The following is based on information contained in the Air Conditioning Contractors of America’s Manual RS: Comfort, Air Quality and Efficiency by Design.

Air distribution design work consists of selecting routes for the duct runs, selecting fittings, sizing the trunk sections (when applicable), and sizing branch run-outs. However, before designers can do this, they must know:

• The results of the heat loss and heat gain calculations (refer to Manual J)

• The primary equipment location (customer survey in Manual RS)

• The supply and return locations (customer survey Manual RS)

• The appropriate location for the duct system (refer to Manual D, Section No. 1)

• The blower’s cubic-feet-per-minute rating (Manual S provides extensive discussion)

• The cfm associated with each supply outlet or return. (See Manual D)

At this point, the designer will select a particular type of distribution system - a trunk-and-branch arrangement, radial layout, etc. - and a fabrication material (sheet metal, duct board, or a flexible-wire helix product).


Duct runs should be routed as directly as possible to minimize the number of fittings along the path. Unless the system is installed in a small attic or a marginally accessible crawl space, this work usually requires a compromise among airside efficiency, encroachment on usable living space, and conflicts with work installed by other trades.

There may be some instances where the designer considers using panned airways or framed chases to create an unobtrusive conduit. ACCA strongly recommends against this practice because of the difficulty associated with sealing the path and the ease of future compromise by another trade group.

Critical circulation path

The blower is normally connected to the conditioned space by multiple supply runs. These supply runs are in parallel - like parallel resistors in an electric circuit. If there is more than one return run, there will be a second set of parallel conduits on the return side of the system.

As far as flow resistance is concerned, the performance of any collection of parallel conduits depends on the total effective length of the longest supply run and the total effective length of the longest return run. This particular conduit pair is referred to as the “critical circulation path.” (Manual D provides instruction and examples for determining effective lengths.)

Figure 2


The performance of the fittings that provide the interface among the duct runs; the equipment; the air distribution devices; and the fittings that are used to turn the flow, to create a branch flow, to merge two flows, or to change the shape of the conduit are characterized by equivalent length values.

An inspection of Manual D’s Appendix No. 3 indicates that there can be a wide range of equivalent length values associated with a particular class of fitting. This variety gives the designer considerable control over the effective length of the critical circulation path. Therefore, when the designer attempts to match the resistance of the duct system to the capability of the blower, fitting efficiency can make the difference between success and failure.

Available pressure

The duct system must be designed to work with the blower that is packaged with the heating and cooling equipment. In this regard, the first task is to determine how much static pressure will be available to move the air through the fittings and the straight sections of duct. This pressure will be equal to a value listed in the manufacturer’s blower performance table, and discounted for the total pressure drop associated with field-added components that are not included in the original-equipment manufacturer ratings.

Note: Components that are included in OEM blower values are usually listed in footnotes located below the blower table. The pressure losses associated with accessory devices are usually compiled in supplementary OEM tables.

For example, Figure 1 indicates that when operating at medium speed, the blower can move 1,250 cfm against an external resistance of 0.49 inches of water column.

In this case, the available pressure does not have to be discounted for the pressure drops associated with a wet coil or a standard filter because these components were in place when the blower was tested. However, an electric heating coil (heat pump application) is an accessory device, as indicated by the note below the blower table. This means that the designer must return to the OEM engineering data to determine the pressure drop across the resistance coil.

As indicated by Figure 2, 0.14 IWC must be subtracted from the blower pressure value. In addition, the available pressure must be adjusted for the pressure drops associated with every ancillary airside device that is in the critical circulation path. (Deductions are always required for the supply outlet, the return grille, and the branch-balancing damper. Other reductions may be associated with an accessory filter, a humidifier, or any other type of pressure dissipating device.)

Variable-speed blowers

Variable-speed blowers can operate over a wide range of flow and pressure conditions. For example, by adjusting the rotational speed of the wheel, a variable-speed-equipped supply fan can deliver 1,250 cfm against system resistances that range from less than 0.20 IWC to more than 0.80 IWC. This flexibility can compensate for poor design work, but it should not be used as a rationale for circumventing the airside design process. (Manual D, Section No. 5 provides more information about variable-speed blowers.)

