About ten years ago we gathered as many duct sizing methods that we could get our hands on. The collection ranged from “Dad’s Duct Sizing” scratched on the back of a clipboard to extensive software programs. When the dust settled, we had over 40 different ways to determine duct sizing. Few were the same and even fewer actually delivered the airflow they claimed. So, how do you check your duct system size?

We’ll stay out of the duct design contest for now. Instead we’ll offer a quick duct size check that will work on almost any residential HVAC system. The test is simple and the test instrument is inexpensive, but the test results are very conclusive.

The Tools

Of course there’s a tool involved. Most of the performance indicators that will tell you if an HVAC system is working well or not are invisible. Temperature, electrical values, airflow and pressure cannot be seen with the human eye, and no, holding up your hand to check airflow still doesn’t work, so we have to use a tool.

This test requires a manometer or pressure gauge. Hopefully, there’s at least one around the shop or in your truck. It should be a low pressure gauge; 0-in. to 1-in. WC if it’s an analog gauge, and around 0-in. to 5-in. WC if you’re using a digital gauge. Accessories should include a test hose and a static pressure tip. You’ll also need a drill and a 3/8-in. bit to drill into the ductwork.

The Test Principle

Here at NCI, we gather a large amount of test data from live operating HVAC systems. There are some mornings that I receive dozens of reports in my email, each telling a different story. By studying these reports over the years, especially the ones that work well, we have found that systems with adequately sized duct work that is installed properly almost always follow this rule:

The pressure drop over the duct system is less than 40% of the fan rated static pressure.

Also, the pressure drop over the supply duct system is less than 20% of the fan rated pressur,e and the pressure drop over the return system duct is less than 20% of the fan rated pressure.

Here’s an Example

Say a fan is rated for .50-in. WC. .50-in. x 40% = .20-in. WC. That means if the pressure drop over the duct system is less than .20-in., your duct system is sized and installed properly. Ideally, the pressure drop over the return duct should be less than .10-in. and the pressure drop over the supply duct system should be less than .10-in.

How to Measure Duct Pressure Drop

Actually, this is normally the easiest pressure reading to take; especially if the filter is installed near the equipment.

Drill a test hole in the supply plenum past the coil. Attach the hose to the positive port of the manometer and insert the other end of the hose into the supply plenum. Read and record the pressure on the manometer. This is the pressure drop over the supply duct system.

Then do same on the return side, just change your hose to the negative port of the manometer.

The Calculation

To find out how your duct system measures up, take the fan rated pressure, listed on the equipment nameplate, times 20%. If the supply duct system pressure drop is less, your duct system is sized and installed properly.

Then do the same thing for the return duct system.

If you find that the duct system pressure drop is 50% or 60% of 70% of the fan rated pressure, your duct system is undersized or poorly installed and needs attention.

When the Test May Not Work
Since pressure is dynamic in an HVAC system and constantly and immediately subject to change, there are occasions when the other defects in the system can negate this test. Sorry, that’s just reality.

Say the old 1-in. disposable filter hasn’t been changed in 7 years. It’s now 3-in. thick. The pressure drop over that filter is now 1.08-in. WC. With this filter in place, the airflow through the system will probably be 50% of what it should be. With 50% less pounds of air squishing through the duct system, the duct system pressure drop will be very low. Less airflow means way less pressure drop through the duct system.

In this case, the pressure drop of the duct system may be nice and low leading you to believe your duct system is sized correctly. But in reality, it’s like me trying to slip on an extra small tee shirt, it’s just not going to work.

Give this test a field run and see what you learn about the size of your duct systems. Of course the final test is a balancing hood test. Simply add up the airflow through each of the supply registers and see what the system is actually delivering. The same test applies to the return side. Remember what my partner Dominick Guarino says, “If you don’t measure, you’re just guessing.”

Rob “Doc” Falke
serves the industry as president of National Comfort Institute a training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in a free procedure on how to measure pressure drops, contact Doc at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.