A mere 20 zones were below 700 square feet per ton. Of the 167 zones, only 53 came in lower than 1,000 square feet per ton. Cooling loads expressed as square feet per ton for houses in hot climates
SIZING HVAC UNIT CALCULATOR MANUAL
We use Wrightsoft’s RightSuite Universal to calculate the ACCA Manual J loads. All the homes included here are in hot climates, which I defined as having an outdoor cooling design temperature of 90 ☏ or higher. The data come from 75 homes, and the numbers plotted here are for the 167 individual zones in the houses. The graph below shows the breakdown for some of the load calculations we’ve done at Energy Vanguard in 2021. Recently, though, I went through our jobs from 2021 and redid the analysis. Also, I didn’t look at the equipment sizing data. I don’t recall how I filtered the data to exclude cold climate homes. I pulled out 40 homes in hot climates and found the average cooling load was 1,431 sf/ton. In a 2016 article, I showed some of our load calculation data from our HVAC design jobs. Basements are often closer to 2,000 square feet per ton-or higher!-for newer houses. But that applies to the above-grade parts of a house. If it’s 1,000 square feet per ton or more, it may be okay. If that calculation comes out close to 500 square feet per ton, I assume the installer didn’t do a load calculation, or if they did, they fudged the numbers. That’s just my quick check to see if a system might be oversized. I don’t mean that’s how I do air conditioner sizing. My rule of thumb is 1,000 square feet per ton for newer houses. Using the Manual J and S protocols, they probably would find the house needs only 2 to 3 tons of air conditioning capacity. In a 3,000 square foot house, for example, the rule-of-thumb installer might put in 6 tons of capacity. That means a contractor using 500 square feet per ton is installing an air conditioner that’s three times larger than it should be.
Even 1,500 square feet per ton or higher isn’t unusual. What we find is that most newer homes-even in hot climates-have square feet per ton numbers significantly higher than 400 to 600. By inverting, we get a nicer number 1,000 square feet per ton. For example, a 3,000 square foot house with a 3 ton air conditioner would have 3 ÷ 3,000 = 0.001 tons per square foot. Then, to factor in the size of the house, we could divide the load or capacity by the floor area, but the numbers would be tiny.
Sticking to BTU/hr, we can divide the cooling load by 12,000 to convert to tons. In the rest of the world, it’s kilowatts. In the US and Canada, it’s mostly BTU per hour. Cooling loads and air conditioner capacities are actually given in the amount of heat gained or lost per unit time. When we talk about square feet per ton, higher numbers mean lower cooling loads or lower air conditioner capacity. load calculation resultsįirst, a quick note on the numbers. It just scales the air conditioning system to the size of the house. It doesn’t account for any of the features of the house that actually have an impact on how much cooling it needs. It doesn’t account for airtightness or the insulation levels. The rule doesn’t account for the window type, orientation, or overhang. The rule is usually specified in square feet per ton (sf/ton).
Usually, the rule is based on the amount of conditioned floor area, and contractors in many areas generally use the rule of 1 ton of air conditioning capacity for each 400 to 600 square feet of floor area. Many contractors use rules of thumb to decide what size cooling equipment to install. It’s time to revisit one of my favorite topics: air conditioner sizing.
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