This article describes small-diameter high-velocity air duct systems.

Our page-top photo shows foil-faced smaller-diameter high-velocity HVAC ducts branching off of a larger main trunk line also covered with foil-faced fiberglass insulation.

Small Diameter High Velocity Air Ducts

High velocity HVAC Duct Advantages

• Improvement in building envelope energy efficiency:
  
  The smaller diameter of high velocity air ducts can permit duct routing through small or confined spaces such as a cathedral ceiling where otherwise larger-diameter ducts would, by occupying more space, interrupt the uniformity and quality of the building's insulation envelope, possibly increasing un-wanted heat loss or heat gain through the roof and cathedral ceiling.
• Pinpoint conditioned air delivery:
  
  Small diameter air ducts can permit pinpoint delivery of conditioned air, improving HVAC performance especially in complex, hard to heat or cool spaces.
• Energy savings?
  
  if the ducts are routed through conditioned space a substantial energy savings is possible for both heating and cooling. (Vineyard 2003)
  
  Other sources confirmed an energy savings with high velocity duct systems, but a smaller one (Payne 2020).
  
  Still other researches found an energy cost not savings (Hassan 2022) so clearly it's important to read through this research in some detail. We provide these articles in PDF form below.
• Installation cost & features:
  
  Material and installation costs of small-diameter ductwork are lower for builders who wish to realize the energy savings of bringing ductwork into conditioned space compared to the cost.
  
  The relatively shallow bulkhead (dropped ceiling) height necessary for bringing small-diameter ductwork into conditioned space is more attractive to builders than traditional bulkheads. (NREL 2017)
• Retrofit cost:
  
  Small diameter HVAC ducts may be easier to install in air conditioning or heating duct retrofit in existing buildings.

High velocity HVAC Duct Warnings

• Air duct noise complaints?
  
  At AIR FLOW RATES in HVAC SYSTEMS we note that the faster the rate of air movement through ducts the noisier will be the HVAC system. At the greater air velocity required to deliver sufficient air through a smaller-diameter duct, occupants may complain of air movement noises
  
  Noise complaints were cited in several of the small diameter high velocity duct studies that we've reviewed. (Neale 2017).
• Air handler blower fan efficiency loss?
  
  Running the small-diameter duct system at high airflow rates significantly reduces fan efficacy. System control should maximize lower speed fan operation and minimize high-speed fan operation. (NREL 2017)
• Draft complaints?
  
  If the air supply registers are not placed thoughtfully in a high-velocity duct system, occupants may complain of being in an uncomfortable supply-air draft, particularly in cooling or air conditioning systems
• Uneven room temperatures?
  
  The small-diameter system did not maintain room-to-thermostat temperature uniformity in all rooms because of insufficient airflow and lack of response to varying solar gains. (NREL 2017) (Poerschke 2017)
• Installation expertise required:
  
  Register placement is more critical for reducing draft-related comfort concerns. (NREL 2017)

High Velocity Small Diameter Duct Installation Errors

The picture just above shows unnecessary lengths of small-diameter flex duct left by the installer. The small diameter of these ducts also tells us that we're looking at a high-velocity air conditioning system that uses a combination of small-diameter ducts and higher air velocity to deliver cooling air to the conditioned space.

In our opinion, excessively-long snaking and twisting flex duct is often found where the air ducts were installed by someone lacking proper training in HVAC ductwork.

Watch out: If you see a length of flex-duct snaking across an area with multiple unnecessary twists and turns, the combination of length and unnecessary bends reduces airflow, with the costs just cited above.

Read more at DUCT ROUTING & SUPPORT

High Velocity Ductwork Performance Research & Sources

• ANSI/ASHRAE Standard 111-2008, Measuring, Testing, Adjusting, and Balancing of Building HVAC Systems, PP 5.2.4
• Burdick, Arlan. Advanced strategy guideline: Air distribution basics and duct design. No. NREL/SR-5500-53352; DOE/GO-102011-3461. National Renewable Energy Lab.(NREL), Golden, CO (United States), 2011.
• Ghosh, Debashis, Mingyu Wang, Edward Wolfe, Kuo-huey Chen, Shailendra Kaushik, and Taeyoung Han. ENERGY EFFICIENT HVAC SYSTEM WITH SPOT COOLING IN AN AUTOMOBILE-DESIGN AND CFD ANALYSIS [PDF]  SAE International Journal of Passenger Cars-Mechanical Systems 5, no. 2012-01-0641 (2012): 885-903.
  Abstract
  
  Spot, or distributed, cooling and heating is an energy efficient way of delivering comfort to an occupant in the car.
  
