Worth keeping rooftop kitchen fan?

Old 06-23-15, 06:28 PM
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Worth keeping rooftop kitchen fan?

Can anyone answer these questions and/or comment on the kitchen ventilation options Iíve listed below?
1. Can a rooftop extractor fan be damaged from the reduced air flow if itís connected to a 7-inch duct instead of the 8-incher it was designed for?
2. How much would using a 7-inch round duct reduce the air flow?

My kitchen has a restaurant-style exhaust system that was installed more than 20 years ago by a previous owner who was a caterer. She put in a rooftop extractor fan (NuTone RF-1N), about 4 feet of round 8-inch duct (3 feet vertical and 1 foot horizontal) and a big ugly steel range hood. Iím now renovating the kitchen and have pulled down the range hood and the horizontal part of the duct.

The fan has barely been used over our eight years here and I suspect the people we bought from didnít use it either. It never seemed to work right. This spring I went up onto the roof, opened it and discovered it was jammed with wasp nests. I gave it a thorough cleaning and it now works though thereís a screech at maximum speed. My uneducated guess is that a rubber mount went hard or corroded over the years. Will check in a month or so when the mosquitoes are gone.

The NuTone RF-1N is rated for 600 cfm and does a nice job of cooling the house on hot days. Iím replacing the ugly steel hood with a conventional over-the-range cabinet and a residential range hood (Broan QML30SSC). The fan in the new hood is rated for 200 cfm.


Plan A: The new Broan hood takes a round 7-inch duct. Initially I figured Iíd install the hood but without the Broan fan, attach a 7-inch duct running to a reducer that will connect it to the rooftop fan. But does the rooftop fan need x amount of air to avoid overheating? And would the smaller duct reduce the air flow to the point where itís not drawing much more than 200 cfm?

Plan B: Remove the extractor fan from the rooftop housing. Use the new Broan fan for ventilation. Use a reducer to connect the 7-inch duct to the old 8-incher which runs to the extractor's rooftop housing. But would a normal 200 cfm kitchen fan like the Broan have enough power to push air 3-4 feet vertically?

Plan C: Install and use the Broan hood as normal with 3-4 feet of 7-inch round duct running horizontally through the cabinets and then the exterior wall. Leave the rooftop fan in place but cap the 8-inch duct above the kitchen ceiling where it canít be seen. Maybe fill it from the top with foam insulation. Have the fan and duct torn out when we replace the roof in 8-10 years.

BTW, I've seen homes with wall-mounted weather-proof fan hoods that look like metal boxes. But I don't know what they're called and my web searches haven't found any for sale in Canada. Can anyone point me to a Toronto area store or Canadian online vendor?

Old 06-23-15, 06:46 PM
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Since the rooftop fan has such a large cfm draw, I would not connect it to anything that has a fan on it. Was that your plan? Either the contained fan in the hood, or the rooftop mount fan, but not both.
Old 06-24-15, 04:55 AM
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I was going to remove the fan from the hood and then connect the empty hood to the rooftop fan.
Old 06-25-15, 01:44 PM
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If this were my house, I would install a new hood with a separate duct to outside. However, I would LEAVE the rooftop fan in place, and connect it to a suitable grill & use it for general ventilation.
If you decide to connect the new hood to the rooftop fan, remove the hood fan (as you stated). You will NOT hurt the rooftop fan by reducing the duct down to 7", as the rooftop exhaust fan is a centrifugal fan. You will lose some airflow however.
Old 06-25-15, 06:25 PM
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This link will take you to the installation sheet for your fan. Note that NuTone does not recommend reducing the exhaust from 8 to 7 inches.

Old 06-25-15, 06:54 PM
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For whatever combination you arrive at, be aware that large fans can backdraft naturally drafted furnaces and water heaters. This isn't the exact topic so the required details aren't listed, just a heads up.


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