Running a Swamp Cooler During Monsoon Season

Every summer in Arizona, New Mexico, far West Texas, southern Nevada and parts of Utah and Colorado, a shift in wind patterns pulls moisture north from the Gulf of California and the Gulf of Mexico. That is the North American monsoon, and it is the season swamp cooler owners talk about most. A cooler that delivered crisp 68 F air in June starts pushing out 78 F air that feels damp. It is not broken; the air has simply run out of room to evaporate water. Here is how to judge each day and get the most out of your cooler until the dry air returns.

What the monsoon does to your cooler

A cooler's output depends on the gap between the outdoor temperature and the wet bulb temperature. Before the monsoon, low desert dew points sit in the 20s to 40s F. Once the monsoon moisture arrives, dew points jump into the upper 50s and 60s F, and occasionally above 70 F during strong surges. Afternoon highs dip a bit, but wet bulbs rise sharply.

The supply temperature on a humid surge day is only 6 F warmer than in June, but the indoor experience is much worse because the air also carries more than twice the moisture. With aspen pads, add about 3 to 5 F to each supply figure. Check your own conditions with the cooling temperature calculator.

A dew point decision guide

Dew point is the most useful daily number during monsoon because it moves slowly and appears in every NWS forecast. Use this as a working rule for a hot afternoon:

As a historical note, the Phoenix NWS office once defined monsoon onset as dew points averaging 55 F or higher for several consecutive days. That threshold lines up well with where cooler owners first notice a difference.

Hour-by-hour strategy for a humid day

1. Overnight to sunrise: pre-cool hard. Night temperatures in the 80s F with dew points in the 60s F still give a wet bulb several degrees below room temperature. Run high with windows open to cool walls, floors and furniture.
2. Morning: keep running while the gap is useful. As the temperature climbs, the dew point is often stable, so the wet bulb depression grows. Mid-morning can be the cooler's best period of the day.
3. Afternoon: maximize airflow and relief. Open windows on the downwind and far side of the house wider than usual. If you have up-duct relief dampers, make sure they are free to open.
4. Storm approach: shut down. Turn the cooler off when dust or a storm line approaches (see below). Close windows on the windward side.
5. After rain: wait for drying. Air right after a storm is near saturation. Run fan-only if it is cooler outside than inside, then resume normal operation once the dew point falls.

How much relief air you really need in monsoon

The normal rule is about 1 to 2 sq ft of open area per 1,000 CFM. In monsoon, err toward the high end or beyond. More exhaust means more air changes, a smaller temperature rise through the house and lower indoor humidity.

Placement guidance and damper options are in venting and relief air.

Dust storms, rain and power: protecting the cooler

Dust storms (haboobs)

Monsoon outflow winds can raise walls of dust in the low deserts. A running cooler pulls that dust into its pads and sump and, with wet pads, turns it into mud. Turn the cooler off when a dust storm warning is issued. Afterward, rinse the pad faces from inside the cabinet with low-pressure water, flush the sump and check the pump intake screen. A clogged pump inlet is a common post-storm failure; see swamp cooler pump not working.

Wind-driven rain

Heavy rain can blow into the louvers and overfill the sump. A clear overflow line drains the excess. If water appears at ceiling registers after storms, the overflow or the drain line may be blocked, or the roof jack flashing may need resealing; see swamp cooler leaking.

Lightning and power surges

Motors and electronic thermostats can be damaged by surges. Whole-house surge protection at the panel is a job for a licensed electrician. Turning the cooler off during close lightning is a simple precaution.

Pad and water care during the monsoon

Monsoon weeks are hard on cooler hardware. Dust, pollen and organic debris wash into pads and sumps, and warm, humid nights slow pad drying, which encourages algae. A mid-season service in late July or early August pays off: drain and scrub the sump, rinse or replace aspen pads that have turned dark or stiff, clear the spider and trough holes, clean the pump intake and confirm that the bleed-off line is dripping. If your control has a dry-out or purge cycle, enable it. Full steps are in how to clean a swamp cooler.

Indoor humidity and mold during monsoon

A few humid weeks are not usually enough to cause mold in a desert home, but long stretches of indoor RH above about 65% combined with cool surfaces can lead to musty closets, damp bathrooms and condensation on cold-water pipes. Check closets on exterior walls and areas behind furniture. If you see growth, follow EPA guidance; our mold and swamp coolers page explains prevention. For the causes and fixes for a muggy house, see swamp cooler too humid.

Backup cooling options for monsoon weeks

Running both systems at once wastes energy: the cooler adds moisture that the AC then has to remove. Shut the cooler off and close its duct damper before running AC. For a full comparison see swamp cooler vs air conditioner.

When to call it a season

The monsoon usually fades in mid to late September, dew points fall back into the 40s F and coolers become effective again for the warm days of early fall. Once nights fall reliably into the 50s F, start planning shutdown. Follow the steps in how to winterize a swamp cooler, and replace pads that took a beating from monsoon dust before next spring.