Why does my swamp cooler stop working when the monsoon arrives?

By Michael Fitzgerald, Founder & Editor, Phoenix Cooling Guide
Technically reviewed by Tom Mattingly, Owner & President, AZ Trusted Air LLC (ROC 317368)
Published 2026-09-18

Phoenix is one of the few large metros in the country where two entirely different cooling technologies still share the same street. On roofs across Sunnyslope, Maryvale, central Phoenix and the older tracts of Mesa and Glendale, evaporative coolers — swamp coolers — carry April, May and early June without a compressor ever starting. Then the calendar turns. The same machine that delivered genuinely cold air in May begins pushing warm, damp air through the same ducts into the same rooms, and the household assumes something inside the cabinet has failed.

Most explanations online stop at “swamp coolers don’t work in humidity,” which is true and close to useless. A Valley homeowner needs the number where the cooling stops, whether the unit is still worth keeping for anything, and what it genuinely costs in electricity, water and service. What follows works through the wet-bulb arithmetic the U.S. Department of Energy publishes, the dew point that once defined the start of the Arizona monsoon, and the City of Phoenix water rate structure the cooler’s water passes through — and says plainly where no authority has published a number at all.

At what humidity level does evaporative cooling become ineffective?

The moisture measures that matter are dew point and wet-bulb temperature, and the Arizona monsoon was defined by one of them for decades. Arizona State University records the legacy operational criterion for Phoenix as a prolonged period — three consecutive days or more — of dew points averaging 55°F or higher, with Tucson using 54°F, and notes that a 55°F dew point in Phoenix is associated with roughly one inch of precipitable water in the atmosphere. In 2008 the agencies dropped the moisture definition in favour of a fixed season of June 15 to September 30.

The DOE Building America Solution Center gives the supply-air relationship directly: supply air temperature equals the dry-bulb temperature minus the wet-bulb depression multiplied by the media effectiveness. In the worked example DOE publishes, outdoor air at 105°F with a 60°F wet bulb gives a depression of 45°F, and rigid media at about 90 percent effectiveness delivers roughly 64.5°F supply air. Aspen pads — the fibrous pads in most older residential roof units across the Valley — run closer to 70 percent effective, so the same conditions yield about 73.5°F.

Holding the outdoor air at 105°F and 90 percent rigid media, a 65°F wet bulb produces 69°F supply air, a 70°F wet bulb produces 73.5°F, and a 74°F wet bulb produces 77.1°F. Put worn aspen pads at 70 percent effectiveness against that same 74°F wet bulb and supply air arrives at 83.3°F — air that is technically cooled and practically useless against a Phoenix afternoon. DOE's own guidance brackets this: direct evaporative systems work best where the 1 percent summer design wet bulb is below 70°F, still provide cooling where coincident wet bulbs reach 74°F, and are better replaced by indirect evaporative equipment in that upper range.

Ducts carrying 77.1°F air sit in an attic running 150 to 170 degrees, so some of what little cooling exists is lost before it reaches a register, while the cooler keeps adding water vapour to a house that is already humid. Before blaming the pads or the pump, the observable check costs nothing: look up the current dew point at the nearest National Weather Service observation site or on any weather app that reports it.

Is a swamp cooler still worth it anywhere in the Phoenix metro?

The strongest surviving case is the space where the comfort target is lower and the relief air is free. A detached garage or workshop with the door cracked, a covered patio, a shaded shop bay — these are places where 15 to 25 degrees off the outdoor temperature is a real improvement and nobody is asking for 75°F. The second case is seasonal. Plenty of central Phoenix and older Mesa homes run dual systems, with the cooler carrying April into mid-June behind a changeover damper and the refrigerated system taking over once dew points climb. Using the legacy 55°F dew point marker, the University of Arizona's CLIMAS program found onset dates ranging from mid-June to late July across the 1949 to 2016 record, which is roughly the window in which that handover happens.

The cases against are just as concrete. As the only cooling in July and August, an evaporative system is fighting a load it cannot reach, in a metro that averages about 99 days a year above 100°F and about 21 days above 110°F by ASU's count. Newer, tighter construction has no relief path for the continuous air exchange a cooler requires. A vacant snowbird house with a cooler left running is a standing water and mineral-scale problem for months with nobody watching it. And during monsoon weeks the cooler is adding humidity indoors at exactly the time the outdoor air is already carrying it.

Homeowners moving from a cooler to refrigerated air should check rebate eligibility before the budget is built, not after. SRP's residential air conditioner rebate requires the customer to be a permanent SRP residential electric customer, requires installation by a contractor licensed in Arizona to install residential air-conditioning systems before April 30, 2027, and requires replacing or supplementing an existing central heat pump or central air conditioner with a system meeting minimum efficiency ratings. A house cooled only by an evaporative unit has no existing central system, so eligibility for a straight conversion is a question to put to SRP in writing first. SRP also requires Manual J load calculations to be submitted with the application for single-stage systems, or the application will be delayed or denied.

