By Carter Stanfield
It is possible to use a magnehelic gauge or a digital manometer and two static pressure probes to determine the amount of airflow a blower is moving. You can read the static pressure difference across the blower and compare it to the manufacturer’s data to determine the blower CFM. It does make a difference what type of blower you have and what type of motor the blower uses. A typical residential blower uses a forward curved centrifugal blower and a PSC motor. The airflow these blowers deliver decreases as the static pressure the blower is working against increases. You need the manufacturer’s data to compare the measured static pressure across the blower to the fan performance table or curve.
Carter Stanfield is an instructor at Athens Technical College and he’s been teaching HVACR since 1976. He is the co-author of Fundamentals of HVACR. He is a CM member of RSES, NATE certified, and holds a Georgia contractor’s license. You can get to his blog via the link below:
Figure One shows a table from a unit with an air handler with a PSC blower motor.
Note that on high speed at a static pressure difference across the unit of 0.1” wc the airflow is 1150 CFM, while at 0.7” wc across the unit the airflow drops to 775 CFM.
With ECM motors, the airflow varies very little as the static pressure across the blower changes. That is the point of an ECM blower. It recognizes the amount of static pressure it is working against and adjusts the blower motor RPM and power output to keep the same programmed airflow – up to a point. ECM motors do have a programmed speed limit. When they hit their speed limit, they shut off. A key point here is that although the ECM motor can compensate for extra restriction, it does this by using more electricity – which can turn an energy efficient blower into an energy hog. It is far more cost effective to remove the restriction than to pay for enough electricity to shove the air through. At any rate, checking the static pressure across an ECM blower is done primarily to make sure it is operating within its design parameters and in an efficient manner. It does not tell you how much air the blower is moving. A table from an air handler similar to the one above, but with an ECM blower motor is shown in Figure Two.
Note that there is very little change in the CFM as the static pressure across the blower increases. The CFM delivered is determined by the program – A, B, C, or D.
X-13 motors are also electronically commutated, but they are programmed for a specific torque or power output, not a specific airflow. The airflow across an X13 motor does drop off as the static pressure across it increases, but not as dramatically as a PSC motor. They are considerably more efficient than a PSC motor and considerably cheaper than a fully programmable ECM. You can determine an airflow from the static pressure across the blower and the manufacturer’s specifications. Figure Three below shows data from an air handler with an X13 blower motor.
Note that its performance compared to external static pressure is somewhere between a PSC motor and an ECM blower.