Duct Leakage Testing – RETURN SYSTEM

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. 
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 joint.
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 page.
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 it.
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.

Troubleshooting:
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.  You can 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.