The fan turns and the air moves, right? But there are a few basic principles governing airflow with residential constant speed fans that are frequently misunderstood, so let’s get back to the basics and see what we can learn.

Supply CFM = Return CFM
In a duct system, the supply and return airflow are rarely equal when airflow is measured at the grilles. The reason the supply and return airflow is unequal is that every duct system has some leakage.

The difference is not because of the fan. When it comes to the fan, the amount of air it takes in and throws out is always the same.

When discussing system performance, one phrase I often hear is: “Airflow is low on the return side because of high suction pressure at the fan.” Although it sounds good, and most of us may have believed it at one time, it’s not accurate. In truth, airflow at the inlet of the fan and the discharge of the fan are always equal.

Here’s why; take a look at any fan, the physical distance between the inlet of the fan and the discharge of the fan is very small. Actually, you could say that the back side of the fan blade is the inlet and the front side of the fan blade is the divider. There is little or no space for air to leak in or leak out of the fan.

Any difference between supply register airflow and return grille airflow is due to leakage in the duct system. It’s that simple. Of course, intentional fresh or outside air ducted into the system or bypass air are other legitimate reasons for a difference in these totals.

Which Side has the Highest Pressure?
Another idea is that the fan is happier when the return ducts are oversized.
However, we learn from the manufacturer’s fan tables that a fan only reacts to total external static pressure, it doesn’t matter if the pressure is higher or lower on one side of the fan or the other.

Fans are rated based on the total of suction and discharge pressures. Fans are not rated differently if the pressure is higher on the return or supply side of the fan.

Compare the fan to a train locomotive. The locomotive can push the train, pull the train, or be in the middle of the train pushing and pulling-- it doesn’t matter. The locomotive simply reacts to the load. The same principle applies to a fan.

Dirty Fans
Dirty fans don’t move as much air as clean fans. To make this really simple, here’s a visual inspection rule of thumb. We’ve found that airflow will decrease in proportion to the amount of dirt lodged in the cup of the blade of the fan. For example, of if fan blade is filled 10% with dirt, airflow will decrease by 10%. If it’s 25% loaded with grease, airflow will suffer by 25%.

Basically this rule of thumb only teaches that if a fan is dirty, clean it. Also, if a fan is dirty, measuring fan static pressure and fan speed, and then plotting airflow from the manufacturer’s fan table won’t even get you close to fan CFM.

These principles are learned as you gain experience measuring fan performance. I hope my experience has moved you a little farther up the fan curve.
Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested receiving a free .80” generic variable speed fan table, contact Doc at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.