My new range hood calls for 5" pipe. I was planning to use a 5"/6" reducer to convert to 6" pipe for going through my attic to the roof. I know that using a smaller pipe than what is called for is problematic due to restricted airflow. But is there any problem with using a larger pipe than what is called for?

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No problem up sizing and this is typically done if the run is longer than normal to minimize friction losses.

I have several HRV sheet metal runs that go from 7 up to 8" where the runs were quite long.

If you push your question out to the extreme end of the spectrum and imagine hooking up to an infinite size pipe then you've really just put the pipe end outside of your structure.

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