How Much Water Does a Swamp Cooler Use? Gallons per Hour, per Day and per Season
Swamp cooler water use explained with the evaporation formula, bleed-off by water hardness, gallons per hour and day by cooler size, and what the water costs.
Short answer
A swamp cooler evaporates about CFM x temperature drop / 8,186 gallons per hour. A 4,500 CFM whole-house unit cooling air by 20 F evaporates about 11 gallons per hour, and bleed-off adds 25 to 100 percent, so total use is roughly 14 to 22 gallons per hour, or about 140 to 220 gallons over a 10 hour day.
Key takeaways
- Evaporation is fixed by physics: every BTU of cooling needs water evaporated, so more cooling always means more water.
- Whole-house coolers commonly evaporate 3 to 15 gallons per hour, rising above 20 on the hottest, driest afternoons with large units.
- Bleed-off adds 25 to 100 percent depending on water hardness; it is the part of water use you can actually control.
- At an example $6 per 1,000 gallons, a typical whole-house cooler uses roughly $0.80 to $1.30 of water on a hot 10 hour day.
- Portable coolers use far less, often 1 to 4 gallons per hour, which is why their tanks need refilling several times a day.
On this page
A swamp cooler's water use is not a mystery or a marketing number. It follows directly from how much cooling the cooler does. Every pound of water that evaporates absorbs about 1,060 BTU of heat from the air, so if you know the airflow and how much the air cools, you know the evaporation. Add bleed-off, which flushes minerals out of the sump, and you have the total. This page gives you the formula, ready-made tables, the cost, and the ways to trim waste without losing comfort. For your own numbers, use the water usage calculator.
The evaporation formula
Here is where it comes from:
- Sensible cooling. The heat removed from the air is 1.08 x CFM x temperature drop, in BTU per hour.
- Water evaporated. Each pound of water evaporated absorbs about 1,060 BTU. Divide the cooling by 1,060 to get pounds per hour.
- Convert to gallons. Water weighs 8.34 pounds per gallon. Divide by 8.34.
- Combine. 1,060 x 8.34 / 1.08 = about 8,186. So gallons per hour = CFM x drop / 8,186.
The temperature drop is the difference between outdoor air and the air leaving the pads. It depends on how dry the air is (the gap between dry bulb and wet bulb temperatures) and on pad efficiency:
On a 100 F afternoon with a 65 F wet bulb, aspen pads at about 70 percent efficiency give a drop of 0.70 x 35 = 24.5 F. Rigid media at 88 percent give 0.88 x 35 = 30.8 F. Better pads cool more and, by the same physics, evaporate more water. Learn more in wet bulb temperature explained, or check a day's numbers with the cooling temperature calculator.
Gallons per hour by cooler size
| Cooler airflow | 10 F drop | 15 F drop | 20 F drop | 25 F drop | 30 F drop |
|---|---|---|---|---|---|
| 1,000 CFM (portable) | 1.2 | 1.8 | 2.4 | 3.1 | 3.7 |
| 3,000 CFM (window) | 3.7 | 5.5 | 7.3 | 9.2 | 11.0 |
| 4,500 CFM | 5.5 | 8.2 | 11.0 | 13.7 | 16.5 |
| 5,500 CFM | 6.7 | 10.1 | 13.4 | 16.8 | 20.2 |
| 6,500 CFM | 7.9 | 11.9 | 15.9 | 19.9 | 23.8 |
Whole-house coolers commonly fall in the 3 to 15 gallons per hour range over a day. The upper columns apply to the hottest, driest afternoons, and the rated CFM is often more than the actual delivered airflow once ducts and relief air are accounted for.
How use changes through the day
Evaporation tracks the gap between dry bulb and wet bulb, which is widest in mid-afternoon. Here is an illustrative desert day for a 4,500 CFM cooler with aspen pads at 70 percent efficiency:
| Time | Dry bulb | Wet bulb | Wet bulb depression | Temperature drop | Evaporation |
|---|---|---|---|---|---|
| 9 am | 85 F | 60 F | 25 F | 17.5 F | 9.6 gal/h |
| Noon | 95 F | 63 F | 32 F | 22.4 F | 12.3 gal/h |
| 3 pm | 102 F | 65 F | 37 F | 25.9 F | 14.2 gal/h |
| 6 pm | 98 F | 64 F | 34 F | 23.8 F | 13.1 gal/h |
| 9 pm | 88 F | 62 F | 26 F | 18.2 F | 10.0 gal/h |
The average across these readings is about 12 gallons per hour, close to the 20 F planning figure used below. Mornings and evenings use noticeably less than the afternoon peak, and switching to low speed at those times cuts use further because less air passes through the pads.
Portable coolers: tank and refill math
Most portable evaporative coolers have no float valve or bleed-off. They draw from a built-in tank, so the useful question is how often you refill it:
Worked example: refilling a portable on a dry afternoon
A 1,000 CFM portable with a 15 F drop evaporates 1,000 x 15 / 8,186 = 1.8 gallons per hour. With 5 usable gallons in the tank, it runs about 5 / 1.8 = 2.8 hours before it needs topping up. A large 3,000 CFM portable at the same drop uses 5.5 gallons per hour, so even a 15 gallon tank lasts under 3 hours.
That is why many larger portables accept a garden hose with a float. Without bleed-off, minerals stay in the tank, so drain and rinse it every few days in hard-water areas. See sizing portable and outdoor coolers.
