How many CFM does a 120 sq ft sunroom need?
| Airflow needed120 ft² × 1.8 CFM per ft² for a sunroom | 216CFM |
|---|---|
| Room factorglass makes this the highest load per square foot in the house | 1.8CFM/ft² |
| Supply registersabout 120 CFM each before they start to whistle | 2registers |
| Branch ductround, at 600 fpm — the quiet band for a branch | 9″ |
| Share of a 3-ton systema 3-ton system moves about 1,200 CFM in total | 18% |
| Bedroom comparisonthe same 120 ft² as a bedroom | 108CFM |
Notes
- **Split by load, not by area.** Two rooms of 120 ft² do not need the same air if one faces west behind glass and the other sits in the middle of the house. A sunroom runs at 1.8 CFM per square foot against 0.9 for a bedroom — dividing the system evenly across rooms is the single most common reason one room is always uncomfortable.
- **2 registers, not one big one.** Around 120 CFM is where a residential supply register starts to whistle. Pushing 216 CFM through a single boot buys noise and a draught in one part of the room while the far corner stays still.
- **A closed door turns a bedroom into a pressurised box.** Air pushed in has to get out. Without a return, a jumper duct or an undercut door, the room pressurises, the supply air slows, and the room ends up short of the 216 CFM the duct is delivering to the boot.
- **9″ branch at 600 fpm.** Smaller is cheaper and noisier; bigger is quieter and eats ceiling. If this run ends in flex rather than metal, size it as flex — the corrugated wall costs real static pressure.
- **This is a check, not a design.** The right number comes from a room-by-room Manual J, which accounts for window area, orientation, insulation and infiltration rather than a factor per square foot. Use this to sanity-check a drawing or a quote — a contractor who cannot show a room-by-room load is guessing too, just less transparently.
A 120 sq ft sunroom
216 CFM, delivered through 2 supply register2 on a 9″ branch duct.
Split by load, not by area
Two rooms of 120 sq ft do not need the same air if one faces west behind glass and the other sits in the middle of the house.
A sunroom runs at 1.8 CFM per square foot against 0.9 for a bedroom. Dividing a system evenly across rooms is the single most common reason one room is always uncomfortable.
Why more than one register
Around 120 CFM is where a residential supply register starts to whistle.
Pushing all 216 CFM through one boot buys noise and a draught in one part of the room while the far corner stays still.
A closed door turns a room into a pressurised box
Air pushed in has to get out.
Without a return, a jumper duct or an undercut door, the room pressurises, the supply slows, and it ends up short of the air the duct is actually delivering to the boot. This is why bedrooms are the rooms people complain about.
The branch
9″ at 600 fpm — the quiet band. Smaller is cheaper and noisier; bigger is quieter and eats ceiling.
If the run ends in flex rather than metal, size it as flex. The corrugated wall costs real static pressure, and flex that is not pulled tight costs more again.
This is a check, not a design
The right number comes from a room-by-room Manual J, which accounts for window area, orientation, insulation and infiltration rather than a factor per square foot.
Use this to sanity-check a drawing or a quote. A contractor who cannot show a room-by-room load is guessing too — just less transparently.