If your new air conditioner or heat pump never performed the way the brochure promised, the duct work is the first place to look. Equipment gets replaced every fifteen years or so. Ducts often stay for the life of the house, and many were undersized from the day they went in. Here is how duct sizing works.
Ducts are sized for airflow, not tonnage
A cooling system needs a certain volume of air moving across the indoor coil, measured in cubic feet per minute, or CFM. A common rule of thumb is around 400 CFM per ton of cooling, with the right number depending on the equipment and the climate. That means roughly:
- 3 ton system: about 1,200 CFM
- 4 ton system: about 1,600 CFM
- 5 ton system: about 2,000 CFM
The ducts have to carry that air to the rooms and back to the unit without too much resistance. The industry method for designing that is ACCA Manual D, which works from the room-by-room loads in a Manual J calculation.
Why there is no simple duct size chart
You will find tables online matching duct diameters to CFM. They are useful as a sanity check, but the right size also depends on:
- Duct type. Flex duct has more resistance than smooth metal, and much more when it is sagging, kinked or compressed.
- Length and the number of turns. Every bend adds resistance.
- How much air each room needs. A west bedroom needs more than a north hallway.
- The blower’s capability. Every blower has a limit on how much resistance it can push against.
Static pressure: the number that tells the truth
Technicians measure total external static pressure, the resistance the blower is working against. Every air handler has a rated maximum. When the ducts are too small or restrictive, static pressure goes above that rating, airflow drops, and you get:
- Rooms that never get enough air
- A noisy system, often whistling at the return
- A frozen coil in summer
- An overheating furnace in winter
- A shorter equipment life
Measuring static pressure takes a few minutes and should happen before any replacement quote.
The most common problem: undersized returns
In Sacramento, the problem we see most is not the supply side. It is the return. Many homes, especially 1950s and 1960s ranches in Carmichael, Arden-Arcade and Citrus Heights, and some tract homes, have a single return grille that is too small for the system. When someone installs a larger air conditioner, the return becomes the bottleneck. Adding a second return, or enlarging the existing one, often fixes noise, airflow and comfort at once.
When you increase tonnage
If a replacement is larger than the old system, the ducts must be checked. A 4 ton unit on ducts built for 3 tons will not deliver 4 tons of cooling. It will run noisy, restricted and inefficient. Usually the better answer is a correctly sized system, which often means smaller, not larger. See what size AC you need.
Leaks matter as much as size
Ducts that are the right size but leak into a 130 degree attic lose both airflow and cooling. In most Sacramento homes, sealing the ducts improves comfort more than resizing them. See duct sealing versus replacement and upstairs hot, downstairs cold.
What we do
We measure static pressure and airflow, inspect the ducts in the attic, and check the return size before quoting any replacement. If the ducts are the problem, we say so and quote that work separately. Our duct work services page covers repair, sealing and replacement. Estimates for replacement are free.
Frequently asked questions
What size ductwork do I need for a 3 ton AC?
A 3 ton system typically needs roughly 1,200 CFM of airflow. The actual duct sizes depend on duct type, length, number of turns and each room’s needs, which is what a Manual D design works out.
How many CFM per ton?
A common rule of thumb is around 400 CFM per ton of cooling, adjusted for the equipment and climate.
What happens if ductwork is too small?
Static pressure rises, airflow drops, rooms get too little air, the system gets noisy, the coil can freeze and the equipment wears out faster.
What is static pressure in HVAC?
The resistance the blower works against in the duct system. Every air handler has a rated maximum, and measuring static pressure shows whether the ducts are too restrictive.
Do I need new ductwork with a new AC?
Not always. If the ducts are sized reasonably and in good condition, sealing them may be enough. If the new system is larger than the old one, the ducts must be checked.
Why does my return vent whistle?
Usually the return is too small for the system, or the filter is clogged. An undersized return is common in older Sacramento homes.