External Static Pressure Testing

Typical Fan Curve

Example: The External Static Pressure for the furnace is 125 Pascals. 125 x.004 = 0.5 in.wc. Thus the fan is moving approximately 895 cfm.

External static pressure is defined as that pressure external to the system as shipped. As a result, air conditioners and heat pumps include the coil in their ESP test and gas furnaces with air conditioners attached do not.

Total Static Pressure


Air Handlers are like water pumps. The greater the pressure they operate against, the less air they can move. All manufacturers provide fan curves. These fan curves show how much air a particular fan can move against a set amount of static pressure. The greater the pressure, the less the air flow.

Most systems are optimized around 0.5 inches water column. Variable speed fans can maintain needed air flow up to 0.8 water column inches.

Prepared by Delta-T, Inc. 185 Beebe Rd., Goldendale, WA 98620. (541) 517-5779. Graphics courtesy of Saturn Resource Management of Helena, MT.

Measuring Airflow by Device Static Pressure Drop
All devices such as filters and coils produce a drop in static pressure as air passes through them. Manufacturers of coils and filters produce tables matching pressure drop with airflow. Below is a typical wet coil pressure drop table for a 3-Ton system.
cfm 1000 1100 1200 1300 1400
DP I.W.C. .23 .27 .31 .36 .41
A flow plate is a device that fits into the filter slot. It contains calibrated orifices and a pressure sensing grid. This device accurately measures air handler flow quickly.

Measuring Register Flows
Several instruments are designed to measure airflow at registers and grilles. Results can vary tremendously between different instruments depending on diffuser type, velocity, turbulance, calibration, and the skill of the technician. Regardless of the type used, it is essential that the user is familiar with the strength and weakness of each type. Instruments that do not utilize capture hoods require the net free area of the grille or register. This is the actual space between the blades. Use engineering data when possible. If not available, use these multipliers to go from the nominal area to effective area.

Round Diffuser X .6

Four-way Diffuser X .4

© 2000-2009 Delta-T, Inc.

Turbine flowhood funnels air through a known area. Measures air velocity by revolutions of a propeller. Excellent at low flows. Poor on returns.

Vane anemometer measures velocity by revolutions of a propeller. Must average several readings per grille. Requires net free area of register. Can measure wide range of velocities and is not restricted by register size.

Hot Wire flowhood uses a capture hood to funnel air past a hot wire anemometer grid. Very fast to use. Sensitive to register type.

Hot Wire anemometer infers velocity by how much heat is removed from the resistor. Requires multiple, time consuming measurements.

Volumetric capture hood a.k.a. garbage bag. Technician records length of time required to fill a plastic garbage sack of a known volume. To achieve cfm divide the volume of the bag in cubic feet by the time in minutes it took to fill the bag.

Rectangular Diffuser X .6



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This page was last updated on 02/01/2009

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