Unless carefully planned and installed, ductwork can ruin the performance of a good ventilation unit. "Shorter is better," Rammien says. Indeed, a duct run should be limited to 30 ft. or less to keep the air moving. It must also be vented to the outside. Unfortunately, in some older homes, the ductwork terminates in the attic or basement, dumping the moisture and cooking effluent back into the house. Turns, which require elbows in the ductwork, and transitions from one type or size of ductwork to another should be minimized or avoided altogether because they restrict airflow. Another rule of thumb, Rammien explains, is, "Never downsize the ductwork in the middle of a run. This provides a place for grease and particles to collect." The dimensions of the ducts vary according to the size of the range hood. Units up to 600 cfm requires 3 1/4 x 10-in. or 7- or 8-in. round ductwork. Joints should be carefully sealed with professional (not cloth-backed) duct tape to keep air from leaking out of the ducts. A ventilation system is an important component in any kitchen. To make sure you end up with the right one, choose a range hood that is not only large enough to handle your needs but also attractive enough to complement the rest of the kitchen. Sizing a Ventilation System Conventional Range or Cooktop
•Install hood 24 to 30 in. above cooking surface •Hood should extend 3 in. on each side of cooktop •Fan should move 120 cfm for average-size range for light cooking and up to 200 to 400 cfm for heavy cooking
Island Cooktop
•Install hood 24 to 30 in. above surface for conventional equipment •Hood should be same size as cooktop •Minimum 180 cfm; 450 to 600 cfm is preferred
Commercial Equipment
•Install hood 30 to 36 in. above cooktop •1 cfm for every 100 Btu delivered by all of the burners

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