How to Find Duct Obstructions with a Manometer

Restriction to airflow comes in all shapes, sizes and origins. Obvious airflow-killers include common causes such as interior duct liner hanging in the airstream due to missing pin-spotters and glue, and other interior duct damage. Whatever the cause, our job is to pinpoint the location of the duct obstruction and remove it so the room can get the airflow it needs so comfort can be restored.

One problem we all face sooner or later is solving the issue of a duct without airflow. The fan is on, the other registers have airflow, but for some reason, one room has little or no airflow. You’ve inspected for any obvious duct leakage, so by now, you’ve come to the conclusion there’s something in the duct blocking airflow. Let’s take a look at how you can “see” the duct obstruction and pinpoint its location, in just a few minutes, using an inexpensive manometer.

What’s in that Duct?
Restriction to airflow comes in all shapes, sizes and origins. Obvious airflow-killers include common causes such as interior duct liner hanging in the airstream due to missing pin-spotters and glue, and other interior duct damage. There may be left behind construction materials or an occasional misdirected rodent looking for a new tunnel or place to call home. Wayward teenagers often find that ducts are a safe place to hide magazines or other taboos-of-the-day from their parents as well.   

Most of the time, low duct airflow is caused by some construction defect that may have been invisible to the original installer. An obstruction may be new damage caused by a cable guy crawling over or under a duct that unfortunately found itself in the path of a TV signal or the Internet.

Whatever the cause, our job is to pinpoint the location of the duct obstruction and remove it so the room can get the airflow it needs so comfort can be restored.

Tools Needed
The tools required to do this testing are not too expensive and are available across the industry. If you’re a service tech, installer or salesperson, you should have these tools or access to them already. A good quality manometer kit that will do the job will cost you less than $200. A manometer kit should contain the following:

•    A low pressure manometer
•    ¼-in. tubing or hose to carry the pressure from the duct to the manometer
•    A static pressure tip or probe to insert into the duct to collect pressure samples
•    A cordless drill and a 3/8-in. drill bit
•    3/8-in. hole plugs or tape to seal the test holes you will drill into the duct.

Pressure Rules to Learn
Before testing to find the duct obstruction, let’s review a couple of basic principles of pressure in a duct system.

Rule Number One- Pressure is the highest near the fan. As air moves away from the fan and gets farther down the ductwork, the lower the pressure will be.

Rule Number Two – If number one is true; the pressure will be higher nearer to the fan, than on the far side of the duct obstruction.

In other words, if there’s an obstruction in the duct and you measure pressure on both sides of the obstruction, there will be a significant change in pressure. The pressure closer to the fan will be higher than the pressure on the opposite side of the obstruction away from the fan.
 
How to Test to Find the Obstruction
Let’s assume you have a 25-ft. long supply duct leaving the plenum that terminates in the living room.

Step One - Drill a test hole in the duct a few feet away from the plenum and take a pressure reading in the supply duct. Lets say the pressure is .15-in. w.c.

Step Two – Working away from the plenum, make a test hole every 3-4 feet down the duct, measuring the pressure in each test hole. Drill a test hole in metal duct, or poke a test hole in flexible duct with the static pressure tip. Be sure to repair each test hole after you complete the testing and diagnostics.

Step Three – Measure pressure in each test hole until there’s a significant change in pressure from one test hole to the next. Yahtzee! You have found the obstruction.

Here’s what’s happening. The obstruction is blocking airflow. The pressure from the fan is pushing on the obstruction, but the obstruction will not let the airflow or pressure pass. The result is a higher pressure before the obstruction and a much lower pressure afterwards due to the lack of airflow. The larger the obstruction, the larger the pressure difference you will measure.

Example: The test holes in the duct as you move away from the fan are: .15-in., .15-in., .14-in., .14-in. .13-in., and .03-in. The obstruction is located in the duct somewhere between the .13-in. pressure test hole and the .03-in. pressure test hole.

Using this test you can “see” the obstruction through your manometer.

If your test holes were, for example four feet apart, you can hone in on the exact point of the blockage by drilling more test holes back towards the fan in one-foot increments until you pin point both sides of the obstruction by pressure difference.



There will be times when you find no specific point where the pressure changes from one test hole to the other. You will not be able to identify one single significant blockage. Most likely what is happening is that the duct is poorly installed or seriously undersized. It may be the combination of too many elbows, bends around structural lumber, or a poor installation job along the entire length of the duct that’s causing the problem.

Example: The test holes in the duct as you move away from the fan may be: .15-in., .13-in., .10-in., .07-in. .05-in. and .03-in. As you can see, there’s no single point of significant pressure change. This steady decline in pressure usually indicates a poor quality installation throughout the entire length of the duct or undersized duct.

How to Repair Duct Blockage
If your testing reveals one point of significant pressure change along the length of the duct, you have a single blockage. Mark the spot, cut the duct open and remove the obstruction.

Repair the duct, assuring not to leave another obstruction to airflow. Seal the duct airtight. You may need to add additional strapping or support because the repair may have weakened the duct. If insulation was removed, be sure to apply adequate external insulation to the duct.

If there was not a single significant pressure change from one test hole to the next as described in the second example, you’ll have to evaluate installation conditions or duct sizing and make appropriate repairs.

Measure the airflow after the repairs are complete to verify adequate CFM and temperature are being delivered at the grille or register.

Enjoy the hunt.
 
Rob “Doc” Falke
serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free Duct Obstruction Test Procedure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.

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