This summer has been so hot that I’ve even entertained the idea that refrigerants might be contributing to a hole in the ozone layer. Not really, I was just kidding. However, it has been hot enough that many of our tech support calls are from hundreds of you dealing with unsatisfied customers who just can’t get comfortable enough.

With extended periods when temperatures have exceeded the outdoor conditions we use in our ACCA Manual J calculations, our ability to test and diagnose our systems' performance have been put to the test.

One call this week was particularly memorable. A service manager on the East Coast was at his wits’ end. His company had installed replacement equipment at the start of the summer. The system had satisfied the homeowner until early July, when the constant high temperatures hit. The temperature and the customer had not let up since.

More than a dozen callbacks later, nearly every diagnostic option had been exhausted, and the manufacturer’s field service representative (FSR) was the last hope. He was willing to investigate a rare compressor issue that would – it was hoped -- end the siege. On the third tech support call to NCI, and after a 20 questions session, this service manager was sure he had “stumped the swami.” I asked him to perform one last Extreme Weather Duct Diagnostic Test, and he agreed.
When temperatures are unusually hot or cold, this duct performance test has provided the needed information to solve many otherwise hopeless cases this summer. This test isolates the effect the ducts have on the system. If you’ll take the time to study it carefully and then implement it, the test can become a valuable weapon in your service and diagnostic arsenal.

The Test

As the name indicates, you should perform this test in the heat of the day, because this is when the conditions are the worst, and this is the time of day when cooling is needed the most. Or, if you’re testing in winter months, measure in the morning when the weather is the coldest and the heating is needed the most.

During summer months, turn the system off until the coil warms up to room temperature. Next, set the thermostat to operate with the fan in high speed but with all cooling off. You may want to set the thermostat to normal cooling mode with the fan on, but pull the electrical disconnect on the outdoor unit so no cooling functions are operating.

Before turning the system back on, measure and record the outdoor ambient air temperature and the air temperature where the ducts are located. We test these numbers so we can compare results when we test out accurately after the repair work has been completed.

When the fan is turned on, measure the following temperatures:

  • The average air temperature entering the return grilles
  • The average air temperature leaving the supply registers

Average grille temperatures are found by measuring several grilles in different locations throughout the system and then averaging the temperatures. Accuracy increases if you’ll use a fast-acting thermometer or a thermocouple with a test instrument that displays temperature to within 1/10th of a degree.

Calculate the temperature change through the system by subtracting the average return grille temperature from the average supply register temperature. Ideally, this temperature change should be zero, since there are no cooling functions operating in the system.

What would you think was the problem with your system if the supply register air was 6F degrees warmer than the return grille air temperature?

A few minutes of pondering the situation will reveal to all of us that our duct system is experiencing some serious heat gain, and that can’t be good. This heat gain comes partly from return air duct leakage, but is added to in great measure by the heat penetrating the duct and the duct insulation. This warms the air passing through the system.

Remember, this same process continues when our cooling system is operational. Actually, with cooler air in the ducts, the delta T between the duct air temperature and the attic or crawlspace air temperature increases, and heat transfer increases at a greater rate. So the situation actually gets worse.

Take the Next Step

If the situation warrants more testing, or if you’re just curious, a few more numbers and some simple calculations can quantify how much heat your system is gathering.

With an air balancing hood, measure the supply airflow being delivered out of the supply registers. Just shoot each register and add all the CFMs together. Then simply multiply the supply airflow, times the temperature change through the system that you just measured, times the sensible BTU constant of 1.08. The answer is the sensible BTU the duct system is gaining.

If we simply look at equipment operation alone, the effect of the duct system may be completely overlooked, as it is more than 85% of the time. It is not uncommon for equipment performance to be near perfect, while the system is gaining 50% of the rated equipment capacity.
If your skills are up to snuff, you can measure wet bulb temperatures at the same locations, convert to enthalpy and calculate the total, sensible and latent BTU effect of the ducts on the system. The latent effects of duct gain problems often pose a greater negative effect on system performance than the sensible losses, even in dry climates, but that’s another article.

So, if a 3 ton system has a duct gain of 1 ton (which is quite common) how much cooling capacity is left over to cool the conditioned space? How often will 2 tons do a 3 ton job? No, this isn’t a good reason to skip ACCA Manual J and oversize your equipment as a safety net. However, it is a good reason to test, fix, and balance the duct system.

Oh, But Our Ducts are Inside the Envelope

I was at a North American Technician Excellence meeting recently, and I was speaking with a prominent distributor who had hosted one of our NCI air diagnostic seminars. He commented how he enjoyed the training, but said it didn’t apply so much to his part of the country because the ducts are all inside the envelope.

I pulled a recent duct diagnostic report from my computer that a contractor in his area had sent to me. We studied a system “with all the ducts apparently inside the envelope.” This system was suffering a 32% loss of the equipment cooling capacity from the duct system.

We had a great discussion about why although ducts are physically contained inside the building, the air and thermal losses of many duct systems are straight from the attic and outdoors due to envelope breaches and duct leakage.
The test results were sufficiently compelling to alter his opinion, and all of us were encouraged to try the Extreme Temperature Duct Diagnostic Test . Consider it the next time a poor comfort situation has you at your wit’s end – even in a home in which all the ducts are inside the envelope.

The Solution
Measure and test the performance of the entire HVAC system. Servicing equipment alone doesn’t cut it anymore. Once system performance has been measured by implementing airflow, static pressure and temperature testing, a new vision of the system carefully directs a technician or salesperson to go past the boxes and into improving the performance of the duct system as well. That’s where the difference is made.

Ducts often need to be repaired or renovated. Once adequate system airflow is assured, only then can room-by-room airflow be delivered. Often, additional duct insulation is needed. Frequently, additional duct runs must be cut in to be sure the job is done right.

Finally the industry is measuring delivered BTU and beginning to rate installed system capacities and verifying the HVAC system is performing as it should. This is light years ahead of the typical clean-and-check mentality we have held to for decades.

So, remember that system that’s been giving you trouble all summer? Give it an Extreme Weather Duct Diagnostic Test and find where the real problem is. You’ll find with testing a few more temperatures throughout the system that you can pinpoint the defects in the duct system and easily repair it and bring it to top operating condition, something your customers will love you for.


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 a Extreme Weather Duct Diagnostic Test Procedure, contact Doc at
robf@nationalcomfortinstitute.com or call him at 800/633-7058. Go to NCI’s website at www.nationalcomfortinstitute.com for free information, technical articles and downloads.