Images courtesy of the author.

w w w . r s e s j o u r n a l . c o m26 RSES Journal NOVEMBER 2013

B y C a r t e r S t a n f i e l d , C m S

Using a differential pressure gauge or digital manometer and two static pressure probes to determine the amount of air flow a blower is moving.

CFM Blower External

Static

table 1

a static pressure probe tip and close-up.

PSC Blower External Static

Motor Speed 0.1 in. wc 0.2 in. wc 0.3 in. wc 0.4 in. wc 0.5 in. wc 0.6 in. wc 0.7 in. 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

[Editor’s Note: The following information was recently posted on Carter Stanfield’s blog, “Fundamentals of HVACR.”]

It is possible to use a differential pressure gauge or a digital manometer and two static pressure probes to determine the amount of air flow a blower is moving. Technicians can read

the static pressure difference across the blower and compare it to the manufacturer’s data to determine the blower cfm.

The type of blower and the type of motor the blower uses make a difference. A typical residential blower uses a forward-curved centrifugal blower and a permanent-split-capacitor motor. The air flow these blowers deliver decreases as the static pres-sure the blower is working against increases. The manufacturer’s data is needed to compare the measured static pressure across the blower to the fan performance table or curve.

Table 1 is 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 in. wc the air flow is 1,150 cfm, while at 0.7 in. wc across the unit the air flow drops to 775 cfm.

With electronically commutated motors, the air flow var-ies very little as the static pressure across the blower changes. That is the point of an ECM blower. It recognizes the amount

vs.

ECM Blower External Static Motor

Program0.1 in. wc 0.2 in. wc 0.3 in. wc 0.4 in. wc 0.5 in. wc 0.6 in. wc 0.7 in. wc

A 630 625 625 620 610 605 600

B 895 885 875 865 845 825 815

C 1,030 1,020 1,005 995 970 945 935

D 1,185 1,175 1,160 1,145 1,120 1,090 1,080

X-13 Blower External Static Motor

Program0.1 in. wc 0.2 in. wc 0.3 in. wc 0.4 in. wc 0.5 in. wc 0.6 in. wc 0.7 in. 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

NOVEMBER 2013 RSES Journal 27w w w . r s e s j o u r n a l . c o m

CFM

taking readings at the proper locations ensures proper measurement of static pressure.

table 2

table 3

of static pressure it is working against and adjusts the blower motor rpm and power output to keep the same programmed air flow—up to a point.

ECMs do have a programmed speed limit. When they hit their speed limit, they shut off. A key point here is that although the ECM 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 state how much air the blower is moving. Table 2 is from an air handler similar to the example just described, but with an ECM blower. 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 and D.

X13 motors are also electronically commutated, but they are programmed for a specific torque or power output, not a specific air flow. The air flow 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. Technicians can determine an air flow from the static pressure across the blower and the manufacturer’s specifications. Table 3 shows data from an air handler with an X13 blower motor. Note that its perfor-mance compared to external static pressure is somewhere be-tween a PSC motor and an ECM blower.

Carter Stanfield, CMS, is the Program Director of the Air Con-ditioning Technology Department at Athens Technical College in Athens, GA, where he has been teaching the fundamentals of HVACR since 1976. He has a bachelor’s degree in education from the University of Georgia, with a major in technical educa-tion. He can be reached via e-mail at hvacrfundamentals@gmail.com. For more information and to sign up to receive Stanfield’s blogs, visit hvacrfundamentals.blogspot.com.