Airflow Per Ton Targets and What Moves Them
Why this matters
"400 CFM per ton" is the first number most techs learn and the one most often applied where it does not belong. It is a reasonable middle value, not a target. Set a 3-ton system in a humid coastal house to 400 per ton and it will hold the thermostat at 75 F while the customer complains the house feels clammy. Set the same system in a dry climate to 350 and you give away sensible capacity the customer paid for. The airflow a system should move is a choice the equipment maker publishes a range for, and the climate and the job pick a point inside that range. This card owns that choice. How to measure what you actually got is in Airflow Measurement and Balancing Reference and the sibling how-tos named below.
The nominal range and what it means
For residential split-system cooling, makers commonly publish a cooling airflow range of about 350 to 400 CFM per nominal ton, with some equipment allowing up to about 450. A nominal ton is 12,000 BTU per hour of rated cooling, so a 3-ton system sits at about 1,050 to 1,200 CFM across the common range (3 x 350 and 3 x 400), and 1,350 CFM at 450.
Those numbers are per nominal ton of the matched system, as the maker rates it. They are not per ton of actual delivered capacity on a given afternoon, and not per ton of the house's load calculation. When the indoor unit's installation instructions publish an airflow table for this outdoor unit, that table governs. The per-ton range is the sanity check that tells you whether a tap setting you are about to choose is in a sane neighborhood.
What airflow does to the coil
Airflow decides how cold the evaporator runs and therefore how the system's capacity splits between sensible (dropping air temperature) and latent (pulling water out).
Less air across the coil means each pound of air spends longer on it and the coil runs colder. More of the air passing through drops below its dew point, so more water condenses: a larger share of the capacity goes to latent, the sensible heat ratio falls, and the house feels drier at the same thermostat setting. Total capacity drops somewhat and efficiency drops with it, and taken too far the coil surface goes below freezing and ices.
More air across the coil does the opposite. The coil runs warmer, sensible capacity and efficiency rise, latent capacity falls, and above some coil face velocity the airstream can lift condensate off the fins and carry it into the duct. The face velocity limit is in the coil maker's data.
So the per-ton choice is a dehumidification choice first and an efficiency choice second. Selecting equipment whose sensible and latent capacity fit the load, per ACCA Manual S, happens upstream of it; the airflow setting fine-tunes the split the selection already made.
Where in the range to set it
| Situation | Where makers commonly point you | Why |
|---|---|---|
| Humid climate, or a house with a known latent problem | Low end, about 350 per ton | Colder coil, more water removed |
| Equipment with a dehumidify mode or a dehumidify input | Below the normal cooling setting, by the amount the maker's setup table states, only while humidity is above setpoint | Latent removal when needed, full sensible otherwise |
| Mixed climate, typical house | Middle, about 375 to 400 per ton | Balance |
| Dry climate | Upper end, about 400, up to about 450 where the maker allows | Sensible load dominates; latent removal is wasted |
| Low stage of a two-stage or variable-capacity system | The maker's low-stage setting, a reduced airflow sized to low-stage capacity | Keeps the coil cold enough at part load |
Heating airflow is a separate setting. A heat pump's heating airflow is set from the maker's table, often equal to or close to the cooling airflow; running it lower raises supply-air temperature, which customers like, and raises head pressure, which the compressor does not. Electric heat strips carry their own minimum airflow per kit, owned by a sibling; see related: Heat Strip Kit Sizing and Circuit Field Card. A gas furnace's heating airflow is not set per ton at all. It is set so the measured temperature rise lands inside the range on the furnace rating plate; see related: Temperature Rise and the Sensible Heat Airflow Formula.
What moves airflow off the setting
Choosing a tap or DIP switch setting is choosing an intent. What the blower actually delivers depends on the system around it.
External static pressure. The resistance of filter, coil, ducts and grilles. A PSC (permanent split capacitor) blower's airflow falls as static rises; an ECM (electronically commutated) constant-airflow blower ramps speed and torque to hold its programmed CFM, up to the limit of its fan table, then it falls off too while drawing more power. Total external static is judged against the rating plate; see related: Static Pressure Budget Across a Residential System.
Filter loading. A clean filter and a filter at the end of its life can be a large share of the system's static budget apart. A setup done with a fresh filter drifts down as it loads on a PSC system and costs watts on an ECM one.
