How to Measure Real Duct Leakage

If a plumber installs a leaky piping system, they’re in trouble. Houses explode from gas pipe leaks and ceilings cave in from leaking water pipes. However, when we install pipe and ducts for air, we get away with leakage all the time. The question isn’t if duct systems leak, but how much do they leak? Wouldn’t it be great to really know the answer?

HVAC contractors may talk about duct leakage, but few really have a grip on how to measure it in an operating HVAC system. While many of today’s duct systems aren’t built in a manner that can be easily tested, they can still be effectively measured using proven methods that are older and more reliable than the typical duct leakage pressurization techniques used in government and utility programs.

We’ve also noticed that when homeowners have a dependable test they understand and trust they become extremely motivated to have the work done to get real energy savings and increased comfort.

Two Tests, One Result
The first test is to establish fan airflow. When we determine how much air the fan is moving, it becomes our baseline from which we can measure duct leakage. The second test is to measure actual delivered airflow at the registers using an air balancing hood. The last step is to calculate the actual duct leakage.

Here’s the formula: Fan Airflow - Grille Airflow = Duct Leakage. That’s too simple, isn’t it? Just measure the live duct leakage on the supply side and then measure the duct leakage on the return side while the system is in a normal operating state. Add the duct leakage on both sides of the fan to find total system duct leakage. This identifies how the system actually functions, minute after minute, hour after hour, day after day, and year after year.

How It Works In the Field
Let’s take a look at a four-ton cooling system that we recently tested in the field. The system had a long straight return drop approaching it. We drilled a series of test holes in the return duct and traversed the airflow to measure it at 1542 CFM. The ducting was tight between the return and the equipment, so we determined this was equal to our fan airflow.

We verified the fan airflow by measuring the fan’s total external static pressure at .84-in. and then we verified the fan speed setting on the dip switches. We used that information to plot the fan airflow on the manufacturer’s engineering data for that piece of equipment. The two airflow values agreed so the baseline fan airflow was 1542 CFM.

We then took an air-balancing hood and measured the airflow being delivered from each of the supply registers. We added up the total of all the supply registers and the airflow was 1232 CFM. We subtracted the 1232 CFM supply register airflow from the 1542 CFM fan airflow and found 310 CFM of supply duct leakage.

We thought that number was too high. Would you like to lose 310 CFM of expensive conditioned air to the attic of your home? Airflow that doesn’t make it to the rooms that need it is substantially wasted.

Then we measured the airflow at the return grilles. The results from that weren’t any better. We added them up and they totaled 940 CFM. We did the math and found that the fan airflow was 1542 CFM and the return grilles airflow was 940 CFM. Somebody has lost this game and unfortunately it’s the homeowner. We subtracted the two airflows to find return duct leakage of 602 CFM.

The last step was to simply add supply duct leakage of 310 CFM and return duct leakage of 602 CFM to find a total of measured and verified duct system leakage of 912 CFM. The fact that we did the measurement under live conditions confirms what’s been happening in this home every minute the fan is running.

The Rest of the Story
Fortunately, we took the homeowner with us and he participated in the entire testing process. There was no explanation needed when we were done.

He looked at the contractor and asked if his duct system was repairable. “Sure it is,” the contractor said. “You’ll be delighted with the difference this makes in your home and in your utility bill. We’ll be able to downsize your replacement equipment if we fix your duct system too. Give me a few minutes to figure out a total cost, just to be sure this is what you want to do.”

A price was quoted a few minutes later and a duct renovation was added to the equipment change out. Now the manufacturer’s equipment efficiency numbers could actually be delivered and energy savings estimates would be real.

This was one of a thousand systems that had live duct leakage tested that day. It was a revelation to the homeowner, but just one more in a constant stream of poorly installed duct systems for the contractor to fix. Sure it was more money than the homeowner was hoping for. But without it, what real efficiency could be expected of the equipment?

Measuring live duct leakage CFM delivers real measurable benefits for homeowners and contractors alike.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a technical procedure to help you Measure Live System Duct Leakage, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, technical articles and downloads.

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