Air conditioner sizing rules of thumb must die. That’s what I wrote in my last article. The most common rule of thumb is to use 500 square feet per ton to determine the size of air conditioner needhttps://www.energyvanguard.com/blog/more-on-air-conditioner-sizing-rules-of-thumbed. Or 400 or 600 or some other number in that general vicinity. In my article, I showed a graph of air conditioner sizes, given in square feet per ton, for 40 homes that we did load calculations for. The average of the 40 was 1,431 sf/ton. Now, let me say a bit more about that.
Here are a few points that came up in the comments to the article and on LinkedIn and Facebook as well as some takeaways that I feel need to be emphasized.
1. No matter the number, you can’t use square feet per ton to size air conditioners. I posted the square feet per ton results we got from 40 Manual J load calculations in hot and mixed climates. The average was 1,431, but you can’t use that to size air conditioners. You have to do an actual load calculation. Those 40 results ranged from a low of 624 to a high of 3,325 sf/ton.
2. If you tell me your load calculations average 400-600 square feet per ton, I assume you’re not doing them correctly. Is it possible that homes meeting current building and energy codes need that much air conditioning? Yes. If they have a lot of window area, they face west, and are in states with weak codes. My friend Mike MacFarland of Energy Docs in California gets 1,500 sf/ton for retrofits and achieved 3,350 sf/ton for a new zero energy home. And their design temperature is 102° F so don’t tell me this doesn’t apply in Florida or Arizona.
3. You need room-by-room load calculations to get the air flow right. Contractors who use whole-house rules of thumb often don’t get the right amount of conditioned air for individual rooms.
4. The square feet per ton you get from Manual J still gives you an oversized system, even when you do it correctly. Mike MacFarland says he finds it 20-40% too much capacity. David Butler says 15%. In my own condo, for which I’ve measured AC runtimes for the past two years, I’m seeing about 75%. (I’ll write more about my condo soon.)
5. It’s easy to get whatever load you want when you do a Manual J load calculation. I’ve written about this before. Putting in the wrong values for windows is an easy way to add load, as is putting in too many people, using exaggerated design temperatures, and the wrong orientation. If you want 500 sf/ton, it’s not hard to produce a Manual J load calculation that gives you that number.
Designing an HVAC system starts with proper sizing. Look at the square feet per ton number you get to see if you’re in the ballpark. If the number is less than 1,000 sf/ton, there’s a good chance the number is wrong.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the fall of 2022. You can follow him on Twitter at @EnergyVanguard.
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