How to Clean a Swamp Cooler: Pan, Pads, Pump, Distribution and Scale

Cleaning a swamp cooler is mostly about water: draining the concentrated, mineral-heavy water that collects in the pan, removing the scale and sludge it leaves behind, and making sure water flows freely through the pump and over every pad. A routine cleaning takes 30 to 60 minutes with a shop vac and a scraper. A neglected cooler with crusted pads and a scaled pan may take 2 to 3 hours, and sometimes the honest answer is that pads, pump or float valve need replacing rather than cleaning.

The procedure below fits roof-mounted downdraft and side-draft coolers. Window and portable units follow the same logic on a smaller scale.

What you are cleaning, and why it gets dirty

Every gallon that evaporates leaves its dissolved minerals behind, so pan water gets harder through the day. Dust pulled through the pads settles in the pan as silt. Warm, still water in shade grows biofilm. Together those produce the three kinds of dirt you will find:

Tools, time and safety

Time

30 to 60 minutes for routine cleaning; 2 to 3 hours for heavy descaling

Difficulty

Easy to moderate (roof work)

Tools

Wet/dry shop vac, plastic putty knife or scraper, stiff nylon brush, old toothbrush, small wire or 1/8 inch drill bit for holes, nut driver or sockets for panels, bucket, garden hose with a gentle spray nozzle (or a pump sprayer if no hose reaches the roof)

Supplies

White vinegar, clean rags or sponges, rubber gloves, safety glasses, any pan coating your manufacturer approves

Step-by-step cleaning procedure

1. Kill power and shut the water. Thermostat off, disconnect off, then close the cooler's water supply valve so the float does not refill the pan while you work.
2. Remove panels. Lift off the pad frames or louvers and lean them somewhere stable. Keep screws in a cup.
3. Drain the pan. Unscrew the overflow standpipe or open the drain. Let it empty completely.
4. Vacuum the sediment. Use the shop vac to pull out silt, leaves and loose flakes. Get under and around the pump.
5. Pull the pump. Unplug it, lift it from its bracket, and set it in a bucket. You will clean it separately.
6. Loosen scale. Spray or sponge a 50/50 mix of white vinegar and water on the crusted areas and let it work for 15 to 30 minutes. Keep it wet.
7. Scrape and scrub. Use the plastic scraper on thick scale and the nylon brush on the rest. Work gently over painted or coated areas. Do not use wire brushes or steel wool.
8. Rinse and vacuum again. Rinse with clean water and vacuum it out. Repeat the vinegar pass if needed.
9. Dry and inspect. Towel the pan dry and look closely for rust spots, pinholes and failed seams, especially under the pump.
10. Touch up. Clean, dry and coat bare metal with a coating your manufacturer approves. Let it cure as directed before refilling.

Cleaning the pump, float and distribution system

Pump

Remove the intake screen or base cover (usually snaps or a few small screws). Soak scaled parts in a vinegar solution for 30 minutes, then brush them clean. Spin the impeller by hand; it should turn freely with no grinding. If the pump was humming without pumping or tripping the breaker before you cleaned it, consider replacing it. See how to replace a swamp cooler pump.

Distribution

Disconnect the pump hose from the spider or distributor. Flush the hose and distributor with a hose. Clear every outlet and every hole in the troughs or drip tubes above the pads with a wire or a 1/8 inch bit turned by hand. Uneven water distribution is the most common reason pads show dry vertical stripes, and dry stripes let hot air bypass the cooling entirely.

Float valve

Wipe scale off the float arm and the valve body. If the valve is crusted or drips when closed, replace it rather than cleaning it; float valves are inexpensive. See float valve adjustment.

Bleed-off line

Clear the small bleed-off tube and its fitting; they clog easily. If your cooler has no bleed-off, adding one usually reduces cleaning more than anything else. See bleed-off and water quality.

Pads: clean, rinse or replace

Aspen pads cannot really be cleaned. Once they are crusted with scale, matted, or smell musty after drying, replace them. Rigid media can be rinsed gently with a low-pressure hose from the air-leaving (inside) face to push debris back out the way it came. Soft scale on the air-entering face may come off with a soft brush and a vinegar rinse; heavy scale that has sealed the flutes means replacement. Never pressure wash rigid media. See how to replace swamp cooler pads and how often to replace cooler pads.

Refill, flush and test

1. Reinstall the pump upright in its bracket and reconnect the hose.
2. Replace the standpipe and open the water supply. Check the level: commonly about 1 to 1.5 inches below the top of the overflow, or per your manual.
3. Run pump only for 10 to 15 minutes to flush the system and wet the pads. Watch every pad for even wetting.
4. Drain and refill once more after a heavy vinegar cleaning, so acidic, dirty rinse water does not stay in the pan.
5. Reinstall panels, restore the blower and listen for normal operation.

Odors, mold and disinfection

A musty or fishy smell is almost always biological growth in stagnant water or old pads. Cleaning the pan and replacing aspen pads fixes most cases. Avoid the temptation to dump bleach into the reservoir: it is hard on galvanized steel, breaks down pads, and is blown straight into your house when the blower runs. If you want to disinfect after a severe growth problem, do it only on the drained, isolated pan with power off, rinse thoroughly, and flush before running the blower.

Well-maintained residential evaporative coolers are generally considered a low risk for Legionella compared with cooling towers, partly because they run on fresh water and drain regularly, but stagnant warm water is exactly what you want to avoid. The CDC recommends keeping water systems clean and avoiding stagnation. If anyone in your household has respiratory concerns, see mold and swamp coolers and talk to a medical professional.

Cleaning portable and window coolers

Portable and window evaporative coolers hold far less water than a roof unit, which means minerals and growth concentrate faster even though there is less to clean. Unplug the unit, empty the tank completely (most have a drain plug at the bottom), and wipe the tank with a cloth dampened in a 50/50 white vinegar solution. Rinse the small submersible pump and its filter screen, flush the water distribution tube or header, and rinse or replace the media per the manual; honeycomb media in portables usually rinses well, while thin fiber pads are replacement items. Let everything air dry before refilling. In hard water, empty and refill at least every few days, and always dry the unit fully before storing it for the season. For product types and features, see the portable evaporative cooler buying guide.

How often to clean

These intervals assume working bleed-off. Without it, cut them roughly in half. Your water utility's annual water quality report lists hardness, often in mg/L (same as ppm) or grains per gallon. Put the cleaning into your yearly maintenance schedule, and if odors persist after a thorough clean, see swamp cooler smells.