Is Your Ductwork Big Enough?

October 15, 2007
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It’s widely acknowledged that in order for a forced-air HVAC system to be truly efficient, attention needs to be given to the air distribution system as well as the efficiency of individual components, such as the condensing unit or furnace. All too often, however, ductwork doesn’t seem to get the respect or attention it deserves.

A new training program from the Air Conditioning Contractors of America (ACCA) could help change that. “Understanding Manual D,” the first in a new “HVAC Essentials” series, is a five-section package from ACCA. The set teaches proper duct design to the contractor who may not have the time or inclination to sit in a classroom away from his business.

According to developer Jack Rise, sole proprietor of Jack Rise Technical Training, the program’s “five bite-sized morsels of information” do a great job of meeting these contractors’ needs in a specific way, with easy portions that can be turned on and off if the contractor needs to do something else. “Time is a big thing with contractors, especially small contractors,” he said.

“If you’re working a two-man shop, you don’t have time to sit in a class. Training is out of the question unless it’s done at night. These CDs get to those guys who can’t get to class.”

The program starts with airflow, then goes into distribution. “We in this industry sell human comfort,” he said. “People want to be warm in the winter and cool in the summer. This program gives you a way to provide that.” There is a prerequisite to the duct training program, he said: a thorough understanding of Manual J load calculation. “The very first thing you do is convert the heat loss and heat gain to Btu.”

REALLY WRONG SIZING

One of the myths dispelled in the CD PowerPoint presentations, Rise said, is “the 0.1-inch static pressure setting for residential duct systems. It doesn’t always work. It was adapted from the commercial system design procedure and the ACCA Wedge,” he said. “Static pressure is really just a measurement of system pressure at some point in the duct. Duct calculators show a friction rate per 100 feet of duct, but in most cases, you’re not going to find 0.1 at any point in that system.”

The practice was handed down from commercial duct sizing, he said. “That’s exactly what you do in commercial work. You assign a static pressure for your duct system, add up all of your other friction and dynamic losses, and then order a blower to overcome the calculated friction rate. You can’t assign a static pressure on a residential system,” he said, “because you only have a limited number of blowers to work with and they’re all nominally rated at 0.5-inch w.c.”

In the residential market, assigning the duct static pressure first is doing it backwards; the duct needs to be sized based on the motor, not the other way around.

“Most duct systems that I experience in the field are way out of range,” Rise said. “The resistance is so high that you hardly have any air delivered.”

Taking into account losses that are due to takeoffs, A coils, filtration, etc., is essential to right-sizing the ductwork. “You add 35 equivalent feet before you even get to the takeoff; and a floor boot or stackhead can be well over 30 equivalent feet. Remember that the 0.1-inch static pressure that you assigned was based on 100 equivalent feet of duct? You already blew that away with three simple fittings on the supply side.

“Saying that the register is too far away from the blower to get the proper airflow is just an excuse for a poorly designed system.”

DUCTS AND VARIABLE SPEEDS

According to Rise, today’s variable-speed motors are especially prone to problems if the duct system has been inadequately designed.

“Duct design is misunderstood and misapplied, and this can cause poor system performance, especially with variable-speed systems, which are less forgiving. They will ramp up loudly to overcome the high resistance caused by undersized ducts. They will become loud and huff and stall in duct systems that are too small. Having a 5-ton variable-speed system trying to pull the air through an 8-inch duct, for instance, isn’t going to work.”

In order to fix the problem, “You add up the measured lengths of duct from the top of the furnace to the furthest point, measure that linear footage, add up that return from the furthest return, then add up the feet from the equivalent feet for the fittings.” Manual D includes tables “for every fitting you can imagine.”

Putting it another way, Rise explained that if the blower can deliver 1,200 cfm against 0.5 inch of external static pressure, the first thing the air will encounter is an A coil at the top of the furnace; when that coil is wet, it has a pressure drop close to 0.30-inch w.c. “Now you’re down to 0.2 inch of available static pressure. Add an air cleaner and you have nothing left. Everybody wants an air cleaner,” which is good, but keep in mind that “it can have the same resistance to airflow as a wet A coil.

“Done correctly, the duct system gets larger within the ranges the blower can handle,” Rise said. “They get bigger. You can deliver a whole lot more air through a cannon than a soda straw.”

Also, don’t confuse volume (amount of air) with velocity (speed of the air). “You can’t measure volume with a wet finger, he said. “You need a flow hood, an anemometer. We should care a whole lot less about velocity than we do about volume.”

AFTERMARKET TOOL

While many parts of the training set are directed at new designs and installations, Rise said it also is applicable to existing systems that aren’t functioning properly. In short, the information should be incorporated into system troubleshooting, especially when homeowners are complaining of zone problems or motor noise.

“Permanent split capacitor (PSC) motors will slow down if you increase the external static pressure,” said Rise. “The amp draw and rpm will decrease. Variable-speed motors will ramp up to overcome that static pressure; that’s where you get the noise. Those kinds of problems won’t show up until you’re at design conditions.”

The information can be invaluable in existing duct applications, and duct repairs can offer an excellent long-term value for consumers. “Very often I’ve found that if I go into a place that gets uncomfortable every time it gets above 90°F, and if the system has been properly sized, then you may find you only have 900 cfm of air being delivered. Now the question is, do you have to change a handful of fittings, is it sealing up the ducts? You’ve got some decisions to make,” but the customer also has solid information to base that decision on.

There are ways to decrease duct resistance. “You can have one fitting in a multistory house where the builder ran a girder down to the basement. The HVAC contractor may have run duct around the girder, “four elbows nice and neat, but that fitting alone is equivalent to 450 linear feet,” Rise explained. “The pressure drop around that girder is equivalent to 450 linear feet. Using mitered 45-degree angles instead of 90-degree angles can knock 400 feet off of the duct.”

Flexible duct is always being abused, he said. “It’s done wrong a lot. It should look like plain round metal duct, not drooping or sagging. It needs to be supported. It shouldn’t just coil around like a slinky.” And duct insulation should not be wrapped so tightly that it creates excessive resistance.

Of course, leaks have a dramatic effect on airflow, he said. Detecting them is discussed in the fifth part of the CD program, which focuses on testing and balancing. “You can’t just design a duct system and walk away from it. If you check the airflow right at the furnace with static pressure tips, you can know what you have at that point.

“If you check outlet-to-outlet volume and it’s different, you have leakage somewhere. If you end up with more airflow at the outlet, that’s a return problem. If you end up short, that’s a supply problem. You can also have leaking in both areas,” he said.

Repairing leaks with mastic - “the real stuff with the tape on top” - can make a tremendous difference. “It will increase the efficiency of the system. If you take a 95 percent AFUE furnace and the ductwork is uninsulated in an unconditioned space, your duct loss could be 30-35 percent.” Ceiling losses can be higher.

In a nutshell, the five-CD set “teaches people how to design residential duct systems,” said Rise. “Everybody thinks they can pick up a duct calculator and assign a static pressure. ACCA has given me the opportunity to get the information out there.” Even though the set is thorough, “don’t make this the end of the learning process. Make it the beginning.”

Visit www.acca.org for more information on the CD set. For information on Rise’s training, visit www.hvactechnicaltraining.com.

Publication date: 10/15/2007

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