I have a roof mounted fan connected to a duct that the microwave exhaust is connected to. It seems that if I'm using the microwave fan only and then turn on the roof fan the microwave fan make a groaning sound and seems to slow down or even stop. Using each fan independently works fine. I just would like to get the maximum exhaust possible. What is happening and any tests I can do? Thanks for your help.
Kitchen Exhaust Fans in Series
