This is part 1 in an ongoing series on HVAC load calculations.

The folks at SNIPS have asked me to do a basic how-to article about HVAC Manual J load calculation as that is allegedly a big part of my field of expertise. As MiTek-Wrightsoft's senior trainer, teaching HVAC contractors correct load calculation is indeed a big part of my job.

However, every time I try to write some tips on load calculation down, I end up with an overly long and significantly worse version of Manual J — which is not what you, SNIPS, or the fine folks at the Air Conditioning Contractors of America (publishers of the Manual J) would want me to do. So, I am going rogue.


What You Should Already Know About HVAC Load Calculation

The following is not step-by-step instructions on HVAC Manual J load calculation. That already exists in ACCA's HVAC Manual J load calculation manual. (However, HVAC contractors in need of a quick primer should bookmark this article.)

What you’re about to get instead is a series of articles (because I am a glutton for punishment, and I hope you are, too) on understanding Manual J Load calculation and how those calculations work for a residential HVAC space.

We will go over some very general things about HVAC load calculations. Then the plan will be to take each of those things and look at them in a little more detail.

Heat out. Heat in. Moisture in. That’s it.


What HVAC Load Calculation Does, What It Doesn't

First things first. An HVAC load calculation does not tell you what size system you need (at least not directly). I’m serious. Load calcs are not about the equipment. A load calculation is about the building.

Once you know about the building then you can make decisions about equipment. And, yes, I am aware that this seems like the kind of pain-in-the-butt technicality that got you ½ credit from Mr. Thompson, your 8th grade science teacher, and drove you crazy.

Doesn’t matter. This distinction is going to be important (really, really important for some, but we’ll dive deeper into that later). It’s enough to say for now, that it’s a good idea to remember that the load tells you three things:

  • How much heat a building loses during one cold hour of a cold day.
  • The heat a building gains during one hot hour of a hot day.
  • Finally, how much moisture the building gains during one hot hour of a hot day.

Once you know how to determine those three things like the back of your hand, you can use them to find the right equipment. However, there are some very important extra steps in between. It’ll be the last thing we talk about in this series, but it’s important to establish it right up front.


Understanding Heat Transfer In Heat Load Calculation

The first horseman of energy loss: Conduction.

Remember Mr. Thompson from a paragraph ago? (Or, in my case, from the Devine Middle School) Good. Do you remember when he taught you that there are three types of heat transfer? Doesn’t matter if you do or don't. There are, and he did.

So yeah, it turns out you are going to use that in real life. And, even if a little piece of you might die inside to admit it, most HVAC contractors use this information regularly. There’s no helping it. So, I’ll make you a deal: I won’t tell Mr. Thompson he was right if you won’t.

(To the real Mr Thompson, we both know you were right. You were awesome, and we owe you a lot).

residential HVAC space heat transfer

Of the three types of heat transfer, conduction is usually the first thing we teach. The math is pretty simple (three numbers multiplied). The concept is pretty basic: If a barrier (like a wall) separates something hot from something cold (like a warm house on a cold day), the heat will move from hot to cold.

How much heat moves depends on how big the surface is, how well it is insulated, and how hot and cold the two sides are.

In the first deep dive in our series, we’ll learn more about conduction: how to check/determine insulation levels in the field. Also, how adding an R2 foam board to an uninsulated wall could make thousands of btus of difference while adding the same R2 to an insulated wall might only change things by a few hundred. Then, what some of the other factors are in calculating conduction such as construction weight.


If You Have An Oven, You Can Understand Convection Energy Loss

The second horseman of energy loss: Convection.

You might be more likely to remember convection from your Home Economics class than a science class. Or, if you’re too young to remember Home Ec, maybe it’ll ring a bell from your kitchen. That fancy new air fryer you bought for Christmas back in 2017? I hate to break it to you, but it’s just a convection oven (a technology that’s been widely available since the 1950s).

Convection is just heat transferred by fluid movement. Stick a fan in a toaster oven and … boom. You’ve got an air fryer (a.k.a. a small convection oven). Why does the fan make a difference? For the same reason the wind makes you feel colder on a winter day. Your body is warm. The air is cold. That means heat is moving from your skin to the cold air around it.

Now where is that heat? In the air immediately around your body. Which means the air next to your skin is now slightly warmer. Which also means your body will now lose less heat to that air (because it’s slightly warmer). Then the wind blows. Bye-bye air, and bye-bye heat in that air. Now the air next to your skin is cold again.

You feel colder because you are colder. You’re losing more heat. The temperature of the air (big picture) outside hasn’t changed, but the temperature of the air next to your skin has.

The same thing happens in an oven in reverse. The thing you’re cooking absorbs heat from the air around it and cools the air immediately surrounding it down. In a convection oven, a fan continually replaces that air with fresh hot air.

What does this have to do with HVAC and load calculations? A lot. Tight v. leaky houses? That’s all convection. Ventilation? Duct leakage? Convection. Indoor humidity? Almost all convection. But there’s some other stuff like improperly insulated bonus rooms and half-story areas that are impacted by convective forces too.


The Third Horseman of Energy Loss: Radiance

I thought it might be funny to write out some of the real physics level formulas for radiance just to scare some people. So I did a little googling. An hour later I had to pour myself a drink, take a cold shower, and have good lie down in a dark room. I’ll be okay. I think. But suffice to say we won’t be discussing too much of the math with this one.

With all due respect to Mr. Thompson, I’m not sure even he could have handled that stuff. But that doesn’t mean we won’t discuss radiance itself (and it should be noted that the Manual J math isn’t nearly as intimidating as the physics class stuff).

ACCA Manual J spreadsheet

Download ACCA’s Manual J load calculation spreadsheet.

In many ways radiance is the most important of the three. Radiance is why bug screens can (but often won’t) shift the load on a building by over 1,000 btus. It’s also why attics are hot, why cold climates tend to put supply registers on outside walls, why big heating temperature setbacks can lead to comfort issues, and at the end of the day why we use software to calculate loads. Ahem like the kind of load calculation and design software we create at Mitek/Wrightsoft.


Other Load Calculation Variables

The fourth horseman of energy loss? I honestly don’t know. Maybe moisture migration? Eh. It’s a reach. Truth is, I got the whole “four horsemen” idea stuck in my head, so I ran with it.

Manual J doesn’t require that you calculate moisture migration if your construction includes a vapor retarder, though it does discuss it. Frankly, in my opinion, if a residential building has enough water passing directly through its surfaces to require taking it into account, you’ve got bigger problems than sizing an HVAC system on your hands.

So, I hope this has been interesting and helps you understand some of the basics involved with load calculation. But mostly, I hope it has made you want to read more on other essential HVACR topics as we learn to be better HVAC contractors together.

Read more on HVACR basics at