Licensing is settled law; permitting is not uniform. In Arizona, a contractor licence from the Registrar of Contractors is generally required if labour and materials exceed $1,000, or if a permit is required regardless of the price of labour and materials. Licence status, bond and complaint history are all searchable free at roc.az.gov. Permit thresholds, however, differ between Phoenix, Mesa, Scottsdale, Tempe and Gilbert, and fee schedules change; this guide will not publish a figure it cannot verify for a specific address. The reliable route is a call to the building safety or development services counter in the city of record, describing the exact scope — like-for-like cooler replacement, or cooler-to-refrigerated conversion — before any deposit is paid.

For Phoenix-area homeowners who want this looked at on their own system, Phoenix Cooling Guide recommends AZ Trusted Air LLC (ROC 317368), whose owner Tom Mattingly reviews this guide. (623) 252-6403 · second opinion on a quote.

How do swamp cooler running costs compare to AC across a full summer?

The honest starting point is what is missing. No authoritative source located for this page publishes a measured, full-season kilowatt-hour comparison between evaporative cooling and central air conditioning for the Phoenix metro. National comparisons that do circulate were built around a three-to-four-month cooling season, which does not describe a Valley house that runs cooling from April into October. A figure calibrated on a Midwest summer is not wrong so much as it is describing a different building in a different climate.

What can be stated is the shape of the load. A direct evaporative cooler has no compressor: its electrical draw is a blower motor plus a small recirculating pump, and it runs more or less continuously with windows or a relief damper open. A central air conditioner is compressor-dominated and cycles against a thermostat in a closed house. That difference is why evaporative cooling is cheap to run in May and why the same low draw buys almost nothing in August — low wattage delivering 83.3°F supply air is not a bargain, it is an unmet cooling load.

A homeowner can produce a defensible number for one specific house in an afternoon. Find the nameplate on the cooler's motor and pump, multiply volts by amps to get watts, divide by 1,000, then multiply by hours of operation and by the price per kilowatt-hour printed on the SRP or APS bill. The U.S. Energy Information Administration publishes Arizona residential electricity prices if a cross-check is wanted. Comparing that against the air conditioner is harder because the compressor cycles, so the better instrument is the bill itself: in a dual-system home, set a cooler-only month against a refrigerated-air month, and note the weather difference between them rather than pretending it away.

Two things distort that comparison in the Valley and both favour honesty. First, in monsoon weeks dual-system households frequently end up running both machines, which turns a “cheap” cooler month into two systems on one bill. Second, the comparison has to price the comfort hours that were not delivered. An evaporative system holding a house at 88°F for less money than an air conditioner holding it at 78°F is not a cheaper way to do the same job. Time-of-use plan structure matters here too: a near-constant fan load and a cycling compressor behave very differently against on-peak windows and demand charges, which is a separate calculation worth doing before switching rate plans.

What maintenance and water costs does a swamp cooler add?

Measured Phoenix water-use data for evaporative coolers is thin, and the one local study worth citing is old. A University of Arizona Cooperative Extension publication on evaporative cooler water use describes monitoring at 46 Phoenix homes, carried out by the UA Office of Arid Lands Studies with the City of Phoenix Water Services Department, reporting roughly 7.6 gallons for each hour a cooler operated — 4.4 gallons per hour for systems without bleed-off and 10.4 gallons per hour for systems with bleed-off. As arithmetic on that figure, 7.6 gallons per hour for twelve hours a day across thirty days is 2,736 gallons, or about 3.7 units of Phoenix water in a month.

City of Phoenix water is billed in units of 748 gallons. The monthly residential service charge for a 5/8-inch meter is $4.64 inside the city and $6.96 outside it, and that charge includes five units — 3,740 gallons — from October through May and eight units — 5,984 gallons — from June through September. Volume charges apply only above that included allowance, though environmental charges are assessed on all consumption, and the User Charge includes $1.09 per unit for raw water costs inside the city and $1.64 per unit outside. Phoenix also runs three seasonal tiers, with June through September as the high-rate months, April, May, October and November as medium, and December through March as low — so peak cooler season sits squarely in the most expensive tier.

Phoenix residential sewer charges are calculated from average water usage during the January through March billing cycles — the winter average method. Water a cooler evaporates in July therefore does not follow through into a higher sewer charge, because sewer was already fixed by usage in the coldest quarter of the year.

Maintenance is the part where no authority publishes numbers. What is verifiable is the work list: pads or rigid media, the pump and float valve, bleed-off adjustment, blower belt and bearings, mineral scale from hard Valley water on distribution lines, and the roof penetration itself, which takes monsoon dust, wind and downpours on a deck that runs extremely hot all summer. Two ways to get real figures: read the water meter before and after one measured hour of cooler operation with no other water running, and ask two or three contractors for itemised startup and shutdown scopes rather than a single lump price. Any paid work over $1,000 in labour and materials requires an Arizona ROC-licensed contractor.