Evaporative cooler water use calculator

Estimate the water a swamp cooler or desert cooler actually consumes: the water evaporated into the air plus any water drained to control minerals. Water recirculated over the pads by the pump is reused, so it is not counted.

How it is calculated (an estimate)

This is our own derivation from standard psychrometric equations and has not been validated against measured cooler water use. Treat results as estimates.

1. We find the outdoor air's humidity ratio (kg of water vapour per kg of dry air) and wet bulb from your temperature, humidity and elevation using ASHRAE psychrometric equations.
2. Outlet temperature = outdoor − effectiveness × wet-bulb depression. The air leaves along a line of constant wet bulb, and we compute its new humidity ratio.
3. Dry-air mass flow = airflow ÷ specific volume of the inlet air.
4. Evaporation = dry-air mass flow × (outlet humidity ratio − inlet humidity ratio). One kilogram of water is about one litre.
5. Consumption = evaporation + bleed-off + dump-pump drains. Recirculation is excluded.

Worked example

About 8,500 m³/h (5,000 CFM) of 38 °C, 15% RH air at sea level through 80%-effective pads cools to about 23 °C (73 °F) and evaporates roughly 59 L (16 gal) per hour. The calculator shows these as its default values. Eight hours a day gives about 475 L (125 gal) per day before bleed-off.

Why real use varies

• Drier, hotter air evaporates more water per hour. That is also when the cooler gives the most cooling.
• Bleed-off can be a large share. DOE notes a bleed valve can increase water use by 10–50%, and suggests a timed dump pump (e.g. every 8–12 hours of operation) as a more efficient alternative.^[1]
• DOE cites seasonal estimates for a 1,500 ft² (≈140 m²) home without bleed-off ranging from 2,980 gallons in Santa Fe to 12,457 gallons in Las Cruces, New Mexico (SWEEP 2007). That shows how strongly climate and run-hours matter.^[1]
• Ratings are often measured without restrictive pads or ducts, so your real airflow, and therefore evaporation, may be lower.

Put the per-hour figure into the operating cost calculator. Mineral control is covered in pad maintenance & mineral buildup.