  This paper describes an approach to distributed cooling in the vehicle. A two passenger CFD model of an SUV cabin was developed to obtain the solar and convective thermal loads on the vehicle, characterize the interior thermal environment and accurately evaluate the fluid-thermal environment around the occupants.
  
  The present paper focuses on the design and CFD analysis of the energy efficient HVAC system with spot cooling. ...
  
  Spot cooling was achieved by strategically placing multiple nozzles in the vehicle directed at specific body parts. Nozzle design and nozzle locations were paramount to the success of comfort delivery and achieving energy efficiency through spot cooling.
  
  CFD analysis was mostly done in steady state mode for designing the spot cooling system. Based on the results of CFD simulation and heat transfer analysis, spot cooling airflow quantities and temperatures were recommended for implementation in the vehicle and testing in the wind tunnel.
  
  Lower cooling requirement on the conventional HVAC system due to spot cooling is the primary basis for energy savings achieved in AC mode. On a pure heat transfer basis, significant improvement in cooling delivery to the occupant was achieved through a quad combination strategy of spot cooling at significantly lower airflow and cooling assist from the conventional HVAC system.
• Hassan, Z., M. S. Misaran, N. J. Siambun, and M. Adzrie. THE EFFECT OF AIR VELOCITY ON THE PERFORMANCE OF THE DIRECT EVAPORATIVE COOLING SYSTEM [PDF] In IOP Conference Series: Materials Science and Engineering, vol. 1217, no. 1, p. 012016. IOP Publishing, 2022.
  Abstract:
  
  This experimental study aimed to determine the effect of airflow velocity on the performance of a direct evaporative cooling system. Rectangular-shaped honeycomb cooling pads with a length of 34 cm, a width of 25 cm, and a thickness of 3.5 cm are used as cooling media.
  
  The main parameters of the study are low air velocity (2.3 ms−1), medium (3.2 ms−1), and high velocity (3.7 ms−1).
  
  The data collected include dry bulb temperature, wet bulb temperature, output air temperature, input and output air velocity, input and output humidity, and solar radiation. These data are used to determine saturation efficiency, cooling capacity, temperature decreases, and feasibility index. ...
  
  The results showed that the evaporative cooling system could produce output temperatures up to 27.5°C with input 31.4°C at low airspeed, 27.97°C with input 31.47oC at medium speed, and 27.7°C with input 31.30°C at high air speed.
  
  It was concluded that a low airflow rate would add to the cooling efficiency, and the higher the airflow rate, the lower the cooling efficiency.
  
  The results showed that evaporative cooling is achievable with a feasibility index of 19.89 ≤ F*≤ 20.67. The results also affirmed that cooling capability is higher where the feasibility indexes are comparatively low.
• Modera, M. 1993. Characterizing the performance of residential air distribution systems, Energy in Buildings, vol. 20.
• Neale, James Richard. EXPERIMENTAL AND NUMERICAL INVESTIGATION OF NOISE GENERATION FROM THE EXPANSION OF HIGH VELOCITY HVAC FLOWS on board ocean going fast ferries [PDF] PhD diss., UNSW Sydney, 2006.
  
