Blower External Static vs CFM

  

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. Here is 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.

PSC Blower	External Static
Motor Speed	0.1” wc	0.2” wc	0.3” wc	0.4” wc	0.5” wc	0.6” wc	0.7” wc
High	1,150	1,095	1,045	1,025	950	865	775
Medium	890	855	835	775	715	665	605
Low	640	605	565	530	485	440	360

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 listed below. 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, D.

ECM Blower	External Static
Motor Program	0.1” wc	0.2” wc	0.3” wc	0.4” wc	0.5” wc	0.6” wc	0.7” wc
A	630	625	625	620	610	605	600
B	895	885	875	865	845	825	815
C	1030	1020	1005	995	970	945	935
D	1185	1175	1160	1145	1120	1090	1080

X13 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. Below is 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.  

X-13 Blower	External Static
Motor Program	0.1” wc	0.2” wc	0.3” wc	0.4” wc	0.5” wc	0.6” wc	0.7” wc
Tap 1	960	905	890	810	760	720	665
Tap 2	1,060	1,010	995	920	875	835	790
Tap 3	1,150	1,100	1,090	1,025	990	945	905
Tap 4	855	810	775	755	750	720	690
Tap 5	1,470	1,440	1,425	1,405	1,375	1,260	1,315