Why is it essential to provide a load calculation before retrofitting a new heating and cooling system in an older house? To make sure the right size systems get installed, use Manual S. To design the proper distribution system, use Manual D. Neither of these, though, can be performed without having done a Manual J calculation first.

Why does everything have to be so progressive? An industry full of real-world, detached experts tells residential installers that we should perform load calculations anytime we change a system. Sure, I can understand new construction mandates. Everything is drawn out and perfectly planned. But ask anyone who installs retrofit HVAC systems in 20- to 50-year-old homes and we'll tell you nothing ever goes as smoothly as the salesperson told the customer it would.

Let's discuss a few things:

  • I have been in the industry since 2010. Been through a few load calc classes and I still can't get it right!
  • The 1% summer design temperature was 8% in 2022. Customers don't care about a 30-year average.
  • Who has a design specialist at the average "mom-and-pop shop"?
  • The five questions to ask to bridge the gap between design software and the way that house will perform after the new system goes in.
  • Like this article, I have limited time to get in, present, and get out.


It's Not Just Me

Three times, I attended a course from our local utility company on how to perform load calcs with popular design software. Over two days, we learned to draw the house, input R-values, U-values, insulation levels, shading, and other building envelope details needed to perform a Manual J.

I've run a successful, double-digit net profitable business that has grown year-over-year by 15% to 20% each year since it started. So I'm pretty savvy at learning, understanding, adapting, and executing tasks. But for the life of me, I cannot figure out how the numbers they have me enter in class compute to a 2.5-ton system for a 2,000-sqare-foot home.


Cooling Design Temperature

Manual J is designed on the premise that its calculation will condition the home 361 days a year. (OK, it's based on hours.) What about the other four days? Is it not important to install a system that will "keep up" on the hottest days of the year? So in Sacramento, we're supposed to design around a 102° design temperature. According to ACCA, that should condition the house just fine 99% of the year based on a 30-year average. That equals 102° in Sacramento County.

On average, we have 22 days a year over 100° and five days a year over 105. Take a look at 2022's numbers:

  • Twenty-nine days over 100°F
    1. May – 3
    2. June – 5
    3. July – 4
    4. August – 8
    5. September – 9
  • Eighteen days over 102°F (design temperature) — 5% of the year, not 1%.
  • Three days over 110°F

As much as Manual J does for adequately conditioning the house 99% of the year, it's that 1% of days, or in 2022's case, 5%, when that homeowner is going to call you or go online to vent because their system can't keep up. And 2022 wasn't an anomaly. It's only getting hotter each year.

Customers don't care about 30-year averages. They care about today. I refuse to try to explain that I installed the correct system for 95% of the year to someone who spent $20,000.00 with us.

Usually, the first thing I hear back from the design experts is that we have to "educate" them that this is OK and why it's so. In my opinion, that's not logical. Mr. Jones wants to be cool in his house. That's why he spent good money with us to buy a new system.

Another thing I hear is to go ahead and nudge the 102° design temperature to reflect real-world conditions more adequately. If I do this, I'm manipulating the numbers to just put in the size system I want to install anyway. Then why all the extra time, money, and energy spent on training on proper load calculations?


A Fine Line to Walk

Here's what I know for sure: Of all our online reviews on Google, Yelp, Nextdoor, and Facebook, none say anything about us installing a system that can't keep up on those 105 to 110° days. Does that mean I oversized it per ACCA specifications? Yes. But who's getting the review, ACCA or me? Me.

Let's say I did buy into the ACCA-approved Manual J, S, D method for sizing up and installing the right unit and air delivery system. How can I expect any ol' salesperson to join my team and grasp it before their first sale?

I was once chosen by a company I worked for to take the BPI certification class — a week-long course! But it did teach me everything about the home's envelope, tests to perform, building leakage tests, R-values, U-values, and how to make it perform better. Am I supposed to be able to teach this to every new salesperson who works for me?

Adding to that, the majority of residential HVAC companies around America are your average "mom and pop shop”; who really has a design specialist around at? Someone who is basically an expert at using the software to perform not only the Manual J but the S, D, and T as well?

Have you ever heard of the game "Telephone"? How much detailed information will be lost between the notes of the salesperson and the person incorporating them into the Manual J software program and on to the installers at the house?

Just getting my salespeople in and out of a house in around 90 minutes is tough enough without having to come up with the following details:

  • Heat gain through exposed walls.
  • Conduction through doors and windows.
  • Sensible and latent gains for lights, people, and equipment.
  • Sensible and latent infiltration gain at all of the windows, doors, and cracks.
  • Ceiling gain.
  • Gain through attic knee walls.
  • Sensible and latent appliance loads.
  • Fresh air ventilation.
  • Solar gain at windows around the house.


We Have to be Practical.

Five practical questions bridge the gap between design software and the house's performance after the new system goes in.

We can actually ask people these questions instead of assuming the following will be done anytime soon:

  • When this last system was in its prime, did it heat and cool your home to your desired temperatures?
  • Are you happy with the airflow to all of your rooms?
  • Do you plan on blowing in some insulation anytime soon?
  • Have you recently or are you planning on upgrading your home's windows?
  • Are you planning on cutting that big tree in front of your house anytime soon?

Then we can measure the house room-by-room and get the actual conditioned square footage of the house. While doing that, we can visually assess the windows and orientation of the house. We can determine the house's age, the house's siding, and the roofing type. We can pop into the attic and assess the ductwork and insulation levels. This data, plus the questions we've asked the homeowner, make it very easy to accurately determine the size system we will install.


Time Constraints

Like this article, I only have a limited time to get in, present, and get out. Industry experts say it's about 90 minutes for an HVAC sales presentation. We are trying to build value into our proposals and the whole time can't be spent explaining why Manual J is so important and why we are better off doing it. This customer faces a real-world dilemma: "Do I pay for these repairs today or just buy a new system?"

While in the home, we are also trying to build value in our proposals by telling the customer:

  • Who we are;
  • Why we choose the brand we install;
  • Any extras that will come with their job;
  • The reason why we need to enlarge their return air system;
  • The benefits of indoor air quality products we sell;
  • The importance and value of insulation;
  • The importance and value of whole house fans we install;
  • How the installation process goes;
  • Our Bronze, Silver, and Gold system options;
  • Financing options; and
  • Other general questions that arise from someone who is trying to decide between a single-stage, two-stage, or variable-speed system. Or whether they should be switching over to a heat pump to get that rebate or tax credit, or stay with that natural gas design they already know and have been comfortable with for the last two decades.

Do you really want a salesperson in your home for three hours, inundating you with information you hardly grasp? Probably not.

I'm naturally drawn to playing devil's advocate on topics I'm passionate about. I truly want to do what's right for the customer. Does that mean performing a Manual J load calculation is needed in residential retrofit applications? For the industry energy experts sitting behind a desk, yes. For those of us in the real world, it doesn't make sense. I have way more to do with my 90 minutes.