What to Expect During a Manual J Load Calculation

House diagram for Manual J load calculation

This article is Part 3 of our series on Manual J load calculations for new HVAC installations.

So far, our primer on Manual J load calculations has covered why you should get one and how to choose the right contractor. But when it’s go-time, what actually, you know… happens?

Here’s what to expect when a qualified contractor visits your home to perform a load calculation.

1. Your contractor is going to take a lot of measurements.

Measuring Tape

And we’re talking lots. While the bulk of the HVAC industry might recommend an HVAC system based on square footage “rules of thumb,” a heating and air expert is going to use real numbers and real data to find out what size furnace and AC you should have.

Getting numbers and data, of course, requires a lot of tape measure gymnastics. Your contractor will measure things like:

  • Room-by-room or level-by-level square footage
  • Ceiling height in different areas of your home
  • Window and door sizes
  • Insulation R-values in your attic, crawlspace, and walls

In addition, the contractor will note the type of materials used in the construction of your home and how well they’re insulated. A single-pane window, for example, is far less capable of inhibiting the movement of heat than a double-pane window with insulated glass.

And when that window faces West, that makes a difference, too. Sun orientation affects the heat load, and it’s a major consideration in performing the calculations.

Of all the measurements we take during load calculations, ceiling height and attic knee wall specifications are two that have a bigger impact on the load than you might imagine. After all, a living room with a flat, eight-foot ceiling will hold far less conditioned air than a vaulted one that’s eight feet high at one end and twenty-three feet high at the other! And attic knee walls can dramatically alter indoor temperatures when they aren’t properly insulated.

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 with batts? Is it air sealed?
  • The type of roof you have, whether it’s made of asphalt shingles, metal, or something else

Needless to say, it could take a few hours for your contractor to study and measure all of these attributes.

2. You’ll probably need a blower door test.

Blower door test

Blower door tests are almost always necessary for load calculations performed prior to an HVAC retrofit. This test reveals your home’s biggest sources of air infiltration as well as how much air might be infiltrating under different conditions.

This matters because air infiltration impacts how much heat your HVAC equipment will need to produce or remove. Homes with more infiltration might need a larger system. Homes with less infiltration might need a smaller one.

The only circumstances under which a contractor might omit this step include:

  • Load calculations for a single story in a home with multiple HVAC systems
  • Airtight new construction with minimal air infiltration

When you’re only doing one area of a home with more than one HVAC system, the results of the blower door test might not be particularly useful. The blower door pressurizes the entire home, so it’s hard to produce useful readings for a single area.

3. All of the measurements go into a specialized software application.

Entering data into a software application

You’ll probably see your contractor keying data into a mobile device or laptop. That device should be running an ACCA-approved application for load calculations. We use Wrightsoft, but there are others.

Could your contractor just use a calculator and a pencil and paper to do your load calculation? No.

If you see a contractor only using a pencil and paper to complete your load calculation, you’re not getting a real load calculation. Everybody uses special software for Manual J. Otherwise, it would take far too long to actually get the results.

In our world, up-to-date Manual J load calculation software = essential.

4. Old school technology helps, too.

Graph paper

Ok, so you actually might see your contractor using a pencil and paper, but only in addition to the load calculation software.

At PV, our home performance consultants frequently make sketches on graph paper as they work through the calculations. Is it simple and old school? Sure. But it’s also the best way to ensure you’ve properly accounted for all the shapes, lengths, widths, and depths that you’re plugging into the software.

5. Designing the system for defaults vs. designing it for you

Smart thermostat set to 72 degrees

Here’s a sign you’ve chosen a good contractor: They ask how cool you’d like your home to be in peak summer and how warm you’d like it to be in peak winter.

You see, the ACCA’s Manual J is set up to calculate the right size HVAC system for certain default temperatures. If you go by the book, you’ll end up with a system that cools your house to 75 degrees on the hottest day of the year and heats it to 70 degrees degrees on the coldest day.

That’s fine for most people. But if it’s not fine for you, your contractor should consider your preferences.

Stricter comfort preferences may require that you get a larger system than Manual J specifies. Or not. Regardless, your contractor should definitely ask.

6. Peace of mind that you’re getting the right size HVAC system

Thumbs up

That’s the final result of your Manual J load calculation. You’ll know exactly what size AC, furnace, and/or heat pump is right for your home.

We find that people are often surprised that the Manual J suggests they purchase a smaller system than the one they have today. But in an environment where HVAC oversizing is rampant, it’s a pretty common outcome.

Whatever the result, you’ll know you’re making a smart buy – not crossing your fingers that an HVAC contractor who eyeballed your home knew which system was the right system.

You’ll also be as comfortable as possible in the years to come.

Image credit (blower door): Tõnu Mauring

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