The Fan Laws in Plain Terms

The three fan laws explained for techs — how CFM, static pressure, and motor horsepower each change when you change blower speed, why small RPM increases cost big power, and how to apply them in the field.

What it is

The fan laws (also called the fan affinity laws) describe how a blower's airflow, the pressure it develops, and the power it draws all change when you change its speed. They're three simple relationships, and once they click, a lot of blower behavior stops being mysterious — why bumping the speed up a little uses a lot more power, why a small RPM change moves more air than you'd expect on pressure, and why you can't just "spin it faster" forever to overcome bad ducts. These hold for a fixed system (same ductwork) when you vary fan speed.

How it works

Three relationships, all tied to the change in fan RPM:

Fan Law 1 — Airflow follows speed directly. CFM changes in direct proportion to RPM. Increase fan speed 10% and you get about 10% more CFM. Double the speed, double the airflow. This one is linear and intuitive.

Fan Law 2 — Static pressure follows the square of the speed. The pressure the fan develops changes with the square of the RPM change. Increase speed 10% and pressure goes up about 21% (1.10 × 1.10 ≈ 1.21). Double the speed and pressure quadruples (2² = 4). Pressure climbs much faster than airflow does.

Fan Law 3 — Power follows the cube of the speed. The horsepower (and watts) the motor must deliver changes with the cube of the RPM change. Increase speed 10% and power jumps about 33% (1.10³ ≈ 1.33). Double the speed and power goes up eight times (2³ = 8). This is the big one — power explodes as you raise speed.

So as you crank a blower up: airflow rises a little, pressure rises more, and power draw rises a lot.

In the field

Why a small speed bump costs so much power. Because power follows the cube, nudging the blower up even one speed tap can raise the motor's draw substantially. That's why "just turn it up" to fix airflow isn't free — you pay for it in amps, heat, and motor stress, and you can overload a motor by overspeeding it.

Why you can't outrun bad ducts. If the duct system is restrictive (high static), Fan Law 2 is working against you — pressure climbs with the square of speed, so the faster you spin, the harder the system fights back. You burn cube-law power (Law 3) for diminishing airflow gains. The right fix for high static is bigger ducts/returns, not more fan speed.

Estimating a speed change. If you know you need, say, 15% more CFM, Law 1 says spin ~15% faster — but then check that the motor can handle the cube-law power increase (1.15³ ≈ 1.52, roughly 52% more power) and that the static the duct will throw back (Law 2: 1.15² ≈ 1.32, ~32% more) is within the equipment's limits.

ECM context. A constant-CFM ECM is essentially applying the fan laws automatically — it senses load and adjusts torque/RPM to hold the target CFM as static changes, but it's still bound by the same physics. Against very high static it ramps power up (cube law) until it hits its ceiling, then it too falls short of CFM.

Normal values & targets

Law 1 (CFM ∝ RPM): +10% RPM ≈ +10% CFM; ×2 RPM ≈ ×2 CFM.
Law 2 (Static ∝ RPM²): +10% RPM ≈ +21% static; ×2 RPM ≈ ×4 static.
Law 3 (Power ∝ RPM³): +10% RPM ≈ +33% power; ×2 RPM ≈ ×8 power.
Practical CFM target you're trying to hit: ~350–400 CFM per ton in cooling — use the fan laws to estimate the speed change, then verify the actual airflow.

Common faults & what they mean

Motor running hot / tripping after a speed increase: cube-law power. Overspeeding pushed the motor past its rating. Back the speed down or address the real restriction.
Turned the blower up but airflow barely improved: the duct/return is so restrictive that Law 2 ate the gain — pressure shot up and choked off the added CFM. Fix the ducts.
High amp draw on the blower motor: check whether someone over-tapped the speed; the cube relationship makes power draw very sensitive to speed.
ECM ramping to max and still short on CFM: the ECM is fighting excessive static; it's hit the cube-law power ceiling. The ducts/return are undersized.
Belt-drive RTU blower (commercial): you change CFM by changing the pulley/sheave — same fan laws apply to the RPM change, including the cube-law power jump that can overload the motor if you overspeed it.

Tech tips & gotchas

"Just turn it up" is rarely free or even effective. Airflow rises linearly, but pressure rises with the square and power with the cube. A restrictive system punishes added speed. Fix restriction first.
The cube law protects motors — respect it. Overspeeding a blower (wrong sheave, too-high tap) can overload the motor because power balloons. Don't chase CFM past the motor's rating.
The fan laws assume the same duct system. They describe what happens when you change fan speed on a fixed system. Change the ductwork and you've changed the operating point too.
Use them to sanity-check, then measure. The laws give you a good estimate of how a speed change will play out, but always confirm actual CFM, static, and amp draw afterward.
They explain belt-drive RTU sheave changes. On commercial belt-drive blowers, opening or closing an adjustable sheave changes blower RPM — and the same square/cube relationships govern the resulting pressure and power. A small sheave adjustment can have an outsized power effect.

Safety / code notes

Overspeeding a blower can overload the motor (cube-law power) — confirm the motor's full-load amp rating isn't exceeded after any speed or sheave change. De-energize before changing speed taps or sheaves. These are air-side mechanical relationships; no refrigerant work involved.