Evaporative Cooler: How Swamp Coolers Work in Colorado

We may earn commission from links on this page. Lead-form submissions are forwarded to local inspector partners. How we research and review.

Drive through older neighborhoods in Denver, Pueblo, Colorado Springs or Grand Junction and you will spot boxy metal units sitting on rooftops or hanging out of windows. Those are swamp coolers, and for decades they have been a popular way to cool homes in the dry West. An evaporative cooler uses water and a fan instead of refrigerant, which typically means lower purchase and operating costs than central air conditioning, but also more hands-on seasonal care and a few risks that buyers should understand. This guide covers how swamp coolers work, why they suit much of Colorado, the main unit types and pad options, startup and winterizing, water use, mold and roof leak concerns, how they compare with central AC and heat pumps, and what they tend to cost. Costs below are general estimates that vary by home and contractor.

Part of our Heating, Cooling & Ventilation guide — start there for the full picture →

How Does an Evaporative Cooler Work?

An evaporative cooler pulls warm outdoor air through wet pads. As water evaporates from the pads, it absorbs heat from the air, and the air leaving the cooler is noticeably cooler and more humid than the air going in. A blower then pushes that cooled air into the house, either through a single ceiling or wall register or through ductwork.

The basic parts of a typical unit are:

• Cabinet: a metal or plastic box with louvered side panels.
• Pads: absorbent media mounted behind the louvers.
• Water reservoir: a pan at the bottom of the cabinet, refilled through a float valve connected to the home’s water supply.
• Pump and distribution lines: a small pump lifts water from the reservoir to a trough or spider that soaks the pads.
• Blower and motor: a squirrel-cage fan, usually belt- or direct-driven, that moves air through the pads into the house.
• Bleed-off or purge system: a small drain or timed dump that removes mineral-heavy water to slow scale buildup.

One key difference from air conditioning: a swamp cooler is an open system. It constantly brings in fresh outdoor air and needs a path for that air to leave. That is why homeowners crack windows on the side of the house away from the cooler, or install relief vents. Running a swamp cooler with every window shut usually makes the house feel muggy and does little cooling.

Why Swamp Coolers Fit Colorado’s Climate

Evaporative cooling works best when outdoor air is hot and dry, because dry air can absorb a lot of moisture. Much of the Front Range and Western Slope sees low summer humidity for most of the cooling season, which makes the approach practical. Many Colorado homeowners find a well-maintained cooler keeps the house comfortable through typical summer weather, especially in homes with good shade and insulation.

The limits show up on humid days. During the summer monsoon season, when afternoon thunderstorms roll in and humidity rises, swamp coolers lose much of their effectiveness, and the air they deliver can feel damp rather than cool. Very hot spells also stretch their capacity. Homes in the eastern plains and some lower-elevation areas may see more humid days than foothill or high-desert locations. A cooler is often a good fit for a home that only needs moderate cooling and where occupants can tolerate a few warm, sticky days each summer.

Cooling effect is often described in terms of the difference between the dry-bulb temperature, which is what a normal thermometer reads, and the wet-bulb temperature, which reflects humidity. The wider that gap, the more cooling a swamp cooler can provide. Manufacturers’ sizing charts use these figures, so a contractor’s recommendation should consider local summer conditions rather than square footage alone.

Types of Evaporative Coolers

Roof-Mounted (Down-Draft) Coolers

The classic Colorado setup is a large down-draft unit on the roof, sitting on a curb or stand and blowing cooled air down through a duct into a central ceiling register or a duct system. These units can cool an entire small to mid-sized home and keep the equipment out of sight from the street. The downsides are roof access for maintenance, a roof penetration that can leak and a direct path for heat loss in winter if the duct is not closed off.

Side-Draft and Ground-Mounted Coolers

Side-draft units sit on the ground or on a wall bracket and push air horizontally through a wall or into a duct. They are easier to service than roof units and avoid a roof penetration, but they need space beside the house and a protected path into the living area.

