# Hey Ed, Will a Furnace With a Three-Ton Drive Really Move Three Tons of Air?

In This Edition of “Hey Ed,” Ed Discusses Will a Furnace With a Three-Ton Drive Really Move Three Tons of Air

Hey Ed, will a furnace with a three-ton drive really move three tons of air? It might. It might not. It depends on how much resistance to airflow this furnace actually has. Start from the beginning and talk about a three-ton drive. Three-ton drives that I'm going to talk about right now are going to address a standard induction motor, something that you might refer to as a PSC motor. We're not seeing any new furnaces like that come off the production line. More than likely, what we're going to find is a constant torque motor. It is an ECM product that is much more efficient than the standard induction motor but has similar airflow characteristics to that standard induction motor. Static goes up, airflow goes down. On the ECM product, not as significantly, but there are some similarities between the two. If we're somewhere around a half inch of external static pressure on a three-ton drive, I would expect to see around 1200 CFM going with a nominal 400 CFM per ton. If I see a small number, airflow could be 10 or 15, 20% higher. But when I see a big number, now, I'm going to define a big number of something around maybe .7 or .8 inches of external static pressure. I'm going to see airflow that is much less than that nominal 400 CFM per ton. So, at .7 or .8, I might see eight or 900 CFM out of that three-ton drive. Don't be mistaken with the model numbers. Look at the lower performance charts to find out what the volume of airflow coming out of that drive will actually be, and that's the way I see it.

In This Edition of “Hey Ed,” Ed Discusses Will a Furnace With a Three-Ton Drive Really Move Three Tons of Air

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

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Posted In: Hey Ed, Technical Tips

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