CFM Explained: What Swamp Cooler Airflow Ratings Really Mean

CFM stands for cubic feet per minute, the volume of air a fan moves. On an evaporative cooler it is the headline spec, and for good reason: it determines how fast the cooler can flush heat out of a house. But the number on the box and the air that comes out of your registers are often not the same thing. This page explains where the difference comes from, how airflow turns into cooling, and how to measure what your cooler really delivers.

If you need to work out how much airflow your home requires, start with how to size a swamp cooler or the cooler size calculator.

What CFM measures

Airflow is speed times area:

Air moving at 1,000 feet per minute (FPM) through a 2 ft by 3 ft duct (6 sq ft) is 6,000 CFM. The same 6,000 CFM squeezed through a 4 sq ft duct must travel at 1,500 FPM, which is louder and takes more fan pressure. That relationship is behind nearly every airflow problem in evaporative systems.

For sizing, CFM ties directly to air changes. A house of 12,000 cubic feet supplied with 6,000 CFM gets a full air change every 2 minutes, or 30 air changes per hour. The site's recommended rates are 40 per hour for hot desert climates, 30 for moderate dry climates and 20 for mild or coastal-dry areas.

Rated CFM vs delivered CFM

Manufacturers often rate cooler airflow at zero static pressure: the blower discharging into open air, with no duct, no elbow and no grille. That is a legitimate test condition, but no house works that way. Every installation adds resistance:

• The discharge transition and roof curb or wall sleeve
• Duct length, especially flexible or undersized duct
• Elbows and tees, each adding the equivalent of several feet of straight duct
• Dampers, registers, ceiling diffusers and grilles
• Back pressure in the house when relief openings are too small

That resistance is called static pressure and is measured in inches of water column (in w.c.). Residential evaporative systems commonly run somewhere around 0.1 to 0.4 in w.c., depending heavily on the ducts. As static pressure rises, the blower moves less air. Ducted installs commonly deliver 10 to 30% less than the zero-static figure.

Real fan tables vary by blower wheel, motor and pulley setting, but the shape is typical: airflow falls faster as pressure rises. When a manufacturer publishes airflow at 0.2 or 0.3 in, compare coolers at that pressure, not at zero.

The fan laws: what happens when you change blower speed

On belt-drive coolers, the adjustable motor sheave (pulley) sets blower speed. Opening the sheave halves slows the blower; closing them speeds it up. The fan affinity laws describe the result for the same fan and system:

Airflow

Proportional to blower speed (RPM)

Static pressure

Proportional to speed squared

Motor power

Proportional to speed cubed

The practical rule: after any pulley adjustment, measure motor amp draw on high speed with a clamp meter and compare it with the nameplate full-load amps. If it exceeds the nameplate, back off the adjustment. Procedures are in belt and pulley adjustment and the swamp cooler motor guide.

How CFM turns into cooling

The cooling an evaporative cooler delivers to your rooms depends on both airflow and how much the room air is warmer than the supply air:

The 1.08 constant comes from the density and specific heat of standard air multiplied by 60 minutes per hour. At high altitude, air is less dense and the constant is somewhat lower, so the same CFM carries a little less heat in Denver than in Phoenix.

Supply temperature depends on the outdoor wet bulb and pad efficiency. Estimate it for your weather with the cooling temperature calculator. Airflow also drives water use, because every BTU removed by evaporation takes water; the water usage calculator converts CFM and temperatures into gallons per hour.

Why evaporative coolers need so much more air than AC

Central air conditioners are commonly designed around roughly 350 to 400 CFM per ton of cooling. A 3 ton system for a 1,500 sq ft house moves about 1,200 CFM. An evaporative cooler for the same house in a moderate dry climate needs about 6,000 CFM (1,500 x 8 x 30 / 60), five times as much.

The reason is temperature difference. An air conditioner delivers air around 55 F, so each cubic foot can absorb a lot of heat. An evaporative cooler delivers air in the upper 60s or 70s F, so it needs far more cubic feet to carry the same heat. That is why evaporative ducts and registers are larger, why AC ductwork is usually too small for a cooler, and why relief openings matter. See ductwork for evaporative coolers.

Many installers keep main duct velocity around 1,000 FPM or lower to limit noise and pressure loss. At that speed a 6,000 CFM system needs a trunk of about 6 sq ft, for example 24 in by 36 in.

Relief air: CFM has to leave, too

Every cubic foot the cooler pushes in must leave through windows, doors or relief vents. If the openings are too small, the house pressurizes, static pressure on the blower rises and delivered CFM falls. A common field rule is roughly 1 to 2 sq ft of net opening per 1,000 CFM; at 6,000 CFM that is about 6 to 12 sq ft spread across the rooms you want cooled. Where you open windows controls where the air goes. Details are in venting and relief air.

How to measure your cooler's actual airflow

You can estimate delivered CFM with a vane anemometer, available from tool suppliers for modest cost.

1. Set up. Run the cooler on high with pads wet, normal relief windows open and all registers in their usual positions.
2. Measure the opening. Record each register or diffuser's face size in square feet. Grilles block part of the face, so estimate the free area (often listed by the grille maker) or use the effective area if known.
3. Traverse. Hold the anemometer flat against the face and take readings at several evenly spaced points, at least 6 to 9 for a large diffuser. Average them.
4. Calculate. Average FPM x free area = CFM for that outlet. Add all outlets.
5. Compare. Divide the total by the rated CFM. Under about 70% points to restrictive ducts, a slipping belt, a low blower speed or too little relief air.

Expect this method to be accurate to perhaps 10 to 20%. It is good enough to tell a healthy system from a choked one. If delivery is low, check belt tension and pad condition before replacing anything; swamp cooler not cooling walks through the diagnosis.