Swamp cooler calcium buildup: keeping a Phoenix cooler clean

Why Phoenix water crusts up cooler pads

The City of Phoenix says it directly: evaporative coolers are affected by the hardness of the water, and hardness, mainly calcium, can build up on the pads and make the cooler less effective. Its 2025 annual water quality report puts the city’s hardness at 10 to 17.6 grains per gallon, which is very hard. Other cities differ; see the hardness in your Valley city.

The U.S. Department of Energy’s Federal Energy Management Program explains why. When water evaporates from the pads, the minerals stay behind as scale. Scale cuts the air flowing through the pads, and the cooler cools less. Every gallon your cooler evaporates leaves its calcium in the cooler.

Bleed-off: the swamp cooler’s built-in scale control

A bleed-off line lets a little water run to drain all the time, so fresh water keeps diluting the minerals left in the pan. Phoenix Manufacturing, which makes coolers in Phoenix, says bleed-off helps stop scale building up. Its residential coolers come with a bleed-off tee and tube, to be run to a proper drain. If yours is missing, look for a swamp cooler bleed-off kit that suits your model.

Don’t let it run wide open. The Department of Energy says most small coolers need less than a few gallons per hour for each 1,000 cubic feet per minute of air flow. The University of Arizona’s Water Wise program says to drain no more water than is necessary and to follow the maker’s setting.

Bleed water is salty. Seeley International says it can go on salt-tolerant gardens but not on salt-sensitive grasses or plants. Our page on softened water and plants explains why salt and Valley soil don’t mix.

Should a Phoenix evaporative cooler run on softened water?

No, according to two makers. Phoenix Manufacturing’s owner’s guide says: “Evaporative coolers should not be connected to a ‘soft’ water system.” Champion Cooler’s manual for its industrial and commercial coolers says: “Do not use water supplied from a water softener.” Seeley’s manual and Breezair’s warranty say nothing either way about softened water, so check the guide for your own model.

If you are choosing a water softener in Arizona and have a cooler, ask the installer which lines will stay on hard water, and keep the cooler line among them if your cooler’s maker says so.

Swamp cooler descalers and treatments: what the makers allow

Many cooler makers’ warranties exclude descalers and water treatments. Use only a product your cooler’s own maker approves. Here is what the makers we checked say:

• Phoenix Manufacturing: anode devices, chemical additives or cooler cleaner treatments void its warranty. For its portable coolers, any additive or water treatment other than bleed-off voids the warranty and shortens the cooler’s life.
• Breezair (Seeley International): its 2024 warranty says “Do not use cooler cleaners, cooler treatment, or other additives to your cooler. The use of any of these products will void your warranty.”
• Champion Cooler: its warranties say it is not responsible for damage from water softeners, chemicals or descaling materials. Its commercial cooler manual also says not to add any type of water treatment chemicals, and recommends a bleed-off kit instead.
• Portacool: the exception. Its portable cooler manuals say that if hard water is a problem, owners can buy the company’s own hard water treatment from Portacool or an authorized distributor.

So the safe order is bleed-off first, then pads and cleaning, and a treatment only if your maker names one.

How to clean a swamp cooler: Phoenix Manufacturing’s steps

These steps are from Phoenix Manufacturing’s owner’s guide for its residential coolers. Always disconnect power first.

1. Take out the pad frames and set them aside.
2. Remove the overflow standpipe from the drain bushing and let the pan empty.
3. Clean the inside with a cloth and clean water. Don’t use wire brushes or anything that scratches the paint, and no chemicals other than soap or detergent.
4. Rinse the bottom pan thoroughly.
5. Clean the pump screen, and clear anything stuck in the adapters at each end of the pump hose.
6. Touch up scratches or bare spots inside with a suitable cooler coating.

For the end of the season, the University of Arizona suggests getting minerals out of the bottom tray, noting that vinegar can dissolve them, then disconnecting and draining the water line before freezing nights. For mineral spots on the outside of the house, see our guide to removing hard water stains.

Vinegar or CLR on a swamp cooler?

None of the maker documents we read recommends CLR. Phoenix Manufacturing allows only soap or detergent inside its coolers. Portacool’s manual refers owners to its online instructions for rinsing pads with household white vinegar. If your manual doesn’t say, follow the cooler maker’s cleaning instructions or ask them.

Replacing swamp cooler pads in the Valley

Phoenix Manufacturing says to service a cooler at least once a year, including new pads, and more often if needed. The University of Arizona suggests new pads each spring (aspen fiber or what the maker recommends) and a mid-summer check: if pads carry heavy mineral deposits, replace them.

To replace aspen pads, Phoenix Manufacturing’s steps are:

1. Lay the pad frame flat with the wire retainers up, and remove the retainers.
2. Throw away the old pads and clean the frame, without a wire brush or harsh chemicals.
3. Check that the slots in the trough at the top of the frame are open.
4. Fit new pads of the correct size and put the retainers back.
5. Wet the pads thoroughly with a hose before reinstalling.

Removing calcium deposits from rigid pads

Rigid media pads are different. For its portable coolers, Phoenix Manufacturing says to hose off the inlet face and lightly brush the inlet edges with a stiff bristle brush, never a wire brush, to remove stubborn scale. If the scale won’t come off, replace the pad, because scale restricts air flow.

Water level, monsoon humidity and other checks

• Water level: the University of Arizona says it should sit about an inch below the top of the tray and below the top of the overflow pipe. Phoenix Manufacturing’s residential guide sets the float for 2½ inches of water. Use your own manual’s figure.
• Pump and float: the Department of Energy suggests checking the pump and water level controls during the cooling season.
• Humid days: Seeley says coolers cool less when outdoor humidity is high, so expect less from yours during monsoon weeks.

Where these facts come from

• City of Phoenix Water Services Department · annual water quality report (2025) and water quality monitoring page · hardness range; hardness and evaporative coolers
• Phoenix Manufacturing · owner’s guides for its residential (2017) and portable (2016) coolers · softened water, bleed-off, warranty on additives, cleaning and pad steps, service schedule
• U.S. Department of Energy, Federal Energy Management Program · water efficiency best practice page · undated · how scale forms, bleed-off rates, pad and pump checks, humidified air
• University of Arizona Water Wise · swamp cooler maintenance page · undated · pad checks, bleed-off, water level, vinegar in the tray, winterizing
• Seeley International · evaporative cooler owner’s manual (2021) and Breezair limited warranty (2024) · bleed system, salty drain water, humid days; cleaners, treatments and additives void the warranty
• Champion Cooler · ducted cooler warranty (2014) and industrial and commercial cooler manual (2015) · no softened water, no water treatment chemicals, bleed-off kit; damage from softeners, chemicals or descalers not covered
• Portacool · owner’s manuals for its portable coolers · 2020 and 2025 · the maker’s own hard water treatment; vinegar rinse instructions