ducted range hood: vibration and air coming out of top vent
We just installed a Broan Allure QS2 hood. It's a ducted installation with the duct coming out of the back of the unit, making an immediate 90-deg turn up and going straight through the roof. When I turned it on I therefore expected to have little or no air to come out of the vent on the top of the unit. But the mount of air is pretty significant. I checked the installation and according to the manual all seems to be fine, including the properly turned baffle. I also noticed significant vibration of the whole unit at the top speed. I called Broan and they told me that the reason for both things is my ductwork since the imemdiate turn generates a lot of turbulence. My questions are:
1. Could there be another reason?
2. Suppose I open up the unit and cover the top vent from the inside with silver tape or something similar. Is there any reason not to do this?
3. Obviously at this point I am not going to change my ducting. Is there any other way to reduce vibration?