What Is A “Manual J” Calculation and Why Do I Need One?

What is an ACCA Manual J load calculation?

Manual J is the book created by the Air Conditioning Contractors Association that holds all the tables, charts, measurements, and definitions for determining the heat load in a living space. A Manual J calculation is simply running a heat load for your home. 

What Is A Heat Load?

A heat load is a measurement of how many BTU’s (British Thermal Units) of heat need to be added or removed from a structure to maintain a specific design condition. Put more simply, it is how much heating or cooling we need to provide your home to keep you comfortable. 

Different Variables We Must Consider

Every dwelling leaks air to some degree or another, which means that heat gets in during the warmer seasons and cold air gets in during the colder seasons. The question is, how much?

Things like the type of windows, how many panes of glass, what the frame is made of, how old they are, the type and size of doors, how much insulation, do you have a fireplace, etc. are all factors. 

According to Manual J a single fireplace leaks 20 cubic feet a minute of air into and out of the home. That’s 600 cubic feet of air an hour. This absolutely affects the ability of your heating and cooling system’s ability to keep you comfortable. 

When trying to determine the correct size of the system for your home we need to take all of this into account. 

We also have to account for anything that adds heat within the home as well, such as appliances. Do you have a large kitchen with a double oven that is used frequently? 6 burner stove top instead of 4? Do you have any terrarium with heat lamps for any pets? All of these add extra heat that needs to be factored in. 

Not to mention other factors like ceiling height, attic insulation, is your floor a cement slab on the ground or is it pier and beam that is raised off the ground?

All of these play into the final calculation to help us determine how much heating and cooling to add to your home.

Hasn’t This Already Been Done for My Home?

We get asked all the time why we need to do this again when the home already has an HVAC system installed and since the builder did this when they constructed the home. 

This is a great question. Many companies look at your existing system, find that it is a 4 ton, and just replace it with a 4 ton. We don’t.

The reason we run a heat load calculation every time we change out a system is because homes deteriorate and conditions change. You may have changed the windows, remodeled, changed the floor plan, etc. When we change out your system we warranty our work, which means we are warrantying your comfort. This means we do not want to assume the builder, or last HVAC company, or previous homeowner, did it correctly. We want to do it ourselves. This way we can have confidence that the correct system was installed and that you will always be comfortable. 

If you have any concerns about the proper sizing for your home’s heating and cooling system give us a call, 817-200-7215.

If you are wondering whether you should repair or replace your existing HVAC system check out our blog here to get some ideas about what questions you should ask.

Choose Prestige Air to Keep Your HVAC Running Optimally

If you experience problems with your heating and cooling call Prestige Air. Our expert service professionals have the training and experience to keep your HVAC system in tip-top shape all year long. Call (817) 200-7215 to make sure your heating and cooling are ready for the season!

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