Bleed-off: the controllable part
When water evaporates, dissolved minerals stay behind. Without removal, they concentrate in the sump until they scale the pads, pump and pan. Bleed-off (or a timed purge) removes some concentrated water so fresh water dilutes the rest. The amount is set by how many times you let minerals concentrate, called cycles of concentration:
| Water type | Typical cycles | Bleed-off added | Total use multiplier |
|---|---|---|---|
| Soft (or rainwater) | 5 | About 25 percent | 1.25 x evaporation |
| Moderately hard | 3 | About 50 percent | 1.5 x evaporation |
| Very hard | 2 | About 100 percent | 2.0 x evaporation |
Much of the Southwest has moderately hard to very hard water, which is why scale is the top maintenance complaint there. Setting bleed-off and choosing between continuous bleed and a purge pump is covered in bleed-off and water quality.
Daily and seasonal use, with cost
Worked example: a 4,500 CFM cooler on a hot day
Evaporation at an average 20 F drop: 4,500 x 20 / 8,186 = 11.0 gallons per hour.
With moderately hard water (50 percent bleed-off): 11.0 x 1.5 = 16.5 gallons per hour.
Over 10 hours: 165 gallons per day.
Cost at an example $0.006 per gallon ($6 per 1,000 gallons): 165 x 0.006 = about $0.99 per day.
Over a 30-day month: 4,950 gallons, about $29.70. Over a 120-day season at the same average: 19,800 gallons, about $119.
| Cooler | Evaporation per day | Soft water (+25%) | Moderate (+50%) | Very hard (+100%) |
|---|---|---|---|---|
| 1,000 CFM portable | 24 | No bleed on most portables | No bleed on most portables | No bleed on most portables |
| 3,000 CFM window | 73 | 92 | 110 | 147 |
| 4,500 CFM | 110 | 137 | 165 | 220 |
| 6,500 CFM | 159 | 199 | 238 | 318 |
In practice, daily totals are usually lower than these because the morning and evening hours have smaller temperature drops. The full electricity and water picture is in swamp cooler operating cost.
Putting it in perspective
A heavy-use day of 165 gallons sounds like a lot, and over a season it adds up: a whole-house cooler can account for a meaningful share of summer household water use, especially in hot, dry regions where it runs long hours.
Coolers are also most water-hungry exactly when they work best: on hot, dry afternoons. In humid weather the possible temperature drop shrinks, so evaporation falls, but cooling falls too. See best climates for swamp coolers for where that balance works.
Note: Some water utilities in drought-prone areas publish guidance on evaporative coolers, such as limits on continuous bleed-off or rebates for purge systems. Check your utility's conservation page.
How to measure your actual use
- Turn off all other water in the house, including irrigation and the ice maker.
- Read the meter (most read in gallons or cubic feet; 1 cubic foot is about 7.48 gallons).
- Run the cooler for one hour at a steady speed during a typical afternoon.
- Read the meter again and subtract. That is your total, evaporation plus bleed-off.
- Measure bleed-off separately by catching the bleed line in a container for 1 minute and multiplying by 60.
- Compare evaporation (total minus bleed) with the formula. If the total is far above what the formula plus bleed predicts, look for a float valve or pan leak.
Ways to cut water use
| Measure | What it saves | Impact on cooling |
|---|---|---|
| Tune continuous bleed to actual hardness | Often a large share of bleed water | None if scale stays under control |
| Switch to a timed purge pump or dump valve | Water dumped only on schedule, not constantly | None; often cleaner sump |
| Fix float valve and pan leaks | Can be hundreds of gallons a day if severe | None |
| Run on low or vent overnight | Less airflow and smaller drop at night | Small |
| Shade the cooler and supply line | Slightly less wasted heat gain | Slight improvement |
| Turn off when nobody is home | All hours avoided | House warmer on return |
What you cannot do is reduce evaporation while keeping the same cooling. If a product promises the same cooling with dramatically less water, it is either moving less air or delivering a smaller temperature drop.
Frequently asked questions
How much water does a portable swamp cooler use per day?
A 1,000 CFM portable cooling air by 15 F evaporates about 1.8 gallons per hour, or roughly 15 gallons over 8 hours. Most portables have no bleed-off, so that is close to total use.
Does a swamp cooler use more water in humid weather?
Less, actually. Higher humidity shrinks the possible temperature drop, so less water evaporates. The trade-off is that you also get less cooling.
Why does my swamp cooler use so much water?
The usual culprits are a float valve that never fully shuts off, a bleed-off set too high for your water, or a leak in the pan or supply line. Compare a one-hour meter reading with the evaporation formula to find out.
Can I reduce swamp cooler water use without losing cooling?
Yes, by tuning bleed-off to your water hardness, switching from continuous bleed to a timed purge, and fixing overflow leaks. Evaporation itself cannot be reduced without reducing cooling.
Do swamp coolers increase my sewer bill?
It depends on the utility. Many cities base residential sewer charges on winter water use, so summer cooler water does not raise sewer fees; others bill sewer on every gallon. Check your utility's billing method.
Can I use rainwater or well water in a swamp cooler?
Yes, as long as it is clean enough to avoid clogging the pump and distributor. Well water is often hard, so it may need more bleed-off; rainwater is soft and needs very little.
Sources and further reading
- Evaporative Coolers, U.S. Department of Energy, Energy Saver
- ASHRAE Handbook: Fundamentals (psychrometrics), ASHRAE
- Evaporative Coolers (fact sheet), Colorado State University Extension
- National Weather Service, NOAA