Per ANSI/ASHRAE Standard 90.1-2010, “positive pressure leakage testing is acceptable for negative
pressure ductwork”.  Even so, you will see specifications that may require return air or exhaust
systems to be leak tested under a vacuum (negative pressure). To perform a proper duct leakage test
under negative pressure using an ORIFLOW duct leakage tester, perform the following steps:

  1. Use the orifice plate with the recommended leakage range for your job. Use our online
    to help determine which orifice plates you can use with the Lynx and Cobra model testers.
  2. Make sure you have adequate electrical power for the fan. DO NOT OVERLOAD extension
    cords. Make sure you use one with the proper gauge.
  3. Cover all outlets with plastic bags and seal with duct tape.
  4. Allow all joints and seams that were sealed with duct sealer to cure for at least 24 hours or the
    required cure time by the manufacturer, whichever is greater.
  5. If possible, position the ORIFLOW duct leakage tester in a location where there will be a
    straight path from the orifice tube outlet to the location where you will feed air into the system.
  6. Drill a 3/8″ diameter hole no closer than two or three feet away from the flexible duct to system
  7. Place the pressure tubing into the previously drilled hole; extend 6 to 12 inches into the duct.
  8. Seal the tubing penetration joint with putty or tape.
  9. On the gauge marked “Duct System”, relocate the pressure tubing from the “Supply System”
    pressure port to the “Return System” port. Refer to the note on the figure at the bottom of this
  10. Zero both gauges. Locate adjusting screw on center front of gauge. Use a small slotted
    screwdriver. Turning clockwise increases the pressure reading, counter-clockwise decreases
  11. Close the fan’s inlet damper so the duct system does not get over-pressurized by the tester.
  12. Turn on the tester blower.
  13. Adjust inlet damper until the reading on the gauge marked “Duct System” is at the system
    pressure required for performing the leakage test.
  14. Once the duct system pressure has been reached and stabilized, note the reading on the
    gauge marked “Orifice Tube”. This pressure drop reading will correspond to the amount of air
    you’re feeding into the duct system, which is the amount of air that is leaking out.
  15. Refer to the calibration table that came with your test rig. Find the pressure reading and note
    the corresponding leakage rate.


If the “Orifice Tube” gauge exceeds the maximum range, the system is probably leaking too much air.
Investigate system for missed outlets or unsealed joints. You can also use a larger orifice or a gauge
with a larger range.

If you cannot obtain any pressure (or very little pressure) with the blower on and the inlet damper wide
open, it could be due to one of the following problems:

1. Using too small of an orifice plate
If pressure drop across orifice plate is more than 2 in.wg., use the next greater sized
orifice plate.  Use our online program to determine which orifice plate you should use
based on the maximum allowable leakage. You can also refer to graphs for each
tester/plate combination.

2. System has more leakage at test pressure than is allowed
When using the Cobra or Lynx tester, use the 4-inch plate to pressurize your system
and refer to the following table to determine if your system is leaking more than the
maximum capacity of the tester.

Maximum Leakage Capacity (cfm)*
Model0.1 in.wg.1 in.wg.2 in.wg.4 in.wg.6 in.wg.7 in.wg.8 in.wg.9 in.wg.10 in.wg.12 in.wg.14 in.wg.16 in.wg.
Lynx570 cfm550 cfm540 cfm455 cfm290 cfm160 cfm
Cobra680 cfm650 cfm645 cfm600 cfm465 cfm405 cfm350 cfm250 cfm
Panther1400 cfm1350 cfm1320 cfm1210 cfm1065 cfm970 cfm875 cfm745 cfm615 cfm270 cfm
Rhino1700 cfm1630 cfm1550 cfm1420 cfm1330 cfm1255 cfm1185 cfm1110 cfm1035 cfm895 cfm660 cfm480 cfm

* – Using the largest size orifice plate. Note each tester comes with one plate. Other orifice plate sizes are available for accurate testing of different flow ranges. Use smaller orifices for smaller flow ranges; largest orifice for maximum flow range.

Also check your flex-duct connections Wrap duct tape tightly around the joint and feel
for leaks. Rectangular duct joint corners can be typical leaky points.  If fire damper
and/or heater coils are part of the system you are testing, they also can be significant
leak contributors. A smoke machine may be used to help locate leaks; contact us for
more information.

3. Equipment not working properly
Make sure the equipment is working properly by doing the following steps:

  • turn off blower.
  • observe blower wheel rotation direction when it slows down (view from looking
    through inlet safety screen); it should be rotating clockwise (if not, contact us.
  • completely close inlet damper.
  • disconnect the flex-duct from the tester.
  • turn on blower.
  • Slowly open inlet damper and verify that you can feel air coming out of the orifice
  • the orifice tube gauge pressure drop reading should be maxed out.
  • you can verify gauges are working properly by removing pressure tubing and
    carefully apply pressure by gently blowing into tube end as if trying to inflate a