Airflow Basics Part 2: The Lost Art of Choosing a Blower
You are going to change out an existing furnace that has a cracked heat exchanger. You find a functioning 3-ton evaporator connected to a 3–ton condensing unit. The cooling equipment seems to be in good working order. That size furnace is available with a 3–ton, a 4–ton or a 5–ton blower. What size drive will you order for the new furnace?
Did you connect a magnehelic or inclined manometer across the furnace to see what the external static pressure was? The most common cause of heat exchanger failure is inadequate return airflow. This would be a great time to do that. But that’s another can of worms we can open at another time.
If you’re going to choose a blower, then you need two things. You need the external static pressure that your blower will be operating against and the blower performance chart for that blower.
External static pressure is the measurement of all the resistance in the duct system against which the blower has to operate. The duct system offers its own resistance to airflow. Many people assign a value of .10” w.c. pressure drop per 100 feet of duct to account for that loss. Although this is acceptable in certain commercial design work, it is a serious error to assign any value to the friction rate in residential design work. But that is yet another can of worms that we will deal with in Part 3.
For now, let’s use .10” w.c. as our friction rate for both the supply and the return ducts for the purpose of demonstration. Let’s also use .30” w.c. pressure drop across our wet evaporator coil. Let’s use a media filter with a .20” w.c. pressure drop. Finally, we will use .03” w.c. pressure drop across our return grille, our open supply register, and our wide-open, branch–line balancing damper.
Now, if we add up all of the system resistances, it comes to .79” w.c. (see Figure 1). Next, go to the blower performance chart and get in the .80 column and drop your eyes down until you see the volume of air you want to handle (lets use 1200 cfm). There it is at 1232. By the way, that’s a 4-ton blower operating on medium speed, not a 3-ton blower.
Here’s the thing, all blowers are nominally rated at .50” w.c. on high speed. We weren’t operating against .50” w.c. We were working against .79” w.c. external static pressure.
If we had chosen the 3-ton blower in Figure 1, how much air would we have been handling? It would be about 1101 cfm on high speed, not 1200 cfm. Remember a lack of air means a lack of Btu’s. The Btu’s get to the areas of the home to be heated and/or cooled via the airstream. That’s a “not enough cooling call” waiting for the first hot day. Choosing the correct blower will save you those unfortunate calls.
Next time we’ll cover the myth of .10” w.c. duct system design.
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