Friction-rate-sizing value

Duct friction charts and duct sizing slide rules correlate duct sizes and flow rates (cfm) with a friction rate (pressure drop in IWC per 100 feet of duct, or F/100 value). To size duct runs, the designer must use a value other than available pressure. But, available pressure is one of the two factors that determine what friction rate will be used to size the duct runs. The other is the total effective length of the critical-circulation path. The following equation shows how these two factors are used to generate the design friction rate value. (Refer to Manual D for instruction and examples pertaining to the use of the friction rate worksheet.)

F/100 = Available pressure x 100/Total length of critical circulation path

Design cfm values

The cfm that must be delivered to a room depends on how the size of the room load compares to the total load on the central equipment. For example, if the room load is equal to 15 percent of the total load, the room should receive 15 percent of the blower cfm. Of course, there may be two loads (heating and cooling), in which case two cfm values would be generated, with the larger value dictating the design condition.

Note that the room cfm value may not always equal the branch-duct flow rate as large rooms (requiring a substantial amount of air) generally require multiple supply outlets. The cfm associated with each branch run will depend on the air-distribution plan.

On the return side, the flow rate associated with a room and its attendant branch run is dictated by the supply outlets that can logically be grouped with the return. This means that the designer must identify open areas, areas that can be coupled by a transfer duct or grille, and isolated rooms.

For example, all of the supply cfm would be routed through a large, central return if the home features a completely open floor plan. An area return might serve two or more rooms; a proprietary return should be installed in an isolated room. The flow rate through a trunk duct is cumulative as the observer looks from a branch duct along the trunk toward the blower. This means that the cfm associated with any section of supply trunk duct is equal to the sum of the downstream branch flows, and the cfm associated with any section of return trunk is equal to the sum of the upstream branch flows.

Noise and velocity

Initially, the size of a duct run is determined by the flow rate and the design friction rate value. This practice produces a design that will be compatible with the blower’s capability, and if generated noise were not a consideration, the sizing exercise would be complete. However, objectionable noise can be generated in a duct run if the velocity of the airflow is too high. When this is the case, the size of the offending run will have to be increased. In this regard, the velocity limit (refer to Manual D, Table 3-1) depends on the location of the duct with respect to the blower (supply or return), the type of run (trunk or branch), and the duct material.

Final design

The final design work begins with a matrix of round sizes that are compatible with the friction rate design value and the cfm associated with the duct sections. When necessary, the duct slide rule can be used to convert round sizes into rectangular sizes that have an equivalent friction rate.

Then the duct slide rule can be used to check the velocities associated with each size (use the ACCA slide rule for round and rectangular ducts). If a velocity is too high, the duct section is resized to comply with the recommended limit. Refer to Manual D for instruction and examples pertaining to the use of the duct-sizing worksheet.

Balancing dampers

Balancing dampers must be installed in each run-out duct. This is necessary because the flow rate associated with a run that is not in the critical path, or any run that has been re-sized to satisfy a velocity limit, will be excessive unless the pressure drop is adjusted to match the critical-path pressure drop. In this regard, registers are not suitable balancing devices because they generate noise in the hard-throttle position.

Airside balancing

Airside balancing is not a design task, but since this work mediates conflicts between installed system performance and design objectives, it falls within the overview of the system designer.

This means that balancing is required even when the design calculations are comprehensive and the installation procedures are perfect. All controls must be checked and adjusted, fan-speed adjustment made when required, and all balancing dampers adjusted and locked.

Quality installation

Customers deserve a quality HVAC installation to deliver clean healthy air in a comfortable home in an energy efficient manner. Residential air distribution systems design is just one step in the whole design process. The design process is just one step in a quality installation. Good contractors must invest time in a good design, make the time to install it correctly, measure their work to verify proper operation, and then give all the appropriate documentation to their customer.

These four essential items are explained in a new specification for a quality installation. The specification is aimed at helping contractors provide a properly designed, well-installed HVAC system that will meet their customers’ requirements. For a free downloadable copy of the “HVAC Quality Installation Specification for Residential and Commercial Applications,” go to .

For information on ordering the ACCA’s Manual RS, write to Air Conditioning Contractors of America, 2800 Shirlington Road, Suite 300, Arlington, VA 22206; call (888) 290-2220 or see on the Internet.