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Building Science

How to Read Manual J Reports

If you know what to look for, it’s easy to check for accuracy

A typical Manual J report. This screenshot shows some of the data you'll get with a load calculation report. It's easy to be overwhelmed, but if you focus on the items discussed in this article, you can make sense of it.
Image Credit: Energy Vanguard

When you enter the world of building science — whether through building a house, becoming a home energy rater/building analyst, or just hanging out in cyberplaces like this — everyone talks about the importance of getting actual heating and cooling load calculations based on ACCA Manual J. A great number of HVAC contractors sell and install oversized equipment with air distribution systems that don’t work because these contractors base their choices on rules of thumb.

OK, but what if you hire a contractor or third-party designer to do Manual J load calculations and you’re not an expert and don’t want to be? Suddenly you’re faced with a bunch of seemingly indecipherable reports. How do you know if they’re accurate or not?

Fear not, dear reader. I’ve got some help for you today.

Don’t confuse load with capacity

I don’t think I can make this distinction often enough. Heating and cooling loads are not the same as the equipment capacity needed. I just did it in my last article, and now I’m doing it again. It’s that important. The first thing you need to know is that the term loads refers to how much heating and cooling the building needs and capacity refers to how much heating and cooling the equipment can supply. Here in the U.S., both are measured in British Thermal Units (BTU) per hour.

When you look at Manual J reports, you’ll see the loads. They’re shown separately for heating and cooling, and cooling is further divided into sensible and latent. When the contractor or designer picks a piece of equipment, they’ll have to go through a “derating” process to match the equipment’s performance specifications with the building’s loads. (For more on this, see “Manual J…

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  1. Jeremy Good | | #1

    Smart thermostat for additional baselining
    Reports from a smart thermostat can also help sanity-check a Manual J report when replacing an existing system when there aren't other major changes to the house.

    I recently got a Man J performed prior to choosing a replacement A/C unit. It was very helpful in terms of considering part-load dehumidification and, of course, confirming the system size. But the other big reason I got a load calc was for future boiler replacement. I know that my current one is oversized, but by how much? The report put heating load at about 60% of the net output of my current boiler. I used the Ecobee data from the coldest stretch this past winter to calculate the actual gas use. There's some error in this method since I wasn't measuring actual heat output, but it should be in the ballpark. (For single-stage appliances this is easy math.) There was good agreement between my calculated heat input and the design-day load for similar outdoor temps.

    Point being, the duty cycle info from a smart thermostat can be valuable in baselining a load calculation report or for a rough gauge of system performance.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Jeremy Good
    For more information on using historic fuel use data (or historic run-time data) to help size new equipment, see Dana Dorsett's article on the topic, Out With The Old, In With The New.

  3. Jeremy Good | | #3

    That's a good article,
    That's a good article, thanks. I've been in my house less than a year, so thermostat data gave me a much more precise estimate and in a target outdoor temp range. I have 12 months of the previous owner's utility info, though I have no idea what their habits were.

  4. Cramer Silkworth | | #4

    equation typo
    the equation should be AC capacity = CFA divided by sf/ton, not multiplied, no?

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Cramer Silkworth
    You're right. I've made the correction. Thanks for pointing out the error.

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