  Abstract excerpt:
  
  This thesis details a study of strategies used to limit the flow generated noise encountered in the outlet diffusers of high velocity heating, ventilation and air conditioning (HVAC) duct systems. The underlying noise rating criterion is drawn from the specifications covering ocean going aluminium fast ferries. A
  
  lthough directed primarily towards the fast ferry industry the results presented herein are applicable to other niche high velocity HVAC applications. Experimental tests have been conducted to prove the viability of a high velocity HVAC duct system in meeting airflow requirements whilst maintaining acceptable passenger cabin noise levels.
• NREL, A. Poerschke: IBACOS BUILDING AMERICA CASE STUDY: HIGH-VELOCITY SMALL-DIAMETER DUCT SYSTEM [PDF] Pittsburgh, Pennsylvania [PDF] (2017) National Renewable Energy Lab. (NREL), U.S. Department of Energy, Golden, CO (United States) NREL/FS-5500-64162; DOE/GO-102017-4665
  
  Abstract
  
  This study tests the performance of a variable airflow small-diameter duct heating, ventilation, and air conditioning (HVAC) system in a new construction unoccupied low-load test house in Pittsburgh, Pennsylvania. The duct system was installed entirely in conditioned space and was operated from the winter through summer seasons.
  
  Measurements were collected on the in-room temperatures and energy consumed by the air handler and heat pump unit. Operation modes with three different volumes of airflow were compared to determine the ideal airflow scenario that maximizes room-to-room thermal uniformity while minimizing fan energy consumption. ...
  
  Measured results indicate the small-diameter, high velocity airflow system can provide comfort under some conditions. Solar heat gains resulted in southern rooms drifting beyond acceptable temperature limits.
  
  Insufficient airflow to some bedrooms also resulted in periods of potential discomfort. Homebuilders or HVAC contractors can use these results to assess whether this space conditioning strategy is an attractive alternative to a traditional duct system.
  
  The team performed a cost analysis of two duct system configurations: (1) a conventional diameter and velocity duct system, and (2) the small-diameter duct system.
  
  This work applies to both new and retrofit homes that have achieved a low heating and cooling density either by energy conservation or by operation in a mild climate with few heating or cooling degree days. Guidance is provided on cost trade-offs between the conventional duct system and the small-diameter duct system.
• Payne, William, and Brian P. Dougherty. A SIDE-BY-SIDE ENERGY USE COMPARISON OF A VARIABLE-SPEED SMALL DUCT HIGH VELOCITY AND TWO-STAGE CONVENTIONAL DUCTED HEAT PUMP IN A NET-ZERO ENERGY HOME [PDF] NIST Technical Note 2101 (2020).
  
  Abstract
  
  Two air-source, split system heat pumps were installed in a residential, net-zero energy home that was constructed as a laboratory on the campus of the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland USA.
  
  The first heat pump was a two-stage, 7 kW (2 ton), 15.8 seasonal energy efficiency ratio (SEER), 9.05 heating seasonal performance ratio (HSPF) conventionally ducted system, and the second heat pump was a variable-speed, 10.6 kW (3 ton), 14 SEER, 8.35 HSPF, high velocity ducted system.
  
  These two systems operated side-by-side, using separate supply ducts and a common return duct, on a weekly alternating schedule to condition the home that was operated with very consistent, simulated thermal loads.
  
  We wanted to know if the high velocity system could provide comparable energy use efficiency to the conventional system.
  
  The results of this study showed that it did meet the required loads while doing so with slightly greater efficiency; the average cooling coefficient of performance (COP) was (0.40±0.11) higher, and the average heating COP was statistically equal.
  
  A new firmware was provided at the end of the heating season which greatly improved the performance of the high velocity system; its average heating COP went from (1.8±0.9) to (2.5±1.1) at a 95 % confidence level. The new firmware heating COP averaged (1.05±0.23) higher than the old firmware over the same outdoor temperatures.
  
  Defrost performance is very different for these two systems yet they consumed equivalent energy per HDD; the conventional system uses a timed-initiate, temperature-terminate algorithm with auxiliary electric resistive heating while the high velocity system uses calculated evaporator parameters with a hot-gas bypass before a full reverse cycle defrost with no supplementary resistive heat.
  Key words energy use comparison; field test; low load home; net-zero home; small duct high velocity; two-stage heat pump; variable-speed heat pump
• Poerschke, Andrew. MEASURED PERFORMANCE OF A VARIED AIRFLOW SMALL-DIAMETER DUCT SYSTEM [PDF] No. NREL/SR-5500-64161; DOE/GO-102017-4664. National Renewable Energy Lab.(NREL), Golden, CO (United States), 2017.
  
