Hey Ed, What Is the Correct Airflow for an AC?

In this edition of "Hey Ed," Ed talks about the correct airflow for AC.

Hey Ed, what is the correct airflow for an AC? The correct airflow for an air conditioner is...well, is it 400 CFM per ton? So, I’m going to say that for the majority of the people, that was the first number that popped into their head. Could it be? Sure. Is it most likely? No. The correct airflow for an air conditioner will be one that meets the sensible, meets the latent, and doesn’t exceed the total by 15, 20, or 30 percent, depending on the compressor technology. The 15 percent is associated with a single-stage compressor, two-speed for 20 percent, and 30 percent the VRV technology. You look up the expanded performance data for your area design conditions where you will use a 62-degree web bulb or 75 degrees, 50 percent RH in the house. And in one of those columns on that expanded performance data, we’re going to be able to meet those requirements, meet sensible, meet the latent, and not exceed our capacity based off of what Manual S® sets. And that’s what the airflow for that system is going to be. There’s some guidance in Manual S® where it literally says ballparking airflow, and it gives us some math to do. And essentially that math gives us a way to look in the middle column, which is the 400 CFM per ton in the expanded performance data. Then we have a column that has airflow that’s greater than 400 CFM per ton. And then, we have a column that’s less than 400 CFM per ton. Generally, what you’re going to find is a house with a sensible heat ratio of .75 or lower, a leaky house is going to benefit from less than 400 CFM per ton. Typical house in my market is going to come in with a sensible heat ratio. .888.89, approaching .9, it is going to benefit from bigger airflow, greater than 400 CFM per ton. If you’re coming in around a .8, up to about a .85 for the sensible heat ratio, they’re going to benefit from 400 CFM per ton. The calcs that I’ve done from coast to coast, from Canada to down to Texas over the years, a very small minority have come out between that .8 and .85 number. I generally see if it’s new stuff, it’s .9 or greater, bigger airflow is king. Existing houses that’s a train wreck, smaller airflow is king. But that 400 CFM per ton, just generally doesn’t come up in the design work that I do. Your location might be different. If you’re using rules of thumb, you’re probably using 400 CFM per ton. That’s the way I see it.

Ed Janowiak is the Manager of HVAC Design Education at ACCA.

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