HVAC trunk line calculator

Undersized ductwork is why a correctly sized system still cannot cool the back bedroom, and why a brand new variable speed unit sounds like a hair dryer. This works the airflow from the equipment out: nominal tonnage times the airflow per ton your climate calls for, divided by the design velocity you are willing to run, gives the free area the trunk needs, and dividing that by the trunk height gives the width. The same arithmetic sizes each branch, and the return is checked against the total.

Velocity is the comfort setting, not a constant

The velocity you choose decides whether the system is quiet. Residential supply trunks run comfortably at 700 to 900 feet per minute; branch runs want 600 or below, and returns lower still. Push a trunk to 1,200 to save sheet metal and the customer hears it at every register, which is a callback you cannot fix without opening the ceiling again. Commercial work runs faster because the plenum is further from the occupant and the acoustic budget is different.

Airflow per ton is a climate decision

Four hundred CFM per ton is the standard starting point and the default here. Humid climates run nearer 350 so the coil spends longer wringing water out of the air, which is why a Gulf Coast install that is set up like a Denver install leaves the house cold and clammy. Dry climates can push to 450 for more sensible capacity. This is one of the settings most worth changing from the default, because the number it produces cascades through every duct on the job.

The return is half the system and gets a quarter of the attention

The return has to carry the same air the supply does. A system with generous supply trunks and one undersized central return will starve, ice the coil, trip on high head pressure and cost the customer capacity they paid for. The return figure here is simply the total airflow, stated plainly so it is on the same page as the supply sizing rather than being an afterthought at the end of the install.