Quick manual D duct-sizing calculations.
(edited version of Air Conditioning Contractors of America's Bulletin 88)
Air Conditioning, Heating & Refrigeration News | January 21, 1991 | Rutkowski, Hank
Quick Manual D duct-sizing calculations
The ACCA Manual D residential duct sizing procedure guarantees satisfactory duct system performance. This article presents a shortcut method, originally introduced in ACCA Technical Bulletin 41, for making the Manual D duct sizing calculation. This is a refinement of that procedure.
Balancing dampers. Do not expect a properly designed residential duct system to be “self balancing.” A balancing damper is always required in each runout duct. It does not make any difference whether the duct system is designed by this shortcut method or by the more comprehensive Manual D procedure. Refer to ACCA Technical Bulletin 41 for an explanation of why this is true.
The complete Manual D duct-sizing procedure can be summarized on one page. The various steps are explained below.
Available static pressure. It is absolutely essential for the designer to verify how much static pressure is available to move the air through the supply and return ducts. Steps 1, 2, and 3 on the Residential Duct Sizing Sheet provide a method to obtain this information.
Step 1. Use the manufacturer's fan performance data to determine how much external static pressure is produced by the fan when it delivers the design cfm. The design is determined during the equipment selection process.
Remember, it is prudent to base the calculations on the medium range of fan speed. Designing for medium speed provides the ability to adjust the fan performance in the field.
For example, assume that 1,300 cfm is required. The blower performance table in Figure 1 indicates that the fan will provide 1,300 cfm at 0.5-in. when the fan operates at the medium low speed.
Step 2. Subtract the device pressure losses for all the air-side devices that will be installed in the duct system which are not accounted for in the blower table. This is important because the pressure that is dissipated by the air-side devices will not be available to move the air through the fittings and the straight duct runs.
Pressure-dissipating devices such as direct expansion (DX) coils, electric resistance heaters, and filters are normally associated with the fan cabinet. Refer to the footnotes below the manufacturer's blower table for information about which component pressure losses are not accounted for in the blower performance table.
For example, the notes below Figure 1 indicate that the pressure losses that are associated with a wet DX coil and a throwaway filter are accounted for. When the device pressure loss is not accounted for in the blower table, the manufacturer will usually provide another table to provide the necessary pressure loss information.
Figure 1 – Blower performance table
Model 10 Blower cfm
External static pressure
Fan speed 0.40 0.50 0.60 0.70 Low 1,300 1,200 1,100 1,000 Med low 1,400 1,300 1,200 1,100 Med high 1,650 1,500 1,350 1,200 High NA 1,700 1,500 1,300
NOTES: 1) Blower unit was tested with wet coil and throwaway filter in place. 2) The pressure loss across the electric resistance heating coils is not included in this table.
Accessory devices such as special filters and humidifiers are not usually accounted for in the blower table. The exact values for the pressure drops across accessory devices can usually be found in the manufacturer's data. Note that, if a standard throwaway filter is replaced by an accessory filter, the net pressure loss is equal to the difference between the pressure drops across the throwaway filter and the accessory filter.
Devices such as air outlets and balancing dampers also will be installed in the duct system. The exact values for the pressure drops across these types of devices can be found in the manufacturer's catalog data.
However, it is an acceptable and conservative practice to use a value of 0.03 in. for the pressure loss that is associated with these particular devices.
The Manual D, Group O fitting data indicates that a loss of 0.05 in. is associated with flexible duct junction boxes. Note that pressure losses that are associated with boxes in series are additive, and the losses that are associated with boxes in parallel are not additive.
(For any duct run, count the number of boxes between the fan and a given outlet. The pressure loss that is associated with the junction boxes in a particular duct run is equal to 0.05 multiplied by the number of boxes in that duct run.)
The 0.05-in. loss that is associated with the Manual D, Group O junction box also applies to a box plenum that is used to distribute air to a system of radial runout ducts.
Step 3. Subtract the pressure losses from the external static pressure to determine how much pressure will be available to move the air through the fittings and the straight duct sections. Ideally, this value should be equal to about 0.2 in.
Values below 0.1 in. are normally not acceptable unless the total equivalent length (TEL) of the longest run is very short. Values above 0.3 in. wg are usually excessive unless the TEL of the longest run is very long. If this value falls outside the acceptable range (0.15 to 0.25), go back to Step 1, select a different fan speed, and repeat Steps 1, 2, and 3.
