'Ducting' the issue
Welklin is an energy consultant and account manager with Cinergy, a gas and electric utility company in Indiana, Ohio and Kentucky. An expert in energy efficiency, Welklin has spent the last 21 years working with contractors and residential builders, explaining why "one size does not fit all" when it comes to home hvac systems.
It was the focus of his recent workshop, "Duct Design: Learn it in One Hour" at the Air Conditioning Contractors of America's (ACCA) 33rd Annual Conference last March in Las Vegas. The workshop was based on information from the ACCA's Residential Duct Systems Manual D, published in 1995. Welklin explained the manual's method of using a "friction rate" to correctly size ductwork.
He said while an hour won't turn a contractor into an airflow expert, it is long enough to start to grasp the basics and dispel some common design myths. "And everyone in the (hvac) business is capable of understanding the basics," Welklin said. "It's not something that just the gear-heads can figure out. My goal ¿ is to prove that science is not as difficult as most technicians think."
And the training is sorely needed, he added.
"I would guess that duct design today is not done by most home contractors," he said. "A lot of people do the same duct design for every job." Instead of such a cookie-cutter approach, Welkin said the same care that goes into designing an urban freeway should be dedicated to the ducts of a home's hvac system, with a plan that distributes air to all the key points.
Problem easy to findIt's not hard to spot a home where the duct system was not sized properly, he said. Millions of homeowners suffer in a hot upstairs bedroom during the summer - a good sign that there's a duct design problem.
Some contractors attempt to correct the situation by increasing the fan speed or closing all the downstairs registers. But if there's too much resistance in the duct system, such changes will do little, if anything.
The key, Welklin said, is to minimize airflow resistance by using the proper fittings, recognizing and measuring the duct resistance, and sizing the ducts correctly. But how does a contractor know what the correct size is? In Manual D, the ACCA uses simple math and five easy steps to determine friction rate. The friction rate is the speed at which all the available pressure is used in a few hundred feet of duct and can be different for every job.
"The friction rate tells us if we have conserved our valuable pressure in a well-designed duct system or if we have squandered this precious resource and tried to divide it into too many hundreds of feet of duct," Welklin explained.
The first step is to understand the fan specifications for the installed equipment. The manufacturer's fan chart will describe the fan's starting pressure, also known as the external static pressure. Note that the airflow resistance associated with your filter or other components decreases the pressure available to the duct system (device pressure losses).
"Researching these two very important variables is absolutely necessary to determine the pressure leftover and available for the distribution of the airflow," Welklin said. This is also called available static pressure.
As for the duct system, he added, "One of the most important things Manual D teaches is how to recognize a poor duct fitting (one with high resistance to airflow) vs. a good fitting."
Manual D rates each fitting in "equivalent feet." For example, a 90° turn rated at 85 equivalent ft. would have the same resistance to airflow as 85 ft. of straight duct. Replacing the 90° turn with one that is only 20 equivalent ft. helps the airflow considerably.
"This is a simpler method than determining the pressure drop of each fitting," Welklin said. "Duct length and resistance are comparable and easy to understand."
Add the equivalent lengths of all supply and return duct fittings to the actual length of the straight duct runs. The sum is called the total effective length and is a measure of the amount of total resistance to airflow in the ductwork.
Now you can determine the friction rate. Divide the available static pressure by the number of hundred feet of duct. For example, if you have an available static pressure of .4 and 500 total effective feet of duct, your equation is .4/5 (per 100 feet of duct) = a .08 friction rate per 100 feet of duct.
"A well-designed system has a higher friction rate which results in normal sized ducts," Welklin said. "The poorly-designed system or the duct system that tries to go too far will have a low friction rate and require very large ducts to compensate for the extra resistance to airflow."
If a contractor is ignoring these factors in duct design or using an arbitrary fiction rate for all jobs, he or she is not really performing a design service for the customer, he added.
"I don't believe a true design is being done on most homes today," he said. "Many contractors still use the same friction rate on every job. If I can teach just one principle in my one hour training, it's to understand the variables that change your friction rates and the resulting size of the ducts."
The ACCA offers several training courses for individuals interested in learning about residential load calculations, duct systems, equipment and more. The ACCA's Education Program for Instructor Certification (EPIC ) helps contractors gain an understanding for hvac principles they can pass on to their employees or coworkers. Sessions are held throughout the country at various locations. Contact Jeanne Cooper at 703-824-8852 or email: Jeanne@ms.acca.org for information.