Air Flow and Duct Design

Airflow across the indoor coil on residential heat pumps, or straight cooling systems, is always a topic of interest for the more astute members in HVAC technical circles…and rightly so. Proper airflow, or more accurately the lack thereof, may well be the HVAC industry’s most cited culprit for substandard equipment performance and efficiency. The importance of design airflow has received even more attention with the entrance of the 13 SEER standard.

There was a time when I would occasionally call Mfg Tech Reps looking for assistance in diagnosing my daily headaches. The first question they ask…”What’s the airflow?”. Well, actually that was usually the second question. The first question was…”What’s the model number?” (a little humor intended there). One distributor tech rep I know, designed a little pamphlet that is essentially a booklet of FAQ’s…the little black book lists several measurements to take, after which the answer to the question is usually obvious. Mfg’s technical guru’s no doubt spend a lot of time troubleshooting low airflow issues, when their true purpose in life, is more one of addressing questions regarding  the exotic  particulars of the equipment.

Commercial ducting systems are necessarily designed to carry the required volume of air, at the least cost. Least costs involves duct size and fan power requirements, among other things. I assume the engineers find the optimum combination of duct size and fan motor horsepower to produce the needed airflow for the least amount of initial costs and long term operating expense…

Residential systems aren’t designed that way. The equipment mfg’s stick a blower / motor combination in the air handler or furnace and tell you it’ll blow the required air so long as you build the duct system big enough to accommodate the limits of the blower / motor combination…which is usually defined in terms of ESP…external static pressure. So you work backwards finding a duct size that won’t choke down the indoor blower, which is reasonably simple enough if you have a Manual D or reasonable facsimile.

It behooves all of us to have at the very least, some comprehension of rudimentary airflow dynamics within a piece of duct. Just enough to understand why some “rules of duct design” need to be accepted and followed. Following the rules will carry you to the most successful duct system design, minimizing the number of potential  flow headaches to have to deal with afterward. Designing out the headaches to begin with is far better than working them out afterward.

Airflow inside a duct or pipe is all about resistance to flow.  Any time air flows through a piece of duct or pipe, it will encounter some resistance, which consumes some of the energy pushing the air through the duct. The net effect is reduced flow. The factors that produce the resistance are numerous. The best case scenario is round, metal, straight pipe, offering the least amount of resistance. Lined rectangular metal, ductboard and flex get progressively worse in terms of resistance. Then, changes in direction or  cross-sectional area, passage through a register, a filter, electric heat element, heat exchange coil, transition or past a take-off add more resistance. Any deviation from the round, metal, straight pipe scenario will create additional disturbances that negatively affect the airflow, requiring some consideration of the deviation in respect to the total system design. And, the only way to compensate for the distubances in most cases is to reduce the velocity of airflow through the duct…and the only way to do that is increase the duct size.  In some cases, improving the “geometry” of the piece of duct, reduces the disturbance…like substituting a radius elbow for a mitered 90.

Duct design via Manual D has been beautifully simplified for those of us neither mentally equipped nor necessarily interested in navigating through the heavy duty engineering design approach. Most all the “disturbance” factors have either been estimated at some direct loss in inches water column (IWC)  pressure, or converted to a value called “equivalent length”. The equivalent length conversion allows us to simply add a linear value to the the straight duct sections, rather than try to calculate an actual pressure loss value for some duct fitting. For example, a 90 deg radius elbow might have an equivalent length of 30 ft.  That simply means the elbow will create the same resistance as 30 ft section of straight pipe. So, a 15 ft run-out with an elbow stuck in somewhere, would have a total effective length of 15 + 30 = 45 ft.

The gist of the friction loss method is a friction rate factor determined by the longest duct run and available static pressure. Then the friction rate factor is applied to a friction loss chart or ductulator to size the duct sections, based on the volume of air they will carry.  So long as you don’t make any huge errors in estimates or arithmetic, the final results  are almost guaranteed to work.

I have over the years seen statements made by so called intelligent and educated individuals offering arbitrary friction rate values for residential duct design, like 0.1 or 0.08. They are saying essentially, size the duct for a specific friction loss value without any regard to the actual duct system  geometry requirements. The value they offer is probably a number that would indeed result in a duct system adequate for the blower capability. But depending on the actual duct system requirements, the value could  be lower than needed, resulting in oversized duct and a waste of material.

The information in Manual D is all most of us need to both understand the mechanics of airflow and design residential duct systems. It is an excellent reference resource to keep on a shelf in your library.

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