Supply and return static pressure. Step 3 determines how much static pressure is available for both the supply-side (SDS) and the return side (RDS). Steps 4 and 5 split this pressure in proportion to the equivalent lengths of the longest supply-side and return-side runs. (Step 4 corresponds to items J and K on the right side of Form D-1.)
For example, if the longest supply-side run has a TEL of 176 ft and the longest return-side run has a TEL of 135 ft, the total length for both sides is equal to 311 ft. Step 4 indicates that 57% of the available static pressure (from Step 3) should be used to design the supply side, and 43% of the available static pressure should be used to design the return side.
(Note that, by using the actual TEL values, the resulting supply-side/return-side ratios will be more accurate than the ratios found in Manual D, Table A-2.)
Runout size. The Runout f/100 values chart (page ASD-178) summarizes the relationships between the TEL of a duct run, the pressure drop across a duct run and the corresponding friction rate (f/100 value). Note that the friction rates on this chart vary between 0.06 and 0.18. When the friction rate falls in this range the corresponding runout velocity will vary from about 500 to 700 fpm.
Use the runout f/100 values chart to determine the design value for the runout friction rate. The design friction rate corresponds to the point where a vertical line through the design static (SDS or RDS) intersects a horizontal line through the duct TEL value.
For example, if the design static is equal to 0.15 in., a friction rate of 0.1 should be used to size a runout that is part of a duct run that has a TEL of 150 ft.
Any point that falls between the 0.06 line and the 0.18 line is a valid solution. Points located above the 0.06 line indicate that the fan does not develop enough pressure to move the air through the duct run. Points which fall below the 0.18 line indicate that the fan produces too much pressure. When points are plotted outside the acceptable f/100 range (0.08 to 0.18) go back to Step 1, select a different fan speed, and repeat the calculations.
Note that the TEL value is the total equivalent length from the fan to the very end of the duct run. Of course, a different TEL value will probably be associated with each run. To save time, all of the runout sizes can be based on the TEL value for the longest run. This shortcut will cause the shorter runouts to be oversized, but the resulting error is usually not more than one inch in diameter.
A slightly oversized runout will not cause a problem because the balancing damper can be adjusted to compensate for the excess carrying capacity of the over-sized runout. (If the designer wants to take the time, the runout sizes could be based on the actual TEL that is associated with each run, but this extra work is contrary to the spirit of the shortcut procedure.)
The duct slide rule or the friction chart can be used to determine the runout diameter. To determine the runout diameter, set the duct slide rule by aligning the runout friction rate value (f/100 value) with the runout cfm. For example, a 6-in. diameter would be selected for a runout that carries 100 cfm at a friction rate of 0.10.
Trunk Sizes. To prevent duct-generated noise, trunk velocities should not exceed 600 or 900 fpm, depending on the duct material. The recommended trunk velocities for various types of duct materials are listed under Step 7 on the Residential Duct Sizing sheet.
A duct slide rule or a friction chart can be used to determine the trunk diameter. To determine the trunk diameter, set the duct slide rule by aligning the recommended velocity with cfm that flows through the first section (the section preceding the first branch takeoff) of the trunk duct. For example, a 16-in. diameter would be selected for a trunk that carries 1,000 cfm at a velocity of 700 fpm.
Calculating TEL. As noted above, it is only necessary to calculate the TEL of the longest supply run and the longest return run. Do not jump to conclusions. Depending on the equivalent length of the fittings, the run that has the longest TEL may not be the run that has the longest measured length. If there is any doubt about which run is the longest, check the TEL of each likely candidate.
Note that if a return run has a lot of fittings, it is possible for the TEL of the longest return run to be greater than the TEL of the longest supply run.
Calculating runout cfm. The runout cfm is determined by the Manual J load calculations and by the blower cfm. Use the room-by-room heating and cooling loads and the heating and cooling factors (HF and CF, respectively) to determine the heating and cooling cfm values. The design cfm for any given room will be equal to the larger of the two heating and cooling values. HF = Blower cfm/design heat heat CF = Blower cfm/design sensible load
Note that lines 7, 8, 9, 10, and 11 on Form D-1 can be used to calculate the runout cfm values. It is also possible to use computer software to calculate these values. ACCA's RIGHT-J load calculation software has the ability to calculate the room cfm values.
