Tell the owner to get the installation instructions, which should have the maximum lenght information in them. Surprisingly in my manual, it allows up to 90 feet if using rigid metal duct with no 90 turns. With only 1 90 turn, it allows up to 60 feet of venting. However, it recommends no more than two 90 degree turns, and limits the length to 25 feet if there are 4 90 degree turns.

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Jim Robinson
New Mexico, USA