizing? It's true. Look no further than the HVAC system to find it.

That's because HVAC load calculations require time and attention to detail, and most HVAC contractors rely on rules of thumb to determine the sizes of the cooling systems they install. Usually it's based on square footage of conditioned floor area, and contractors in many areas generally use about 500 or 600 square feet per ton as their rule. But every house is different. Even the same house rotated ninety degrees could vary in cooling load by 25% or more.

You may think that having a bigger system is better, but the Texan way doesn't always prevail. In the case of air conditioning, oversized cooling systems result in:

  • A clammy house because they don't run long enough to dehumidify the air
  • Shorter lifetime for the system because it turns on and off frequently (also called short cycling). Since it's so big, it doesn't take long to cool off the house.
  • More expensive to install. That extra size isn't free.

Another reason to installed right-sized HVAC systems is that the ENERGY STAR new homes program requires no more than 15% over sizing of cooling systems.

The correct way to size an air conditioning system is with Manual J, a protocol developed by the Air Conditioning Contractors of America (ACCA). Manual J HVAC load calculations determine how much cooling a house actually needs. It used to be done by engineers with pen, paper, and a slide rule, so you can see why contractors developed rules of thumb. Now it's almost always done with computer programs.

Here at Magnolia Heating and Cooling, we use Right Suite Universal by Wrightsoft, shown in the image below. Contact us by clicking here if you'd like more information about how we can help you with your HVAC project.

HVAC Project 



When we do a Manual J HVAC load calculation, we enter all the relevant data, such as the home's orientation, insulation levels, window types, areas of all the surfaces that gain or lose heat, and more. The output of the calculation is how much cooling and heating the house needs in Btu per hour for each room, each zone, and the whole house.

What we find is that most new homes come in no lower than 800 square feet per ton, referring back to the rule of thumb preferred by many contractors. High performance homes can be as high as 1500 or 2000 square feet per ton. That means that a contractor using 500 square feet per ton is installing an air conditioner that's 2, 3, or even 4 times larger than it should be.

With a room-by-room load calculation, we can use the results and the equipment specifications to select equipment and design a duct system that will perform at its maximum efficiency using ACCA's other design protocols, Manuals S, T, and D.

Contact us and we can help you get an accurate HVAC load calculation for your next project.

Manual J load calculations are the hallmark of a properly designed system. This calculation allows us to specially design a comfort system for your home. When we come out to design your new comfort system we take a look at the whole home.

We check:

  • Your homes orientation to North
  • The size of each room by completing an accurate drawing of your home
  • What type of roof is on your home
  • If vapor barriers are installed
  • The condition of your air delivery system and its r-value
  • What type of insulation you have in the attic and how thick it is
  • The insulation in the walls and how thick the walls are
  • What type of windows you have
  • The size of each window
  • For over hangs above the windows
  • How many people live in the home
  • What type of kitchen appliances you have
  • What kind of floors are in your home

We take all the information above plus a number of other factors and are able to design the perfect system for your home. The software we have selected is wrightsoft. We have consistently designed comfort systems using this software for over 8 years. The results are a perfectly designed system every time.

It is not at all uncommon for me to complete a load calculation on a home and find that the existing system was oversized. I also find that the other contractors recommended the same size system or bigger. (Bigger is not better).

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