A wet coil. Condensate on the fins adds resistance compared with a dry coil. Fan tables are published with the coil wet or dry as the maker states, and setup readings should be taken under the condition the table assumes.
Return and supply restrictions. Undersized returns, closed or blocked registers, crushed flex and leaky ducts all change delivered airflow at the registers even where blower airflow holds. Return sizing is owned by a sibling; see related: Return Grille and Filter Sizing by Face Velocity.
Altitude. CFM targets are volume flow. At altitude air is thinner, so the same CFM carries less mass and less heat. Makers address high-altitude installs in their own instructions; some call for higher airflow or derating, and that adjustment comes from the manual, not from the sea-level per-ton rule.
Worked example: one 3-ton system, two houses
A 3-ton matched system with an ECM air handler. The installation instructions publish cooling airflow taps at 1,050, 1,200 and 1,350 CFM, and a dehumidify input that trims cooling airflow by the percentage stated in the setup table when a humidistat calls. Per nominal ton those taps are 350, 400 and 450 CFM (1,050 / 3, 1,200 / 3 and 1,350 / 3).
House A: a slab ranch near the Gulf coast, customer complaint "cool but sticky".
- Tap choice: 1,050 CFM, the 350 per ton setting.
- Dehumidify input: wired to the thermostat's dehumidify output, so humidity above setpoint trims airflow further, per the maker's table.
- Check against static: total external static measured at the 1,050 tap sits inside the rating plate maximum, with the filter the house will actually run.
- Result: the coil runs colder, more of the 3 tons goes to latent, and the customer's complaint is addressed by the setup rather than by lowering the thermostat.
House B: a two-story house in a high-desert city, low humidity all summer.
- Tap choice: 1,200 CFM, the 400 per ton setting, with 1,350 held in reserve.
- Why not 1,350: at 1,350 CFM, measured total external static on this duct system exceeds the rating plate maximum, so the blower would be at the edge of its table and the extra airflow would come with noise and watts. 1,200 is inside the budget.
- Result: more sensible capacity per hour of runtime, little latent capacity wasted.
What would change the answer. If House A's ducts could not pass even 1,050 CFM inside the static budget, the low-airflow setting would be a symptom of a duct problem rather than a choice, and the fix is the duct, not a lower tap. If House B had a whole-house humidifier running in winter, its heating airflow choice would not change, because heating airflow comes from the heat pump table, not from the cooling decision.
Failure modes this card is about
Low airflow mistaken for a charge problem. Low airflow across a coil lowers suction pressure and lowers superheat on a fixed-orifice system, which reads like overcharge, and on a TXV system it can look like an underfeeding valve. The shelf rule is to confirm airflow before touching the charge, per How to Charge a Fixed Orifice System by Target Superheat and How to Charge a TXV System by Subcooling.
A dehumidify setting left on full time. Wired so the trimmed airflow runs on every cooling call, the system loses sensible capacity all season and can ice on a low-load night. The trim belongs behind a humidity call, as the maker's setup table describes.
The setting chosen from the old unit. A replacement air handler's tap labels do not map to the old unit's. "Medium-high" on one product and another can be hundreds of CFM apart.
A setting that is right on paper and wrong in the ducts. A tap that should deliver 1,200 CFM delivers much less behind a crushed return. The tap is the intent; the measurement is the result.
How to verify the choice
- The chosen airflow is written on the ticket as CFM and as CFM per nominal ton, with the reason (climate, complaint, maker's dehumidify feature).
- The tap or DIP setting matches the maker's table for that CFM.
- Total external static at that setting is inside the rating plate maximum, measured with the filter the customer will run.
- Delivered airflow was estimated by at least one method, per How to Estimate Blower Airflow From Static and the Fan Table or a method from Airflow Measurement and Balancing Reference.
- Heating airflow was set separately from the heating table.
References
- Indoor unit and outdoor unit installation instructions and fan tables (governing airflow settings).
- ACCA Manual S, Residential Equipment Selection (current edition; sensible and latent capacity selection, binding where a jurisdiction adopts it by reference through the energy or mechanical code).
- See related: Airflow Measurement and Balancing Reference; Temperature Rise and the Sensible Heat Airflow Formula; Static Pressure Budget Across a Residential System; How to Set Blower Speed Taps for Cooling and Heating Airflow; Heat Strip Kit Sizing and Circuit Field Card.