Setting Airflow: The 400 CFM Per Ton Rule
Airflow is the foundation every other measurement sits on. Get it wrong and your superheat, subcooling, and capacity numbers all lie to you. Here is how to hit roughly 400 CFM per ton with blower taps or ECM settings — and how to prove you got there.
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In This Guide
Why 400 CFM Per Ton Matters
Air is the medium that carries heat into and out of the coil. If the blower does not move enough of it, the evaporator gets too cold, latent capacity climbs, and eventually the coil ices over. Push too much air and the coil runs warm, the system sheds humidity poorly, and the customer complains that the house feels clammy even when the thermostat is satisfied.
The manufacturer rates the equipment at a specific airflow. When you are off that target, every downstream reading drifts. Superheat and subcooling shift, delta-T lies about capacity, and you can chase a refrigerant charge problem for an hour that was really an airflow problem all along. That is why experienced techs set airflow first and touch the gauges second.
The core relationship
Sensible capacity, airflow, and temperature split are locked together by one formula:
Sensible BTU/hr = 1.08 × CFM × Delta-T
The constant 1.08 comes from air density (0.075 lb/ft³) × 60 min/hr × the specific heat of air (0.24 BTU/lb/°F). Rearranged, CFM = BTU ÷ (1.08 × Delta-T).
The Rule and When to Bend It
One ton of cooling equals 12,000 BTU/hr. The industry rule of thumb is roughly 400 CFM per ton of nominal cooling capacity. Multiply tonnage by 400 and you have your target airflow:
| System Size | Nominal BTU/hr | Target CFM (400/ton) | Dry Climate (350/ton) |
|---|---|---|---|
| 1.5 Ton | 18,000 | 600 | 525 |
| 2 Ton | 24,000 | 800 | 700 |
| 2.5 Ton | 30,000 | 1,000 | 875 |
| 3 Ton | 36,000 | 1,200 | 1,050 |
| 4 Ton | 48,000 | 1,600 | 1,400 |
| 5 Ton | 60,000 | 2,000 | 1,750 |
The 400 figure is a starting point, not gospel. Match it to the climate and the humidity load:
- 350 CFM/ton — hot, humid climates where you want the coil colder to wring out more moisture. Slower air boosts latent removal.
- 400 CFM/ton — the standard middle ground for mixed climates and most residential jobs.
- 450 CFM/ton — dry desert climates or heat-pump heating mode where sensible capacity matters more than dehumidification.
Always defer to the data plate
The rule of thumb gets you close, but the manufacturer's installation instructions and the blower performance table are the authority. Variable-capacity and high-SEER2 equipment often spell out an exact CFM per ton for each operating stage. When the nameplate and the rule of thumb disagree, the nameplate wins.
How to Measure Real Airflow
You cannot set what you cannot measure. There are three practical ways to know what the blower is actually moving, from most to least direct:
- Flow hood or TrueFlow grid — the most accurate method, capturing airflow directly at the registers or across the filter slot.
- Static pressure + blower table — measure total external static pressure (TESP) with a manometer, then read the CFM off the manufacturer's blower performance chart at that static and speed tap.
- Delta-T back-calculation — measure the temperature split and known capacity, then solve CFM = BTU ÷ (1.08 × Delta-T). Fast, but only as good as your capacity estimate.
Before any of this, confirm the system is not choked. Total external static pressure should sit at or below about 0.50 iwc for most residential air handlers. Above 0.80 iwc you have a duct or filter restriction that no blower tap will fix — the air simply cannot get through. Drill your test ports, read the supply and return, and add the absolute values together.
Delta-T reality check
A normal cooling temperature split runs 14–22°F between return and supply air, landing near 18–20°F at design conditions. A split well above that range usually means airflow is too low; a split below it means airflow is too high (or the system is low on charge). Delta-T is your fastest airflow sanity check on any call.
Worked Example: A 3-Ton System
Scenario
A 3-ton straight-cool system delivering 36,000 BTU/hr nominal. The customer says the upstairs never cools. You measure a return air temperature of 76°F and a supply of 50°F — a 26°F split. What is the airflow, and where should it be?
