Broan HRV90H Service Manual download pdf (Page 10)

5.3 CALCULATING DUCT SIZE

Use the table below to ensure that the ducts you intend to install will be carrying air flows at or under the recommended values.

Avoid installing ducts that will have to carry air flows near the maximum values and never install a duct if its air flow exceeds

the maximum value.

NOTE: Examples 5.3.1 and 5.3.2 use imperial measures.

The same calculation applies to metric measures.

5.3.1 EXAMPLE OF CALCULATION:

Problem: My installation requires two exhaust registers (one for the kitchen, one for the bathroom). I will connect these

registers to a main duct which will connect to the unit (high speed performance value of 140 cfm). What size of duct

should I use for the main exhaust duct and for the two end branches leading to the registers? (See Figure above.)

Solution: Simplified method. (For a more detailed method of calculating duct size refer to the ASHRAE or HRAI HANDBOOK.)

Main duct: Table above indicates a 6” Ø duct: Recommended air flow: 120 cfm; maximum air flow: 180 cfm. The high speed air

flow of 140 cfm is close enough to the recommended value (120) and far enough away from the maximum value

(180). Therefore a 6” Ø duct or larger is an appropriate choice for the main exhaust duct.

End branches: Each end branch will have to transport an air flow of 70 cfm (140 divided by 2). Table above indicates a 5” Ø

duct: Recommended air flow: 75 cfm; maximum air flow: 110 cfm. The high speed air flow of 70 cfm is close

enough to the recommended value (75) and far enough away from the maximum value (110). Therefore a

5” Ø duct or larger is an appropriate choice for both end branches.

NOTE: A 4" Ø duct would have been too small because the maximum acceptable value for a 4"Ø duct is 60 cfm.

5.3.2 EXAMPLE OF A DESIGN FOR A FULLY DUCTED SYSTEM FOR A UNIT HAVING A HIGH SPEED PERFORMANCE OF 222 CFM

VI0003

END

BRANCHES

5" Ø,

70 CFM

MAIN BRANCH

6" Ø, 140 CFM

140 CFM

VI0004

6" Ø, 93 CFM

6" Ø, 129 CFM

5" Ø, 65 CFM

5" Ø, 64 CFM

4" Ø, 42 CFM

6" Ø, 84 CFM

7" Ø, 222 CFM

6" Ø, 138 CFM

6" Ø, 96 CFM

DUCT DIAMETER RECOMMENDED AIRFLOW MAXIMUM AIRFLOW

4" 40 CFM 19 L/S 68 M³/H 60 CFM 28 L/S 102 M³/H

5" 75 CFM 35 L/S 127 M³/H 110 CFM 52 L/S 187 M³/H

6" 120 CFM 57 L/S 204 M³/H 180 CFM 85 L/S 306 M³/H

7" 185 CFM 87 L/S 314 M³/H 270 CFM 127 L/S 459 M³/H

8" 260 CFM 123 L/S 442 M³/H 380 CFM 179 L/S 645 M³/H

5. INSTALLATION (CONT'D)

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 27 28

Comments to this Manuals

No comments

Broan HRV90H Service Manual Page 10

Comments to this Manuals

Related products and manuals for Air filters Broan HRV90H