Manual D says the Maximum Return side FPM for Flex duct should be:
Originally Posted by Gary Swanson
Return Grill face velocity - 500 fpm
Filter Grill face velocity - 300 fpm
Trunk and Branch Ducts - 700 fpm
Trunk and Branch Ducts - 600 fpm
Return and Filter Grill stay the same as listed above.
Instead of learning the tricks of the trade, learn the trade.