Ducting for HRV
I’ve been remodelling/refinishing my basement, and I haven’t really been planning to put in an HRV. Thinking about it more recently, an HRV does seem like a good idea to get fresh air into the rooms of my basement which don’t have any windows. Reading the latest issue of Fine Homebuilding, one of the recommended ones was the FanTech VHD 70R, which has 5″ duct ports.
Along the length of my basement I’ve furred down a 2×4 chase for my mechanicals (mainly plumbing, fire sprinklers, and electrical at this point) which I can use for ductwork. I have enough space there to put in a 3.25″-high rectangular duct. Would I be able to transition from the 5″ round duct that the HRV has to a 3.25″ rectangular duct that will then run the length of my basement, and have 5″ round branches coming off of it in the joist bays to each room? A 5″ circle has an area of roughly 19.5 square inches, so assuming it’s simply a matter of area, it sounds like a 3.25″ x 6″ duct will provide the same airflow. I can even make it 7″ to give it a bit more room.
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It is mostly about area but the duct shape also plays a role, as rectangles and ovals have more friction than round ducts. Even more important is the shape of transitions--smooth curves are much better than sharp bends. Adding size can help with both issues.