Step 1: Find the target airflow
3 tons × 400 CFM/ton = 1,200 CFM target
Step 2: Estimate sensible capacity (about 80% of total for cooling)
36,000 × 0.80 ≈ 28,800 BTU/hr sensible
Step 3: Solve for actual CFM using the measured 26°F split
CFM = 28,800 ÷ (1.08 × 26) = 28,800 ÷ 28.08 ≈ 1,026 CFM
Step 4: Compare to target
1,026 CFM vs 1,200 target — roughly 342 CFM/ton, about 15% low
Result: Airflow is starved
A 26°F split and a back-calculated 342 CFM/ton confirm the blower is not moving enough air. Before you touch the speed tap, check static pressure and the filter — a clogged filter or crushed flex return is the usual culprit and needs to be fixed first. If the ductwork is clean and static is reasonable, bump the blower up a tap to bring airflow toward 1,200 CFM and watch the split fall back into the 18–20°F range.
What good looks like
After correcting the restriction and confirming 1,200 CFM, you should read a supply temperature around 56°F against that 76°F return — a clean 20°F split, roughly 400 CFM/ton, with static pressure comfortably under 0.50 iwc.
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Setting PSC Blower Taps
Older and budget equipment uses a permanent split-capacitor (PSC) blower motor with several fixed speeds. Each speed is a colored wire — a tap — landed on the furnace control board or pigtailed at the motor. Moving the active tap from one color to another changes the fan speed, and therefore the airflow, in discrete jumps.
There is no universal color standard, but a common convention runs black (high) → blue → yellow → red (low). Never assume — pull the blower table from the installation manual and match the tap to the CFM you need at your measured static pressure.
| Speed Tap | Typical Use | Relative Airflow |
|---|---|---|
| High (often black) | Cooling on larger tonnage | Highest CFM |
| Medium-high (blue) | Cooling, typical 2.5–3 ton | ↓ |
| Medium-low (yellow) | Heating on many gas furnaces | ↓ |
| Low (often red) | Continuous fan / small tonnage | Lowest CFM |
Cap off the unused tap
When you move a speed lead, the unused motor tap must be capped and parked on the dedicated park terminal. Leaving a live unused speed lead loose or landed can back-feed the motor and destroy it. Always confirm heating and cooling are on separate, correctly landed taps.
Programming ECM Airflow
Electronically commutated motors (ECMs) are the modern standard. Instead of fixed speed taps, they hold a commanded airflow across a range of static pressures — the motor ramps up its own torque to keep CFM constant as the filter loads or a damper closes. That is a huge advantage, but it also means a badly restricted duct system can hide behind the motor while it quietly draws more watts.
There are two common ways to set ECM airflow, depending on the platform:
- Dip switches / jumpers — many constant-CFM boards use a bank of switches to set cooling tonnage and an airflow-per-ton trim (often options like 350, 400, and 450 CFM/ton) plus heat-rise and delay profiles.
- Software or thermostat setup — communicating and variable-speed systems let you enter the exact CFM for each cooling stage, heating stage, and continuous-fan mode directly in the setup menu.
Quick ECM CFM/ton example
For a 3-ton coil you want a middle-of-the-road setting for a mixed climate:
3 tons × 400 CFM/ton = 1,200 CFM
Set the tonnage switch to 3 and the trim to the "normal / 400" position. For a humid climate, drop the trim to the "–" / 350 CFM/ton setting to run the coil colder and pull more moisture; that is 3 × 350 = 1,050 CFM.
Whatever the platform, verify the result the same way you would a PSC blower: confirm static pressure, read delta-T, and check the motor amp draw against the nameplate. Do not assume the commanded CFM is the delivered CFM until the numbers agree.
Common Mistakes in the Field
- Setting airflow with a dirty filter or coil. Restore the system to a clean baseline first, or you will set the blower against a false restriction.
- Trusting delta-T on a low-charge system. A system short on refrigerant also reads a small split. Verify charge and airflow together, not in isolation.
- Cranking blower speed to cure high static. On a PSC motor more speed just moves you further up a bad duct curve; on an ECM it spikes the watt draw. Fix the ductwork, undersized return, or crushed flex instead.
- Ignoring heat-mode airflow. Gas furnaces need enough airflow to stay inside the nameplate temperature rise (commonly 35–75°F). Too little air trips the limit switch; too much drops supply temperature and comfort.
- Forgetting the A2L conversation. New R-454B and R-32 equipment carries A2L handling and leak-detection requirements. Airflow setup is unchanged, but follow the manufacturer's mitigation and ventilation instructions on those systems.
Field workflow in one line
Clean baseline → measure total external static → set blower to target CFM/ton → confirm with delta-T and amp draw → then, and only then, touch the gauges to fine-tune charge.
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