The Tight Duct Myth

Tight ducts are not necessarily right ducts. Although critical to a well-performing system, the disaster of the duct blower craze across the country has produced the single largest inventory of poorly performing duct systems that we have ever had the opportunity to harvest.

HVAC contractors everywhere are targeting residential new construction subdivisions as a place to go to obtain new customers for duct repair work. With HVAC industry proven diagnostic tools and procedures the poorly performing systems can be revealed to homeowners in less than an hour.

I received a call recently from a contractor who had harvested another "energy saving sealed duct system." His customer had participated in a incentive program to have his ducts sealed. The program raised awareness of excessive energy costs and a lack of comfort, but the duct-sealing alone did little to remedy the problem. The home has been declared "highly efficient," but the homeowners were left uncomfortable and furious, until this contractor had come in, added return ducts, increased the duct system capacity, and then balanced the system.

"Tight ducts" failed. "Right ducts" prevailed.

The renovated air conditioning system now had adequate airflow, lowered static pressure and balanced room-to-room airflow. The system worked beautifully, the homeowners were thrilled.

Somewhere along the road, our industry was thrown way off track by someone taking desperate actions. Supported by the idea that any improvement has to be a good thing, consumers everywhere are being promised by laws passed proclaiming that if citizen's ducts are tight, their government can assure them that their HVAC systems are energy efficient.

Much of our problems today go back to the era when blower doors came to light as the primary diagnostic tool for the building envelopes. They work great on building envelopes, so the same concepts were tried on ducts. Everyone knew ducts were in pretty bad shape. And if tight was right for building envelopes, it should work on ducts too. Unfortunately progress stopped right there.

A Solution
More and more individuals are attending air balancing training. These are terrific testers, committed to improving efficiency and providing their customers with comfort. Through a pursuit for excellence, and a search for improved technology, they have found a basic test recommended by every HVAC equipment manufacturer on the planet — measure total external static pressure.

Total external static pressure measurement is the missing link that challenges the narrow tight duct theory. The reason tight ducts are not right ducts are because most ducts are undersized and produce static pressures that the fan cannot overcome. Low airflow does equal low heat transfer and is the primary cause for poorly performing HVAC systems.

The fallacy that the heat still gets transferred anyway defies all manner of good principles of physics.

The simple solution is to measure system total external static pressure and compare the reading to the rated maximum total external static pressure rating printed on nearly every air handler’s nameplate. This five-minute test will reveal weather or not the duct system is undersized or not and if it is a likely candidate for duct sealing, or if additional duct work needs to be added before the ducts can be sealed.

Sounds simple, it is. Give it a try.

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 free technical report detailing how to measure total external static pressure, 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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