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Calculator · Evaporative cooling
Evaporative cooler size calculator
A swamp cooler can only cool the air toward the outdoor wet-bulb, so the wet-bulb sets the size more than the floor area does. At 105°F dry-bulb and 70°F wet-bulb a pad with 0.85 effectiveness delivers 75.3°F air, and a 30,000 BTU/h sensible load at an 80°F room needs about 5,850 CFM. Enter your design air and load to get the airflow. For the load itself, run the Manual J calculator first.
- Type your station's design wet-bulb from the ASHRAE wet-bulb design conditions and the mean coincident dry-bulb that goes with it.
- Add the cooler's saturation effectiveness, the room temperature to hold, the sensible load and the elevation.
- The result shows the air leaving the cooler and the CFM it takes to carry the load, or says the cooler cannot keep up.
Example result for the default inputs
105.0°F dry-bulb and 70.0°F wet-bulb outdoors, effectiveness 0.85, room at 80.0°F.
Cooler airflow
5,850 CFM
Air leaving the cooler
75.3 °F
Room minus supply
4.8 °F
| Wet-bulb depression | 35.0°F |
|---|---|
| Drop across the pad | 29.8°F (0.85 x 35.0°F) |
| Sensible heat factor used | 1.080 BTU/h per CFM per °F at sea level |
| How the airflow was worked | 30,000 BTU/h / (1.080 x 4.75°F) |
| Relief air | The cooler runs on outdoor air only, so open windows or relief vents have to exhaust as much air as it delivers. |
For estimation purposes only. Consult a licensed engineer for final designs.
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How the supply air and airflow are calculated
A direct evaporative cooler pulls outdoor air through a wet pad. Water evaporating into the air lowers its dry-bulb toward the outdoor wet-bulb, and how far it gets is the pad's saturation effectiveness. The room load then decides how much of that air you need.
- Air leaving the cooler
t_supply = t_db - e x (t_db - t_wb)ASHRAE defines the saturation effectiveness e as how far the leaving air gets toward the entering wet-bulb. At 105°F dry-bulb and 70°F wet-bulb, a 0.85 pad delivers 75.3°F.- Airflow for the load
CFM = Q_sensible / (1.08 x altitude factor x (t_room - t_supply))The sensible heat equation, the same one the HVAC CFM calculator uses. For 30,000 BTU/h and a room at 80°F, the worked example needs about 5,850 CFM at sea level.- Altitude factor
p(z) / 14.696, p(z) = 14.696 x (1 - 6.8754e-6 x z)^5.25591.08 assumes sea-level air. The ASHRAE standard atmosphere scales it down with elevation, to 1.038 at 1,100 ft.- When it cannot work
t_supply >= t_roomIf the cooler cannot deliver air below the room temperature, no airflow carries the heat out, and the calculator says so instead of printing a number.
Outdoor conditions and what they do to the airflow
Six cases run through the same function the form uses, 30,000 BTU/h of sensible load each. Where the cooler cannot get the air below the room, the row says so.
| Case | Supply, °F | Room minus supply, °F | Airflow, CFM |
|---|---|---|---|
| Worked example, 105/70°F, 0.85, sea level | 75.3 | 4.8 | 5,850 |
| Same, at 1,100 ft | 75.3 | 4.8 | 6,090 |
| Same, pad at 0.90 | 73.5 | 6.5 | 4,270 |
| Same, worn pad at 0.70 | 80.5 | none | cannot meet |
| High desert, 98/60°F, 0.85, 5,000 ft, room 78°F | 65.7 | 12.3 | 2,710 |
| Humid afternoon, 100/74°F, 0.85 | 77.9 | 2.1 | 13,230 |
At 1,100 ft the same load needs about 4 percent more air, because thinner air carries less heat per CFM. A 0.90 pad cuts the airflow by 27 percent against a 0.85 pad. A worn pad at 0.70 delivers 80.5°F air, above the 80°F room, so it cannot carry the load at any airflow. On the humid afternoon the air leaves at 77.9°F and the airflow climbs to 13,230 CFM, which is the ASHRAE warning about wet-bulbs over 70°F in numbers.
Standards and data sources
- ASHRAE Handbook, Fundamentals (2017), Chapter 14, Climatic Design Information - the design wet-bulb to size with: "Design conditions based on wet-bulb temperature" with the mean coincident dry-bulb, which ASHRAE says are "useful for design of cooling towers, evaporative coolers, and outdoor-air ventilation systems."
- NOAA NCEI Engineering Weather Data, Phoenix/Sky Harbor AZ (period of record 1973 to 1996) - the example in the field hints: design wet-bulb 75°F with a mean coincident dry-bulb of 97°F at 1.0 percent (76/97 at 0.4 percent), against 108°F dry-bulb with a 70°F mean coincident wet-bulb.
- ASHRAE Handbook, HVAC Systems and Equipment, Chapter 41, Evaporative Air-Cooling Equipment - the definition of direct saturation effectiveness, the 50 to 95 percent span for wetted pads, 70 to over 95 percent for rigid media, about 80 percent for clean random media, and the 70°F wet-bulb limit for indoor comfort.