Calculating trunk cfm values. The trunk cfm is determined by adding up all of the corresponding runout cfm values. Sum the heating cfm values, and sum the cooling cfm values, and select the larger of the two totals. The Form D-2 can be used to perform these calculations.
Other sizing shortcuts. Here it is necessary to stop and consider the implications of the duct-sizing guidelines that are printed on some duct slide rules.
The specific guideline to be questioned is the one that recommends that the entire residential system be designed for one friction rate (f/100 value), typically 0.08 or 0.1. The first objection to this shortcut procedure is related to the fact that the design value for the friction rate is not arbitrary.
The f/100 design value is directly related to the available static pressure and to the TEL of the longest duct run. The “Runout f/100 values” chart summarizes this relationship.
For example, this chart shows that a design friction rate of 0.1 is only correct when the duct run has a 200-ft TEL and when 0.20 in. of static pressure is available to move the air through the run.
The cfm will be less than required if the TEL of the run is greater than 200 ft and/or if the available static pressure is less than 0.20 in.
The second objection is related to the fact that trunk duct velocities may exceed the recommended values when the trunk duct sizes are based on an arbitrary friction rate. For example, the velocity in a round trunk duct that carries 1,200 cfm will be equal to 1,000 fpm if the duct size is based on a 0.1 friction rate.
Example problem. An 8-room house will be equipped with a heat pump. The blower and the sheet metal duct system will be located in the basement.
Ten runout ducts will be supplied by one extended plenum trunk duct. Three return air grilles will be installed. One return will be located in the hallway and the other two returns will be located in large open areas. Transfer grilles will provide a return path from the smaller rooms to the hallway. A summary of the design data is shown on opposite page.
Solution to problem. Begin by determining the design friction rate.
The residential duct sizing sheet, Steps 1 through 5, indicates that 0.13-in. static pressure will be available for the supply runs and 0.08-in. static pressure will be available for the return runs. The runout f/100 chart indicates that the supply runouts should be sized for a friction rate of 0.08-in. These calculations are shown in Figures 2 and 3.
Next, calculate the room and runout cfm values. The room cfm values can be determined from the room load calculations. Simply multiply the heating loads by the heating factor and the cooling loads by the cooling factor and select the maximum value.
More than one runout will be required if a room requires a relatively large amount of supply air. The room and the runout cfm calculations are summarized in Tables 1 and 2.
Room Cfm Runout Cfm
1 128 1 128
2 133 2 133
3 88 3 88
4 224 4 112
5 112
5 133 6 133
6 71 7 71
7 184 8 92
9 92
8 102 10 102
HF = 1,000/32,400 = 0.031 CF = 1,000/20,482 = 0.049
Heating Cooling
Room Btuh Cfm Btuh Cfm DSN-cfm
1 4,157 128 1,881 92 128 2 4,299 133 2,299 112 133 3 2,842 88 1,672 82 88 4 7,035 217 4,598 224 224 5 3,603 111 2,717 133 133 6 2,114 65 1,463 71 71 7 5,116 158 3,762 184 184 8 3,233 100 2,090 102 102
The next step is to size the supply runouts. Figure 2 indicates that the supply runouts should be sized for a friction rate of 0.08 in. per 100 ft. The size of each supply runout can be determined by setting the duct slide rule so that the runout cfm value is aligned with the 0.08 in. per 100 ft runout friction rate value. The resulting runout sizes are shown by Table 3. The dimensions in this table have been rounded to a standard size.
Room Runout Cfm Diameter
1 1 128 7
2 2 133 7
3 3 88 6
4 4 112 7
5 112 7
5 6 133 7
6 7 71 6
7 8 92 6
Size the supply trunk next. The first section of the supply trunk carries 1,000 cfm. This section of the trunk is sized so that the velocity is equal to 700 fpm. The duct slide rule indicates that the initial trunk diameter will be equal to about 16 in. The trunk size can be reduced just before the room 5 runout.
At this point, the trunk is only carrying 490 cfm. The second section of the trunk can be sized so that the velocity is increased back to 700 fpm. The duct slide rule indicates that the trunk diameter will be equal to about 12 in. A schematic of the supply duct system is provided by Figure 4.
Finally, size the return trunks. All the return trunks can be sized so that the velocity in the branch and main trunks is equal to 600 fpm. These calculations are summarized below. A schematic of the return duct system is provided by Figure 3. [Figures 2 to 4 Omitted]