PRESSURE AND FLOW
FOR HEATING AND COOLING CONTRACTORS - PAGE 1

Duct Pressure - Static/Total/Velocity

Why do pressures matter? Measuring duct system pressure at the air handler is a necessary part of troubleshooting existing systems and ensuring that new systems are installed according to manufacturers' specifications. Specifically, correct pressures ensure the following:

  • Correct airflow to each zone
  • Correct fan output (necessary for achieving rated delivery of conditioned air; and ensure the rated service life)
  • Acceptable noise levels
Three types of air pressure

1. Static - "bursting" pressure, the pressure exerted against the side of the duct. Must be measured with a static pressure tap.

2. Velocity - The pressure created by the flow of air down the duct. Calculated by subtracting the static pressure from the total pressure.
3. Total - The combined effect of both static and velocity pressure.

Pressure can also be used to measure duct leakage. A duct blower is attached to the duct system. Grilles and registers are taped off and the duct system is pressurized to 25 or 50 Pascals. Because the duct blower acts as a calibrated orifice, by reading the fan pressure, it is possible to calculate the airflow required to pressurize the system. A tight system should have less than 1 cfm of leakage for every 10 sq. ft. of house at 50 Pascals.

Pitot Tube

Air Flow Calculations with Velocity Pressures (All Pressure Measurements in Water Column Inches)
For estimates of air flow, a single velocity pressure measurement at the center of section of straight duct may be sufficient.
First: Calculate velocity V=4,005 X sq. root of the velocity pressure.
Second: Calculate area of duct in sq. feet
Rectangular: Area = L
X W/144
Round Duct: Area =p
X(radius)2 /144
Third: Calculate CFM: CFM = Area (sq. ft.)
X velocity

Duct Diameter Area (Sq. Ft.)
8" .3488
10" .5451
12" .7850
16" 1.3955

© 2000-2009 Delta-T, Inc.

 

Types of Manometers
Differential manometers read the pressure difference between two places. Typically they read in water column inches or Pascals. Tests they are used in include:

  • Gas valve pressures
  • Fan pressures
  • Duct leakage
  • Air flow
  • Device pressure drops
  • Draft


Inclined manometer


Dial-and-needle manometer


Digital manometer

Pressure Conversion Factors

I.W.C.
248
Pascals
I.W.C.
.036
P.S.I.
Pascals
.004
I.W.C.
Pascals
.000145
P.S.I.
P.S.I.
6894
Pascals
P.S.I.
27.7
I.W.C.

Go to Page 2 - Pressure & Flow for Heating and Cooling Contractors prepared by Delta-T, Inc.

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This page was last updated on 02/01/2009

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