Maytag HRV-150, HRV-210 Installation cont’d, Planning of the Ductwork, Calculating the Duct Size

6.0

Installation(cont’d)

6.3PLANNING OF THE DUCTWORK

a)Follow the instructions in Section 6.4 below to determine the appropriate duct diameters for your system.

b)Keep it simple. Plan for a minimum number of bends and joints. Keep the length of insulated duct to a minimum.

c)Do not use wall cavities as ducts. Do not use branch lines smaller than 4” (102 mm) Ø.

d)Do not ventilate crawl spaces or cold rooms. Do not attempt to recover the exhaust air from a dryer or a range hood. This would cause clogging of the recovery module. Use sheet metal for the kitchen exhaust duct.

e)Be sure to plan for at least one exhaust register on the highest lived-in level of the house if it has 2 floors or more.

6.4CALCULATING THE 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.

Duct	Recommended			Maximum
Diameter	Air Flow				Air Flow
4”(102 mm)	40 cfm	19 l/s	68 m3/h	60 cfm	28 l/s	102 m3/h
5”(127 mm)	75 cfm	35 l/s	127 m3/h	110 cfm	52 l/s	187 m3/h
6”(152 mm)	120 cfm	57 l/s	204 m3/h	180 cfm	85 l/s	306 m3/h
7”(178 mm)	185 cfm	87 l/s	314 m3/h	270 cfm	127 l/s	459 m3/h
8”(203 mm)	260 cfm	123 l/s	442 m3/h	380 cfm	179 l/s	645 m3/h

NOTE: Examples 6.4.1 and 6.4.2 use imperial measures. The same calculation applies to metric measures.

end branches

5”ø

70 cfm

main branch 6”ø 140 cfm

140 cfm

VI0003

figure 8

6.4.1Example 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 8.)

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 5ӯ 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 5ӯ duct or larger is an appropriate choice for the 2 end branches.

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

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