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Installing a 36" range hood with an internal 3 speed 630 CFM 3 blower,damper and 6" vent pipe. Installation is in the area of an unused 10" square fireplace chimney flue.

Supposition: Will increase the hoods 6" vent to an 8" vent to increase air flow efficiency.

Option 1. Run vertical venting approximately 20 feet up chimney flue to top of chimney. A run of 8" galvanized pipe would be inserted into the 10" square chimney flue. The hood has a 6" vent and will require two 45 elbows connected together to enable tying to the 8" pipe in the chimney flue. A transition connector increasing the flow from 6" to 8" would also be required just after the elbows.

Option 2. Run venting horizontally approximately 3 feet through the chimney brick and out the chimney's exterior wall. A 90 elbow is required. Air flow could also be increased to 8" round.

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  • Welcome to SE. This question may get a better response after some clarification. Option 1 describes how to get from the range hood vent into the chimney flue for a 20 ft vertical run. I think Option 2 describes escaping out of the chimney flue early for a 3 ft horizontal run. I think you are asking which gets you better performance: two 45 bends plus 20 ft up vs one 90 bend and 3 ft over. Please add a specific question. Also, it would help to know how you intend to terminate each route.
    – Stanwood
    Commented Apr 18, 2018 at 12:18

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A larger pipe will certainly increase the flow as will shorter pipe lengths. Fittings are expressed in equivalent pipe length. A 90 degree elbow is typically 5 feet and a 45 degree elbow is typically 2.5 feet. So you can add up your total length and add 5 feet for each elbow to get your total equivalent length. The smaller that number is the better. The only issue you will have with increasing the pipe size is you will decrease the velocity and that won’t remove solids like grease as well but that is seldom an issue with residential hoods. On another note, all hood manufacturers lie about their CFM. 630 CFM through a 6” pipe would be a ridiculous amount of static pressure and you wouldn’t be able to stand the noise or the electrical bill. I would guess when all is said and done you will be closer to 200 CFM but anything you can do to minimize the total equivalent length of duct work or make the duct bigger will help in maximizing the output.

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