Whole-House Evaporative Cooler Buying Guide: Size, Media, Motor and Install

A whole-house evaporative cooler is a large appliance that you will live with for a decade or more, and most of what determines whether it keeps your house comfortable is decided before it reaches the roof: how much air it moves, what pads it uses, how its air gets into the house and how its water is managed. This guide walks through those decisions in the order a good installer would.

If you are still deciding between a whole-house cooler and refrigerated air, read swamp cooler vs air conditioner first and check your city in the climate index.

Step 1: Calculate the airflow you need

Evaporative coolers are sized by airflow, not tons. The goal is to replace the air in the house often enough that warm air is flushed out before it heats up much. The site's standard method:

Residential coolers are commonly sold from about 3,000 to 7,000 CFM, with larger units available. If your number is far above the biggest unit that suits your roof, consider two coolers serving different parts of the house. The full method, including heat-gain adjustments, is in how to size a swamp cooler, and the cooler size calculator runs it for you.

Step 2: Allow for duct losses

Manufacturer CFM ratings are often measured at zero static pressure, meaning no duct, no grille, no resistance. Real installs add resistance, and the blower moves less air as it rises. A central ceiling diffuser on a short down-duct loses little. A long attic duct system with several elbows and small registers can cut delivered airflow by 10 to 30% or more.

Two ways to handle it: buy a cooler whose rating covers the loss (required CFM divided by about 0.75 to 0.9 depending on the ducts), or reduce the loss by enlarging trunks, cutting elbows and adding registers. The second usually makes the house quieter too. Ask for the manufacturer's airflow table at 0.2 or 0.3 in of static pressure if it is published, and see CFM explained for how to read it.

Step 3: Choose the mount and discharge style

Downdraft units sit on the roof and drop air into the ceiling. Side-draft units usually sit on a ground stand and blow through a wall or up into attic ducts. Single-story homes with a central hallway suit downdraft; two-story homes, tile roofs and owners who want ground-level service often suit side-draft. The full comparison is in downdraft vs side-draft coolers. If you are replacing an existing unit, keeping the same style and footprint saves a lot of installation cost.

Step 4: Pick the pad media

This is the single biggest performance decision after sizing. Supply temperature equals outdoor dry bulb minus saturation efficiency times the wet bulb depression.

The supply figures come straight from the formula: with a 35 F wet bulb depression, 75% efficiency gives 100 - 26.25 = 73.75 F, and 90% gives 100 - 31.5 = 68.5 F. Over a cooler's life, the cheaper pads of an aspen unit add up, while rigid media delivers colder air every day. In hot desert climates, technicians commonly recommend rigid media for anyone who can afford it. Details are in aspen vs rigid media.

Step 5: Compare motor, drive and cabinet

Motor

Two-speed motors are the norm and worth insisting on: low speed for mornings and nights, high for afternoons. Residential units commonly use roughly 1/3 to 3/4 HP, with larger motors on big units. Check whether it is 120 V or 240 V.

Drive

Belt drive with an adjustable motor sheave lets an installer tune blower speed to the duct system; belts need tension checks and periodic replacement. Direct-drive designs have fewer wear parts.

Blower

A larger blower wheel turning slower is typically quieter than a small wheel at high RPM for the same airflow.

Cabinet

Galvanized and painted steel is common and repairable but can rust where water sits. Polymer cabinets do not rust and are used on several premium designs. Check the reservoir pan material and any pan liner.

Water distribution

Look for a distributor or trough that wets pads evenly and is easy to clean; dry stripes on pads mean warm air bypassing the water.

Step 6: Water management and controls

Hard water is the main enemy of a cooler. As water evaporates, minerals concentrate in the reservoir and deposit on pads and pump screens. A bleed-off line (a small continuous drain from the pump outlet) or a purge pump that dumps the reservoir on a schedule keeps mineral levels down. Units with a purge system and a dump valve tend to keep rigid media working for more seasons. See bleed-off and water quality.

For controls, the minimum is separate fan-speed and pump switching so you can run fan-only to dry the pads or ventilate on humid days. A wall control with a thermostat, timer or humidity sensing adds convenience and cuts water and power use. Options are compared in thermostats and controls.

Step 7: Budget for the whole job

The cooler is only part of the cost. A realistic budget includes:

• Equipment: typically about $700 to $2,500 for an aspen-pad steel unit and $1,500 to $5,000 or more for rigid media and premium designs in 2026, varying by size and region.
• Installation: labor, roof curb or stand, duct connection, water line, bleed-off, electrical disconnect, and permits where required. A like-for-like replacement costs far less than a new install.
• Relief air: working windows or up-ducts / ceiling relief vents so air can leave the house.
• Running costs: electricity, water, pads and annual service. Estimate them with the operating cost calculator and the water usage calculator on the tools page.

Installed price ranges are broken down in cost to install a swamp cooler.

Common buying mistakes

Technicians see the same handful of errors on replacement calls year after year:

• Copying the old cooler's size. Many older homes were fitted with whatever was on the truck, and additions, vaulted ceilings or a converted garage have since increased the volume. Recalculate rather than matching the old nameplate.
• Ignoring relief air. A new, stronger cooler in a house with every window shut simply pressurizes the rooms and raises indoor humidity. Delivered airflow, and therefore cooling, depends on how freely air can leave.
• Pairing rigid media with no water management. Rigid pads are expensive to replace and scale quickly in hard water if the bleed-off is missing or plugged. Budget the purge or bleed-off system as part of the upgrade.
• Undersized duct connection. Bolting a 7,000 CFM cooler onto a duct sized for a 4,500 CFM unit adds noise and static pressure and throws away much of the upgrade.
• Buying in a heat wave. Emergency replacements in July limit choice and often mean paying for whatever is in stock.
• Skipping the electrical check. A larger motor may need a different circuit or a 240 V supply. Have a licensed electrician confirm the circuit, breaker and disconnect.

Questions to ask an installer

1. How did you size it? Expect a figure based on house volume and climate, not just "the same as the old one."
2. What airflow will it deliver on my ducts? A good installer considers static pressure and the motor sheave setting.
3. How will water and minerals be handled? Bleed-off or purge, where the drain line goes, and how the overflow is routed off the roof.
4. How is the roof or wall penetration sealed? Curb, flashing and duct connection details for roof units.
5. What about winter? How the water line is drained and how the ceiling or wall opening is closed.
6. Which parts are covered, and for how long? Cabinet, motor, pump and labor coverage often differ.

Brand reputations and parts availability are compared in evaporative cooler brands.