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Building Science

Manual J Load Calculations vs. Rules of Thumb

This sample of Manual J results shows that the old way really needs to die

Oversized air conditioners are often the result of a contractor using rules of thumb instead of load calculations for sizing.
Image Credit: Energy Vanguard

We do a lot of heating and air conditioning system design at Energy Vanguard. Alexander Bell, who goes by Andy, is our design wizard, and I’ve been getting involved with the process again lately.

When I talk to potential clients, a lot of them tell me their contractor wants to size their air conditioner using a rule of thumb. The rule is usually something like this: Install one ton of air conditioning capacity for every 500 (or 400 or 600) square feet of conditioned floor area. How far off are they? Let’s take a look.

A bit of our Manual J data

True HVAC designs always start with a load calculation. So we can look at the data. The graph below shows data for just a few buildings we’ve done in the past few years. Forty of them, to be exact. We’re in the process of putting all our data into a spreadsheet so we’ll have more to show later.

On the horizontal axis, I plotted the conditioned floor area, in square feet. On the vertical axis, I plotted the cooling load divided by the floor area, or square feet per ton. Remember, when HVAC contractors use rules of thumb to size air conditioners, they usually pick a number between 400 and 600 square feet per ton.

Here’s what our data show.

Note that not a single one of these load calculations was as low as the high end of the typical range used in rules of thumb. The low number on that graph is 624 square feet per ton. The majority of the cooling loads shown here are above 1,000 sf/ton. Only eight are below 1,000 sf/ton.

In case you’re wondering, I threw out the data for cooling loads in cold climates when I plotted this graph.…

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  1. Jody Keppers | | #1

    Unit size increments
    What I have often had mechanical contractors tell me is that trying to precisely size mechanical equipment is a waste of time because equipment isn't available that precisely fits the loads anyway. The equipment is sized in 20,000 - 50,000 BTU Increments, they'll say. I'm in Minnesota, so they're usually talking about heating rather than A/C, but the principle is the same. Is available equipment size something that needs to be considered?

  2. User avater GBA Editor
    Allison A. Bailes III, PhD | | #2

    Response to Jody Keppers
    Jody, yes, equipment size definitely needs to be considered. Yes, with low-load homes — and many new homes today fall into this category — getting equipment with a low enough capacity can be a problem. But that's no excuse to skip the important step of proper HVAC design. Here are a few reasons:

    1. Without a full room-by-room load calculation, you don't know how much heating and cooling each room needs. This is a common source of problems with rooms being too hot or too cold.

    2. The same floor plan can vary significantly in its heating and cooling load just by rotating the building and changing the solar gain. Then throw in differences in insulation, airtightness, window specs, and all the other factors that go into the load calculation and the only way to get away with skipping proper design is to greatly oversize the system. But that doesn't solve all the problems and it creates others.

    3. The load calculation is only the first step. Once you know the room-by-room loads, you do Manual S to select the equipment. The actual equipment size is usually different from the load. I wrote about this before here.

    You should never hire any contractor who says that proper sizing is a waste of time. They're wrong and their customers suffer for it.

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