Product review (1 stars): Caveat Emptor
The Broan PN 634 Roof cap advertisement says it is supposed to be for 8" duct, but I disagree. I measured the vent throat and it was only 3.5"x10" or 35 sq in, a 30% reduction from the 50 sq in that a 8" duct needs. The vendor refuses a refund, and hides behind Broan's boilerplate. Topping off our 710 cfm Zephyr hood and 8" duct exhaust system with this vent cap will result in a expensive system (including relays, transformers, exhaust damper and MUA damper, ceiling soffit, drywall, texture, paint, and lights ~ $2500) with the performance of those $350 Chinese specials. Squirrel cage fans aren't positive displacement pumps. Air velocity doesn't increase proportionately at restrictions, pressure goes up and volume declines. Using this vent cap would be like putting retreads on a Ferrari.