Planning the Installation

Planning the Ductwork

warning

•To prevent combustion by-products, smoke or odors from entering the home and to improve efficiency, tape all duct joints securely.

•Use only ductwork deemed acceptable by state, municipal and local codes.

•Range hoods may interrupt the proper flow of smoke and combustion gases from furnaces, gas water heat- ers and fireplaces. To avoid drawing lethal gases into the home, follow the manufacturer’s recommendation for these devices and consult NFPA and ASHRAE rec- ommendations.

•Failure to install proper ductwork may result in a back draft and/or the insufficient venting of smoke and fumes.

•DO NOT install an additional in-line or external blower to increase the length of the duct run. Even small dif- ferences between blower air flow rates can greatly reduce the air draw by the hood.

•CAUTION - To reduce the risk of fire and to properly exhaust air, be sure to duct air outside the house or building. Do not vent exhaust air into spaces within walls or ceilings or into attics, crawl spaces or garages.

•For optimal performance, consult a qualified HVAC specialist when designing the duct system.

•All ductwork materials (including screws and duct tape) must be purchased separately by the customer. When planning new ductwork, always look for the shortest, most direct route to the outside.

elbow (notincluded)
fan

fan

•The exhaust connects to 8-inch dia. round duct. You may increase the duct size over the duct run if desired. To prevent a back draft, never decrease the duct size over the run. If existing ductwork is smaller than 8 inch- es, remove and replace with 8-inch ductwork.

•Do not rely on tape alone to seal duct joints. Fasten all connections with sheet metal screws and tape all joints with certified silver tape or duct tape. Use sheet metal screws as required to support the duct weight.

•To prevent back-drafts, a damper at the duct outlet may be required. Make sure ductwork does not interfere with floor joists or wall studs.

Calculating the Maximum Duct Run Length

The maximum straight duct length for the hood is determined by the type of duct used. See the chart below.

DUCT SIZEMAXIMUM DUCT RUN
8” round60 feet
10” round50 feet
3¼” X 10” rectangular

50 feet

For each elbow and transition added to the ductwork, a certain number of feet must be subtracted from the maximum duct run to compensate for wind resistance. To determine the length the ductwork cannot exceed, subtract all of

the equivalent lengths of the elbows and transitions listed below from the maximum duct run above.

Ductwork Equivalent Lengths

EQUIVALENT LENGTHS (NOMINAL)
PieceSubtract

Piece

Subtract

 

 

 

 

8” 90° elbow

7 feet

10” 90° elbow

5 feet

8” 45° elbow

3 feet

10” 45° elbow

2 feet
3¼” X 10”

 

3¼” X 10” to

 

to round 90°

25 feet

8”/10” round tran-

4 feet

transition

 

sition

 

3¼” X 10”

15 feet

3¼” X 10”

20 feet

90° elbow

90° flat elbow

 

 

3¼” X 10”

7 feet

Roof cap or wall

*

45° elbow

cap with damper

 

 

*The equivalent lengths of roof and wall caps vary with model and configuration. For equivalent length, contact the manufacturer or a qualified HVAC specialist.

Ductwork Design Tips

Wherever possible, reduce the number of transitions and turns to as few sharp angles as possible. Two staggered 45° angles are better than one 90°.

Keep turns as far away from the hood exhaust as possible, and as much space between bends as possible.

For best performance, use round duct instead of rectangu- lar, especially when elbows are required.

If multiple elbows are used, try to keep a minimum of 24 inches straight duct between them.

Avoid “S” or “back to back” use of adjacent elbows.

In regions where the weather gets extremely cold, use thermal breaks, such as a short section of non-metallic duct, to avoid indoor heat loss. Locate the break as close as possible to the outside pass through point.

Do not use flexible metal duct.

Do not use ductwork that is smaller in cross-sectional area than the recommended types above.

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