Window Coolers

Window units cool one room or a small area. They are less expensive and easier to install and remove, which suits renters and homes that need cooling only in a bedroom or office.

Portable Coolers

Portable evaporative coolers roll from room to room and are filled by hand. They provide localized relief rather than whole-house cooling, and they work only with good ventilation. In a closed room they mostly add humidity.

Sizing a Whole-House Cooler

Whole-house coolers are rated by airflow in cubic feet per minute (CFM). Contractors and manufacturers commonly size them by estimating the home’s volume and how often the air should be exchanged, then adjusting for local climate, insulation, shading and ceiling height. An undersized cooler runs constantly without keeping up, while a badly oversized one can push more moisture into the house than needed. Duct size matters too, because a large cooler connected to a small duct or a single undersized register will be noisy and deliver less air than its rating suggests. When replacing an old roof unit, ask the installer whether the existing curb, duct and register suit the new cooler or need modification, since mismatches are a common cause of disappointing performance.

Cooler Pads: Aspen vs Rigid Media

The pads do the actual evaporating, so they have a big effect on performance and maintenance.

• Aspen pads: shredded aspen wood fiber held in a net. They are inexpensive and widely sold at hardware stores across Colorado. They generally need replacement every season or so, can sag and leave gaps, and may shed fibers or develop odors as they age.
• Synthetic fiber pads: polyester or blended pads that last longer than aspen in some cases, with mixed results on cooling efficiency depending on the product.
• Rigid media pads: thick cellulose panels with angled flutes, often several inches deep. They usually cost more upfront and require a compatible cabinet or conversion kit, but they often deliver more cooling, last several seasons and resist sagging.

Hard water, which is common in parts of Colorado, leaves mineral scale on any pad. A working bleed-off system and regular cleaning help pads last longer. Replace pads that are crusted, crumbling, discolored or smell musty.

Seasonal Startup and Shutdown

Swamp coolers need attention twice a year. Many Front Range homeowners schedule startup in May or early June and shutdown in September or October, before the first hard freeze. A service company can handle both if roof access is a concern.

Spring Startup Checklist

1. Remove the winter cover and open the damper or remove the duct plug that closes off the register.
2. Clean out the reservoir, removing sediment, scale, leaves and any dead insects.
3. Install new pads or inspect existing rigid media.
4. Reconnect the water line, check the float valve and set the water level.
5. Inspect the belt, pulleys and motor bearings, and adjust belt tension.
6. Run the pump to confirm water flows evenly across all pads.
7. Check the bleed-off line and look for leaks around the cabinet and roof curb.

Fall Shutdown and Winterizing

1. Turn off the water supply to the cooler and drain the supply line to prevent freezing.
2. Drain and clean the reservoir so standing water does not sit all winter.
3. Disconnect or switch off power to the unit.
4. Close the interior damper or install a register cover to stop warm indoor air from escaping up the duct.
5. Fit a breathable cover over the cabinet to keep out snow and debris.

Skipping the damper step is one of the most common mistakes. An open cooler duct acts like a hole in the ceiling, and warm, moist indoor air can escape and condense inside the duct or attic. Some homes also use ceiling relief vents, sometimes called up-ducts, that exhaust cooler air into the attic and out through the roof vents. If a home has these, the attic must have adequate exhaust, and our guide to attic ventilation covers how that system is supposed to work.

Water Use

Evaporative coolers trade electricity for water. Water use depends on unit size, weather, run time and how much the bleed-off line discharges. A large roof unit on a hot, dry day can use a meaningful amount of water, and in areas with tiered water rates that can show up on summer bills. Ways to manage it include fixing a stuck float valve, adjusting bleed-off to the minimum the manufacturer recommends for the local water hardness, and checking for overflow dripping off the roof. A cooler that constantly overflows wastes water and can damage roofing.

