DUCT DESIGN
Duct Routing and Configuration:
1. Route ductwork, both horizontally and vertically, square and plumb with building walls.
2. Go straight! The straighter the duct system, the both energy and first costs will be lower. When laying out a duct system, reduce the number of bends, turns, and transitions to an absolute minimum.
3. Use round spiral duct wherever it can fit within space constraints. Round duct is less expensive than oval and rectangular duct, especially when routed in long, straight sections. Round spiral duct requires less sealing than rectangular duct.
4. With rectangular duct, if at all possible, avoid the use of square throat elbows with or without turning vanes…use full radius elbows. Never add turning vanes to a full radius elbow.
5. Where space limitations require the use of square throat elbows, provide airfoil configured turning vanes that are sized, spaced, and constructed in accordance with SMACNA standards (see Fig. 4-3, HVAC Duct Construction Standards Metal and Flexible).
6. DO NOT USE DUCT LINER INSULATION. Duct liner, unless provided with foil-faced jacket on the airflow side will ultimately erode and, in the interim, provides an ideal mold breeding ground.
7. Provide opposed blade balancing dampers in low pressure, low velocity ductwork for air-balancing. Do not include air splitter controls or “air scoops”…these generate a lot of noise and provide very little balancing effect.
Supply, Return, Relief and General Exhaust Duct Sizing: Size ductwork for typical comfort conditioning systems using the "equal friction" method and the following criteria:
1. Low pressure, low velocity supply, return, and exhaust ductwork should be sized on the basis of 0.10" wg/100 lf pressure loss.
2. Medium pressure, medium velocity supply ductwork should be sized on the basis of 0.30" wg/100 lf pressure loss.
3. Use the following graphic, a Trane "Ductulator", Carrier Calculator, or equivalent method to determine duct size selection based on friction loss criteria:
4. “Up size” duct as required to limit duct velocities and potential noise in accordance with the following:
Location / Design Room Criteria(RC) / Maximum Allowable
Velocity (fpm)
Trunk/Main Ducts / Branch Ducts
Rectangular / Round or Round Oval / Rectangular / Round or Round Oval
In duct shaft or above a hard (GWB) ceiling / 41+ / 3500 / 5000 / 2800 / 4000
31-40 / 2500 / 3500 / 2000 / 2800
≤30 / 1700 / 2500 / 1400 / 2000
Above suspended acoustical tile ceiling / 41+ / 2500 / 4500 / 2000 / 3600
31-40 / 1700 / 3000 / 1400 / 2400
≤30 / 1200 / 2000 / 1000 / 1600
Exposed ductwork within an occupied space / 41+ / 2000 / 3900 / 1600 / 3100
31-40 / 1400 / 2600 / 1200 / 2100
≤30 / 1000 / 1700 / 800 / 1400
5. Size transfer ducts on the basis of 0.04" wg/100 lf with a maximum air velocity of 750 fpm.
6 Select duct sizes in even inches and, for rectangular duct, maintain aspect ratios (width/height) of 3 or less (preferably, 2 or less) for supply ducts and 5 or less for return or exhaust ducts.
7. Do not reduce duct size downstream of a branch or takeoff unless the flow rate is reduced by 25% or more and the duct size changes by at least 4”…this reduces fitting pressure losses and costs and generates some static pressure regain.
8. Where ductwork must be routed through the webs of bar joists supporting a floor or roof above, the maximum duct (including insulation) sizes that will “fit” are tabulated below:
Joist Depth(in) / Maximum Duct Size
Round
(in) / Square
(in) / Rectangular
(in)
8 / 5 / 4 x 4 / 3 x 8
10 / 6 / 5 x 5 / 3 x 8
12 / 7 / 6 x 6 / 4 x 9
14 / 8 / 6 x 6 / 4 x 9
16 / 9 / 7 x 7 / 6 x 10
18 / 11 / 8 x 8 / 7 x 11
20 / 11 / 9 x 9 / 7 x 12
22 / 12 / 9 x 9 / 8 x 12
24 / 13 / 10 x 10 / 8 x 13
30 / 15 / 12 x 12 / 10 x 14
Duct Construction and Sealing: The guide specifications require that HVAC ductwork be constructed in accordance with SMACNA's HVAC Duct Construction Standards - Metal and Flexible for the following pressure classes:
Application / Construction Pressure ClassReturn Ductwork / -2" W.G.
Supply Ductwork Downstream of
Air Terminal Units / +1" W.G.
Supply Ductwork With Fan Static
Pressure Less Than 2.5" W.G. / +2" W.G
Supply Ductwork with Fan Static
Pressure Greater Than 2.5" W.G. / +3" W.G
Supply Ductwork with Fan Static
Pressure Greater than 5.0"W.G. / +4" W.G.
Smoke Control System Ductwork / 1.5 times the design static pressure, but not less than +/-3" W.G.
The guide specifications also require that HVAC ductwork be sealed in accordance with SMACNA requirements for each seal class as follows:
Duct Construction Class / Seal Class+/- 1" W.G. or less / C
+/- 2" W.G. / B
+/- 3" W.G. and greater / A
These duct design and construction requirements are adequate for the vast majority of HVAC ductwork, but for projects that include higher pressures or more severe duty (such as industrial exhaust ductwork) may require more stringent requirements that the designer must define on the drawings.
Underfloor Air Distribution: Avoid the use of underfloor air distribution (UFAD) for comfort air-conditioning applications.