  Abstract
  
  This study tests the performance of a variable airflow small-diameter duct heating, ventilation, and air conditioning (HVAC) system in a new construction unoccupied low-load test house in Pittsburgh, Pennsylvania.
  
  The duct system was installed entirely in conditioned space and was operated from the winter through summer seasons. Measurements were collected on the in-room temperatures and energy consumed by the air handler and heat pump unit.
  
  Operation modes with three different volumes of airflow were compared to determine the ideal airflow scenario that maximizes room-to-room thermal uniformity while minimizing fan energy consumption. Black felt infrared imagery was used as a measure of diffuser throw and in-room air mixing.
  
  Measured results indicate the small-diameter, high velocity airflow system can provide comfort under some conditions. Solar heat gains resulted in southern rooms drifting beyond acceptable temperature limits.
  
  Insufficient airflow to some bedrooms also resulted in periods of potential discomfort. Homebuilders or HVAC contractors can use these results to assess whether this space conditioning strategy is an attractive alternative to a traditional duct system.
  
  The team performed a cost analysis of two duct system configurations: (1) a conventional diameter and velocity duct system, and (2) the small-diameter duct system.
  
  This work applies to both new and retrofit homes that have achieved a low heating and cooling density either by energy conservation or by operation in a mild climate with few heating or cooling degree days. Guidance is provided on cost trade-offs between the conventional duct system and the small-diameter duct system.
• Siebein, Gary W., and Robert M. Lilkendey. "Acoustical Case Studies Of HVAC System in Schools." ASHRAE Journal 46, no. 5 (2004): 35.
  
  Includes comments on high speed air duct noise.
• Vineyard, Edward A., Randall L. Linkous, and Evelyn Baskin. MEASURED PERFORMANCE OF CONVENTIONAL AND HIGH-VELOCITY DISTRIBUTION SYSTEMS IN ATTIC AND SPACE LOCATIONS [PDF] TRANSACTIONS-AMERICAN SOCIETY OF HEATING REFRIGERATING AND AIR CONDITIONING ENGINEERS 109, no. 2 (2003): 45-51.
  
  Abstract:
  Residential distribution systems are inherently inefficient at delivering heated or cooled air to the conditioned space as the result of poor design and installation practices. Examples of some of the more common problems include heat loss/gain in unconditioned spaces and leakage through supply and return ducts.
  
  These defects can result in significantly increased energy consumption, poor thermal comfort, and high peak electricity demand.
  
  Efforts to improve distribution systems could result in substantial nationwide energy savings since more than fifty percent of existing homes have ducted systems.
  
  In an attempt to quantify the potential energy savings resulting from the elimination of duct losses, a field test was conducted to compare the energy consumption of an attic-ducted system to a no-loss duct system for two types of forced-air distribution systems: conventional and high-velocity.
  
  The no-loss system was achieved by locating the entire duct system and air handler in the conditioned space. The results were compared to predicted energy savings using ASHRAE Standard 152P as a means of validating the procedures used for determining distribution efficiency.
  
  Excerpt:
  The results from the tests indicate that, for the conventional system, placing the ducts in the conditioned space resulted in a measured energy savings of 31% (heating) and 36% (cooling). The predicted savings using ASHRAE Standard 152P were 33% for heating and 15% for cooling.
  
  For the high-velocity system, the measured energy savings were 46% (heating) and 35% (cooling).
• Walker, Iain S. BEST PRACTICES GUIDE FOR RESIDENTIAL HVAC RETROFITS No. LBNL-53592. Lawrence Berkeley National Lab.(LBNL), Berkeley, CA (United States), 2003.
  Excerpt:
  Use register grilles that have reduced pressure drops and noise, and better throw into the room for better mixing. Grilles should be chosen and located such that the high velocity air does not enter the occupied parts of the room directly.
  
  Noise and pressure-drop reduction is aided by selecting grilles with curved blades and by placing air flow control dampers at plenum take-offs (this also reduces duct system pressures and therefore has the potential to reduce leakage).
• Zaatari, Marwa, Atila Novoselac, and Jeffrey Siegel. "The relationship between filter pressure drop, indoor air quality, and energy consumption in rooftop HVAC units." Building and Environment 73 (2014): 151-161.

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