Mold, Bacteria and Indoor Air Quality

A swamp cooler’s reservoir and pads stay wet for months, which makes them a possible site for microbial growth if they are neglected. The EPA lists bacteria and fungi such as mold among common biological contaminants indoors, and its mold guidance stresses that moisture control is the key to mold control. A cooler with stagnant water, slimy pads or a musty smell coming from the register deserves a thorough cleaning or pad replacement before it runs again.

Good practice includes draining and cleaning the reservoir at least at season’s start and end, more often if odors or algae appear, replacing pads on schedule and running the cooler on fan-only for a while before shutting it down so pads dry out. Homeowners who also run central forced-air systems may find our guide to an air filter for mold helpful for the rest of the year. Anyone with asthma or allergies who notices symptoms worsening when the cooler runs should take that seriously and consult a physician.

Humidity Effects

An evaporative cooler adds moisture to indoor air, which many Colorado residents welcome in a dry summer. The trade-off is that a cooler running constantly with poor ventilation can push indoor humidity higher than intended. The EPA recommends keeping indoor relative humidity below 60 percent and ideally between 30 and 50 percent. A basic hygrometer shows where a home actually sits. If readings climb, open more relief windows, reduce run time or switch to fan-only. Our guide on how to lower humidity in the house covers additional fixes for homes that trend damp.

Evaporative Cooler vs Central AC and Heat Pumps

Central air offers more consistent cooling on humid and very hot days, filters recirculated indoor air and keeps windows closed against smoke, pollen and noise. Wildfire smoke days are a real consideration in Colorado, because a swamp cooler pulls outdoor air into the house. Homeowners with central AC can learn about upkeep in our AC maintenance guide. Heat pumps add heating to the mix, and our comparison of heat pump vs furnace explains when replacing both heating and cooling at once makes sense. Some homes keep a swamp cooler for mild days and use AC for humid or smoky stretches.

Roof Leaks and Other Inspection Concerns

Roof-mounted coolers are a common source of inspection findings. The curb or stand where the cooler meets the roof is a penetration, and flashing there can fail, especially when overflow water and scale run across shingles each summer. Common issues include:

• Water stains on the ceiling below or around the cooler register.
• Rusted cabinet bottoms leaking onto the roof.
• Deteriorated or missing flashing at the curb.
• Unsealed or disconnected supply lines in the attic.
• Electrical wiring run without proper protection or a nearby disconnect.
• Abandoned coolers still sitting on the roof with open ducts below.

The ASHI Standard of Practice requires inspectors to inspect central and permanently installed cooling equipment and its distribution, but it does not require them to inspect cooling units that are not permanently installed or that are installed in windows. Off-season inspections may also be limited because a winterized cooler cannot be run. Buyers looking at a home with a roof cooler can ask whether the inspector will evaluate it and how. Our guide to hiring a home inspector covers questions to ask about scope.

Roof Safety

Servicing a roof cooler twice a year means climbing onto the roof twice a year. OSHA notes that falls are among the most common causes of serious work-related injuries and deaths. Homeowners who are not comfortable on ladders and roofs, or whose cooler sits on a steep or two-story roof, are usually better off hiring a cooler service company for startup and shutdown.

Evaporative Cooler Costs

Prices vary by size, brand, installation complexity and region. As rough estimates:

Removing an old roof cooler and patching the roof is another common project when homeowners switch to central air, and its price depends on the roofing work involved.

References

• A Brief Guide to Mold, Moisture and Your Home — U.S. Environmental Protection Agency
• Biological Pollutants’ Impact on Indoor Air Quality — U.S. Environmental Protection Agency
• ASHI Standard of Practice for Home Inspections — American Society of Home Inspectors
• Fall Protection — Occupational Safety and Health Administration

Buying a Colorado home with a rooftop swamp cooler? Send us a note and we can help you find a Front Range inspector to check the cooler, roof curb and attic before closing.