Even with ducted UFAD, one aspect must be clearly understood: there will be temperature variations between the floor and the "stratified height" (typically +/-7 feet above the floor). A temperature gradient of about 10°F can be expected, with 60-70% of that gradient occurring within the first 3 feet above the floor. This may result in some "cold floor" of "cold draft" complaints, especially by women wearing dresses or skirts. After the building is occupied, some relocation of floor outlets may be required to address these complaints.
In an effort to support sustainable design, the General Services Administration (GSA) allowed the use of unducted underfloor air distribution systems (UFAD) in buildings prior to 2007. Several buildings were constructed for the GSA incorporated unducted UFAD between 2001 and 2005 and numerous problems with the design and operation of UFAD soon became evident. In 2006, six buildings with unducted UFAD were selected for detailed study by the National Institutes of Building Sciences. This study identified several significant failures in design and construction, resulting in poor performance by unducted UFAD.
Consequently, in 2007, unducted UFAD was banned by the GSA. The GSA 2010 Design Guidelines for Public Buildings states the following:
"A fully ducted underfloor air distribution (UFAD) system or displacement ventilation system is permitted except in courtrooms, restrooms, cafeterias, kitchens, laboratories, loading docks, mail rooms, U.S. Marshal’s areas, and detention areas. The insulated supply air ductwork from the AHU must be connected directly to each floor diffuser or sidewall grille. Unducted or partially ducted UFAD systems or displacement ventilation systems with pressurized plenums are not permitted. Unducted supply is allowed only in computer rooms. [Emphasis added.]
"Equipment such as air-handling units, VAV boxes, or other equipment that requires maintenance, is not permitted below a raised-access floor."
Based on the GSA findings, the use of unducted UFAD systems was added the HVAC design guidelines as a concept to be avoided.
The No. 1 problem with unducted UFAD is plenum air leakage. While plenum air leakage is an architectural design and general construction issue, it is a mechanical engineering concern to the extent that the capacity of the air-handling unit must be adequately sized to compensate for the leakage at design load and controls must be designed to compensate for the air leakage at partial loads. To minimize these design issues, a tight plenum is required.
Plenum air leakage is defined in two categories:
1. General construction leaks from the plenum into other building cavities. An example would be leakage around and within annular spaces in conduit, openings around electrical device boxes, etc.
2. Leakage through the raised-access flooring into conditioned spaces, such as leaks of conditioned air from the plenum through floor-panel seams and edge closures, in-floor electric-power connections and outlet service units, as well as around air diffusers that are not fitted well and/or do not close tightly.
Plenum air leakage tests at design static pressures conducted in the six federal buildings and courthouses by the GSA ranged from 30% to 200% of the design airflow rates, compared to the design criteria of a maximum 5% air leakage. Construction of an airtight plenum requires strict coordination of 10 to 12 trades. Special construction techniques to guarantee plenum tightness have not yet been developed for concrete, masonry, drywall, millwork, sealant and joint specialists, raised-access floor installers, carpenters, plumbers, electricians, communications and IT installers, etc.
Some of the consequences of plenum air leakage include:
1. Occupant complaints of being cold around the feet and legs from air leakage during partial-load conditions, or when the thermostatic setpoint was satisfied.
2. Energy wasted because AHUs were required to run at higher-than-expected airflow rates and/or static pressures and for longer periods to compensate for air leakage.
3. IAQ issues and housekeeping difficulties caused by accumulation of inert and biological matter and pest infestation within the raised-floor plenum.
The second common problem with unducted UFAD is that the thermal mass of the floor slab and plenum walls is a significant issue for energy management and control. Observations during the GSA study revealed that longer than expected operational periods by the HVAC systems were required to maintain plenum temperatures.
Heat and moisture transmission and condensation in the plenum also are issues because gradients across the plenums resulted in non-uniform temperatures in the occupied spaces, and surfaces within the plenums were more likely to support condensation.
Other problem areas include the following:
1. Occupancy, over time, results in space rearrangement…moving desks and other office furniture, piling books and papers on the floor, bringing that nice throw rung from home to dress up a drab cubical, other things that will negatively impact the design air distribution. Moving floor vents and electrical connections is not easy once the system is under pressure and working. Once you lift a tile to do some work you are losing static pressure for the whole system and if the floor is open for any length of time, temperature control goes out the window. And, even after the floor is re-closed, are the new diffuser locations going to provide satisfactory air distribution and even zone temperature?
2. Maintenance is an issue. Access to the connections and equipment within the floor plenum, once furniture and other fixtures are in place, can create real headaches. Every change within the plenum will introduce the need to reseal leakage pathways because of new or changed electrical, communications, and/or plumbing penetrations. And, finally, there will be a need to routinely check and reseal the plenum cavity as sealants and caulks age and deteriorate, creating new air leakage pathways.
GSA has the right answer…use ducts for air distribution, not spaces constructed of leaky general construction materials! If an Owner insists on using UFAD, the design must be based on using fully ducted UFAD and the Owner must be advised, in writing, that air distribution may require modification as furniture is rearranged, personnel change, etc. over time.
Special Exhaust Systems: Duct sizing criteria for special duct systems is defined in the following sections of these guidelines:
Kitchen Ventilation Systems Application and Design Guide
Industrial Ventilation
Laboratory Exhaust Systems
Engineering Design Guidelines
DUCT DESIGN 6