- ASHRAE Handbook, Fundamentals, Psychrometrics chapter - the sensible heat factor 1.08 for standard air and the standard atmosphere used for the elevation correction.
- Cooler manufacturer's data sheet - the saturation effectiveness at your airflow and pad depth, and the cooler's rated airflow at the static pressure of the job.
Limitations of this calculator
- Sensible load only. The cooler adds moisture to the house, so indoor humidity rises and occupants feel warmer than the dry-bulb suggests. The page does not model that.
- The effectiveness field runs from 0.5 to 0.95. Deep rigid media can rate above 0.95, so enter 0.95 for those, which errs toward more airflow.
- Effectiveness drops as the pad clogs or dries in spots. A cooler sized at its new-pad rating loses capacity as the pad ages.
- The calculator uses the standard-air factor 1.08 scaled for elevation and treats the supply air as dry air. Fan heat and heat picked up in ducts are left out.
- Design wet-bulb is your input. The page does not look up a design wet-bulb by ZIP code, and the design temperatures by ZIP page gives a climate zone estimate paired with the 1 percent dry-bulb, not the station wet-bulb design condition ASHRAE uses for evaporative coolers.
- The psychrometer wet-bulb stands in for the thermodynamic wet-bulb in the ASHRAE definition. The difference is smaller than the error of a field reading.
- It works out airflow only. Pick the unit from the manufacturer's rated airflow at the static pressure of the job.
- Direct coolers run on outdoor air only. ASHRAE says they should not recirculate indoor air and that exhaust should equal the air brought in, so plan relief openings for the full airflow.
Common evaporative cooler sizing mistakes
- Sizing by floor area alone. A square-foot rule cannot see the wet-bulb, and the wet-bulb decides how cold the air leaves the pad. Two houses of the same size, one in the desert and one on the humid Gulf Coast, need very different coolers, and the humid one may not work at all.
- Using a dry-bulb without the wet-bulb that goes with it. The leaving air temperature depends on the gap between dry-bulb and wet-bulb. A hot dry afternoon and a hot humid one at the same dry-bulb give very different results. For a design condition, take the station's design wet-bulb with its mean coincident dry-bulb, the pair ASHRAE gives for evaporative coolers, not the 1 percent dry-bulb with its mean coincident wet-bulb.
- Closing the house up. With no relief path the cooler pressurizes the house, airflow falls off and humidity builds. Open windows or relief vents sized for the airflow.
- Counting on the new-pad effectiveness for years. Scale and dust lower the effectiveness. If the cooler barely meets the load at its rated effectiveness, it will fall short once the pad wears.
- Setting the room temperature too low. The closer the room setpoint is to the leaving air temperature, the faster the airflow climbs. In the worked example, holding 78.0°F instead of 80.0°F raises the airflow by about 73 percent.
Common questions about evaporative cooler sizing
How do I size an evaporative cooler?
Work out the air temperature leaving the pad from the outdoor dry-bulb, wet-bulb and the pad's saturation effectiveness, then divide the sensible load by 1.08 times the gap between the room and that air. At 105°F and 70°F wet-bulb with a 0.85 pad, air leaves at 75.3°F, and 30,000 BTU/h with the room at 80°F needs about 5,850 CFM at sea level.
What size swamp cooler do I need for 2,000 square feet?
It depends on the cooling load and the outdoor wet-bulb, not on the floor area alone. Run a load calculation for the sensible load, then use this page with your station's design wet-bulb and its mean coincident dry-bulb. Some installers size by an air change every two to three minutes, but that rule ignores the wet-bulb, which is what decides whether the cooler can keep up.
How cold will an evaporative cooler make the air?
Supply temperature equals dry-bulb minus effectiveness times the wet-bulb depression. At 105°F and 70°F wet-bulb, a 0.85 pad delivers 75.3°F. In a high-desert afternoon at 98°F and 60°F wet-bulb the same pad delivers 65.7°F.
What is saturation effectiveness on a swamp cooler?
It is how close the leaving air gets to the entering wet-bulb, as a fraction of the gap. The ASHRAE Handbook puts an efficient wetted pad at up to 95 percent, a poor one at 50 percent or less, rigid media at 70 to over 95 percent depending on depth and air velocity, and clean random-media coolers at about 80 percent. Use the number on your cooler's data sheet.
Do evaporative coolers work in humid weather?
Not well. The ASHRAE Handbook says direct evaporative cooling is not applicable for indoor comfort when the outdoor wet-bulb is above about 70°F. The leaving air gets too close to room temperature, and the airflow needed to carry the load climbs past anything practical.
Does elevation change evaporative cooler sizing?
Yes. Air at elevation is thinner, so each CFM carries less heat. The factor 1.08 becomes 1.038 at 1,100 ft, and the worked example goes from 5,850 to 6,090 CFM.
Why do I need to open windows with a swamp cooler?
A direct cooler brings in outdoor air only. ASHRAE notes that these coolers should not recirculate indoor air and that exhaust should equal the incoming air. Without a relief path the house pressurizes, airflow drops and humidity builds.
Used on the same job
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