Air Distribution, Duct Design, Fans, and Indoor Air Quality
Manual D method
Duct design starts with available static pressure: rated blower external static pressure minus the pressure drops of coil, filter, grilles, and diffusers. Friction rate equals available static pressure times 100 divided by total effective length, and TEL counts straight runs plus the equivalent length of every fitting. Residential systems often land near 0.1 inch w.c. per 100 feet, but that is a result, not an assumption. Size each run from a friction chart at that rate, then check velocities to control noise.
Hardware practice
Seal every joint with mastic or listed tape, not cloth duct tape. Support flex duct at code intervals, keep it straight and fully extended, and never crush it into a joist bay - compression cuts capacity and raises static. Insulate to the energy code, size the filter for adequate face area, and verify total external static pressure with a manometer at the unit.
Fans
Fan laws: airflow varies with speed, static pressure with speed squared, brake horsepower with speed cubed. A small speed increase can overload a motor.
IAQ
ASHRAE 62.2 sets residential ventilation; commercial work follows 62.1 and its outdoor-air calculations. Keep indoor humidity roughly 30 to 50 percent, exhaust bathrooms and kitchens at the source, and control combustion byproducts and return-air contamination.
Worked example
A furnace rated 0.5 inch w.c. external static has a coil at 0.22 and a filter at 0.09, leaving 0.19 inch for the duct system and grilles. TEL is 190 feet, so friction rate = 0.19 x 100 / 190 = 0.10 inch w.c. per 100 feet.
Traps
Using 0.1 per 100 feet without computing TEL; ignoring coil and filter drop; crushed flex duct; undersized returns; measuring static in the wrong location.