Swamp cooler water use and calcium buildup in Las Vegas

Why swamp coolers scale

A swamp cooler pumps water over pads and blows desert air through them. The water evaporates and the air cools; the dissolved minerals can’t evaporate, so they stay behind. The Las Vegas Valley Water District’s water quality summary, published in 2026 with 2025 data, gives the valley’s water 280 parts per million of hardness, 16 grains per gallon. In a cooler pan that water is boiled down, in effect, all day.

The University of Arizona Cooperative Extension’s pamphlet on evaporative cooler water use spells out the damage. Mineral deposits and scale from hard water can cause rust and corrosion in metal coolers; it cites estimates that this can shorten a cooler’s life by 50 percent. Scale on the pads also spreads the water unevenly, leaving dry “hot spots” and less air flow, so the cooler cools less.

Swamp cooler bleed-off explained

Bleed-off is the answer most makers give. A small tube or valve on the recirculating line drains part of the pan water all the time, so fresh water keeps diluting what is left. The extension pamphlet describes it this way: draining part of the recirculating water reduces the buildup of hard water minerals.

Newer coolers may do the same job on a timer. Phoenix Manufacturing’s owner’s guide for its Pro-series ducted coolers (2019) says the pre-installed cleaning system drains the cooler water every 5 hours of running and lets it refill with fresh water, “to minimize mineral scale build-up”. Its older and smaller models use a bleed-off tee and tube instead.

How much to bleed? The pamphlet quotes the University of Arizona’s water systems manager suggesting about a third of the makeup water, and lower if no scale is forming on the pads. Your manual’s setting comes first.

Where the drain line ends matters too. Phoenix Manufacturing says to avoid draining cooler water onto a roof, because mineral build-up or roof damage may occur; it says to drain according to local plumbing codes.

How much water bleed-off adds

The best measured figures we found are old but direct. The extension pamphlet (revised 1994) reports preliminary data from a University of Arizona and City of Phoenix study of coolers at 46 Phoenix homes:

Cooler set-up	Water use per hour of running
Without bleed-off	about 4.4 gallons
With bleed-off	about 10.4 gallons
All coolers in the study	about 7.6 gallons

In that study, bleed-off more than doubled the hourly figure. The same pamphlet also cites an estimate that bleed-off raises cooler water use by 10 to 50 percent, and New Mexico’s Office of the State Engineer warns that a bleed-off line can add 50 percent or more, so expect the real difference to depend on how wide the bleed is set. The New Mexico brochure suggests a dump pump, which empties the pan on a timer, as the alternative. It reports that bleed-off coolers used an average of about 8,650 gallons over a cooling season. Phoenix air is a fair match for Las Vegas in summer, but no study of valley coolers exists that we could find.

What the water costs at water district rates

The Las Vegas Valley Water District bills single-family homes in four tiers per 1,000 gallons over a 30-day period, and the Southern Nevada Water Authority commodity charge of $0.67 per 1,000 gallons is added to every tier (under the district’s service rules in force since January 1, 2026). The full rate table is on our home page.

Which tier a cooler’s water lands in depends on everything else your home uses that month; in summer, with outdoor watering, many homes are past the first 5,000 gallons. So the useful number is the price of the last 1,000 gallons you use:

Your tier	Tier price + SNWA charge	Per 1,000 gallons
First 5,000 gallons	$1.61 + $0.67	$2.28
Next 5,000 gallons	$2.87 + $0.67	$3.54
Next 10,000 gallons	$4.27 + $0.67	$4.94
Over 20,000 gallons	$6.33 + $0.67	$7.00

Example (our arithmetic, using the study’s figures). For every 100 hours a cooler runs, bleed-off adds about 100 × (10.4 − 4.4) = 600 gallons. That is 0.6 × $2.28 = $1.37 in the first tier, $2.12 in the second, $2.96 in the third and $4.20 in the top tier. A full season at the study’s 8,650 gallons for a bleed-off cooler comes to 8.65 × $4.94 = $42.73 if it all falls in the third tier, or $60.55 in the top tier. These leave out the daily service charge and the 0.25 percent reliability surcharge on residential bills, which don’t change with use.

Pad scale and pad care

Phoenix Manufacturing’s Pro-series guide asks for a look at the pads at least once a year, best at the start of the season, and more often depending on local air and water conditions. On valley water, plan on more.

• Light scale: with the power off, hose the air-inlet face of the pad. The maker says light brushing of the inlet edges with a stiff bristle brush won’t harm the pad and removes more stubborn scaling. Never use a wire brush.
• Heavy scale: replace the pad. Use the type your model takes; that guide says fitting aspen, expanded paper or another pad type in a rigid-media cooler won’t work and voids the warranty.
• Pan and cabinet: drain the reservoir, then clear mineral scale and hard water deposits with warm water and a soft cloth. The maker warns that scouring pads, steel wool and wire brushes damage the finish and encourage corrosion.
• Every two months of running, the same guide lists checks that include dry spots or streaks on the pads and clean water in the bottom pan, both early signs of scale.
• End of season: drain the cooler and the supply line completely.

Read your manual before adding anything

Descaling tablets and “cooler treatments” are sold for exactly this problem. Phoenix Manufacturing’s guide is blunt: do not use cooler cleaners, cooler treatments, anodes or other chemical additives, and “use of any additives or water treatment other than the furnished bleed-off will void your warranty”. The University of Arizona pamphlet adds that some cooler makers don’t recommend additives because they may damage the cooler’s protective coating, and that chemicals in the pan can be blown into the home.

Softened water isn’t the fix either. Phoenix Manufacturing says coolers should not be connected to soft water systems. If you are adding a softener, see Las Vegas hard water problems for what it does for the rest of the house, and keep the cooler on a hard line. Bleed water is mineral-heavy, so read softened water and plants in Las Vegas before you route it to a garden bed. The district’s mineral breakdown of valley tap water shows what ends up in the pan.

Sources for this page

• Las Vegas Valley Water District · water quality summary (2026, 2025 data), water rates page (current, checked 21 October 2026) and service rules (in force since January 1, 2026) · 280 ppm hardness; four residential tiers; SNWA commodity charge of $0.67; 0.25 percent residential reliability surcharge
• University of Arizona Cooperative Extension · pamphlet on evaporative cooler water use (publication 191045) · 1991, revised 1994 · 4.4, 10.4 and 7.6 gallons an hour; 8,650 gallons a season; 10 to 50 percent estimate; scale, corrosion and hot spots; bleed rate; bleed water on plants; additives; softened water
• New Mexico Office of the State Engineer · water conservation brochure on evaporative coolers (undated; read 21 October 2026 for our research notes) · bleed-off adds 50 percent or more; dump pump as the alternative
• Phoenix Manufacturing · owner’s guide for its Pro-series ducted coolers · 2019 · automatic drain every 5 hours; no soft water; no additives or treatments other than bleed-off (warranty); pad checks and cleaning; draining onto roofs

The cost table and examples are our own arithmetic from the rates above.