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I am replacing the flexible ductwork in my attic, which is a mix of ancient 6" and 7" metal ducting. I'm having a hard time finding 7" flexible ductwork for sale locally, but no problem with 6" and 8". For a smaller, fairly open single story ranch house, would it cause excessive airflow issues to use 6" or 8" instead?

This is going to a room with about 160 square feet by the way. An additional, larger room is about 290 square feet that currently has 2x7" ducts to it.

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up vote 4 down vote accepted

You're probably fine with the larger diameter (I wouldn't go smaller), but I would install a damper where you tie into the main trunk and then have someone come and rebalance the system using flow meters. That way a professional can optimize your entire house so that the air entering each room is the same relative to it's size and heat load, and the house will stay more comfortable as a result.

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This makes sense. I do have some metal sections with dampers. I can probably cut them down to a shorter section with damper included, and put it in-line with the 8". –  Tim Holt Jan 11 '12 at 21:38

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