Evaporative Cooler Glossary
From CFM to bleed-off valves, learn the terminology used by technicians and manufacturers.
A
Air Changes Per Hour (ACH)
The number of times the total air volume in a space is completely replaced in one hour. Evaporative coolers typically aim for 20-40 air changes per hour (equivalent to replacing air every 2-3 minutes) for effective cooling.
Aspen Pads
Traditional evaporative cooler pads made from shredded aspen wood (excelsior). Inexpensive and widely available, but require annual replacement. Best suited for areas with soft water. Also called "wood wool" or "excelsior pads."
Adiabatic Cooling
The scientific principle behind evaporative cooling. When water evaporates, it absorbs heat energy from the surrounding air without adding external heat—hence "adiabatic" (no heat transfer). The result is cooler, more humid air.
B
Belt (Drive Belt)
A V-belt that connects the motor to the blower assembly in belt-driven evaporative coolers. Belts should be inspected annually for cracks, fraying, or glazing. Proper tension is critical—too loose causes slipping and reduced airflow; too tight causes premature bearing wear. Replace belts every 2-3 years or when showing wear.
Bleed-Off Valve (Dump Valve)
A valve that continuously drains a small amount of water from the cooler reservoir to flush out mineral deposits. Essential in hard water areas to prevent scale buildup on pads and components. Typically drains 3-10 gallons per hour.
Blower (Squirrel Cage)
The fan assembly that moves air through the cooler and into the home. Named for its cylindrical shape with curved blades. Measured by diameter and RPM. Larger blowers move more air (higher CFM) but use more electricity.
BTU (British Thermal Unit)
A unit of heat energy. While air conditioners are rated in BTUs, evaporative coolers are typically rated in CFM because their cooling capacity varies with humidity. One BTU is the energy needed to raise one pound of water by 1°F.
C
CFM (Cubic Feet per Minute)
The standard measure of airflow capacity for evaporative coolers. Indicates how many cubic feet of air the unit can move per minute. To size a cooler, calculate your home's volume and divide by 2 for the minimum CFM needed.
Cellulose Pads
See "Rigid Media Pads." Made from specially treated cellulose paper in a honeycomb pattern. More efficient than aspen pads and longer-lasting.
Cross-Ventilation
The practice of opening windows or doors on the opposite side of the home from where cool air enters. Essential for evaporative cooling—without proper exhaust, humidity builds up and comfort decreases.
D
Damper
A movable plate that controls airflow in ductwork. In evaporative cooling, dampers can close off ducts to unused rooms or seal the cooler opening during winter to prevent cold air drafts.
Dew Point
The temperature at which air becomes saturated and water vapor begins to condense into liquid. A lower dew point indicates drier air—ideal for evaporative cooling. When the dew point is below 55°F, evaporative coolers work exceptionally well. Above 60°F dew point, cooling effectiveness decreases significantly. Dew point is often a better indicator of evaporative cooling potential than relative humidity alone.
Direct Evaporative Cooling
The most common type of evaporative cooling where air passes directly through wet media and enters the living space. Adds humidity to the air. Contrasts with "indirect" systems that keep humidity separate.
Downdraft Cooler
A roof-mounted evaporative cooler that blows air downward into the home through ceiling ducts. Most common type for whole-house cooling. Contrasts with "side-draft" or window-mounted units.
Dry Bulb Temperature
The standard air temperature measured by a regular thermometer, unaffected by humidity. Contrasts with "wet bulb temperature," which accounts for evaporative cooling potential. The difference between dry bulb and wet bulb temperatures indicates how much cooling an evaporative cooler can provide—the larger the difference, the greater the cooling potential.
Ductwork
The system of metal or flexible tubes that distribute cooled air from a central evaporative cooler throughout the home. Properly sized ductwork is essential for efficient airflow. Undersized ducts restrict airflow and reduce cooling capacity; oversized ducts reduce air velocity and may not reach distant rooms effectively.
E
Efficiency (Saturation Efficiency)
A measure of how effectively an evaporative cooler approaches the theoretical maximum cooling (wet bulb temperature). Expressed as a percentage. Aspen pads typically achieve 60-70% efficiency, while rigid media pads achieve 80-90%. Higher efficiency means cooler output air but may also mean higher humidity.
Evaporative Cooler
A cooling device that lowers air temperature through water evaporation. Hot, dry air passes through wet pads, causing water to evaporate and absorb heat. The result is cooler, humidified air. Also known as a "swamp cooler," "desert cooler," or "wet air cooler."
Evaporation Rate
The amount of water that evaporates per hour, typically measured in gallons per hour (GPH). Depends on temperature, humidity, airflow, and pad surface area. Higher evaporation rates mean more cooling but also more water consumption. A typical residential cooler evaporates 3-15 gallons per hour.
Exhaust
The path for humid air to exit the home. Evaporative coolers require adequate exhaust (open windows, roof vents, or dedicated exhaust systems) to function properly. Without exhaust, humidity accumulates and cooling effectiveness drops.
F
Float Valve
A valve connected to a floating ball that maintains proper water level in the cooler reservoir. Similar to a toilet tank mechanism. When water drops, the float lowers and opens the valve to refill. A faulty float valve causes overflows or dry operation.
Fan-Only Mode
Operating the evaporative cooler's fan without running the water pump. Used to dry out pads after use (preventing mold), provide ventilation on cooler days, or circulate air when humidity is too high for effective evaporative cooling.
G
GPH (Gallons Per Hour)
The rate of water evaporation or consumption, measured in gallons per hour. Evaporative coolers typically use 3-15 GPH depending on size, temperature, and humidity. Higher GPH means more cooling but also more water consumption. Used to estimate water costs and ensure adequate supply.
GPM (Gallons Per Minute)
The flow rate of water pumps, measured in gallons per minute. Cooler pumps typically range from 150-350 GPM. Adequate GPM ensures all pads stay fully saturated. If your pump's GPM is too low, pads may have dry spots that reduce cooling efficiency and can cause premature pad deterioration.
Grille (Register)
The decorative cover over a duct opening where cooled air enters a room. May have adjustable louvers to direct airflow. Ceiling-mounted grilles (for downdraft coolers) or wall-mounted grilles distribute air throughout the home. Keep grilles clean and unobstructed for maximum airflow.
H
Hard Water
Water with high mineral content (calcium, magnesium). Common in the Southwest. Hard water causes scale buildup on cooler pads, pumps, and distribution systems, reducing efficiency and lifespan. Bleed-off valves and regular cleaning help manage hard water issues.
Humidity (Relative Humidity)
The percentage of moisture in the air compared to the maximum it can hold at that temperature. Evaporative coolers work best below 50% relative humidity. Above 60%, cooling effectiveness drops significantly. Check your local humidity levels to determine if evaporative cooling is right for your climate.
I
Indirect Evaporative Cooling
An advanced evaporative system where air is cooled by a heat exchanger that's been cooled by evaporation, rather than passing directly through wet media. This provides cooling without adding humidity to indoor air. More complex and expensive than direct systems, but works in slightly higher humidity.
L
Louvers
Angled slats on the exterior of an evaporative cooler that allow air to enter while keeping out rain, debris, and direct sunlight. May be fixed or adjustable. Helps protect pads and internal components from weather damage.
M
Media Pads
The water-absorbent material through which air passes in an evaporative cooler. Common types include aspen (wood shavings), cellulose (rigid media), and synthetic materials. Pad quality and condition directly impact cooling performance.
Motor
The electric motor that powers the blower fan. Evaporative cooler motors typically range from 1/3 HP to 1 HP for residential units. May be single-speed or variable-speed. Motors should be lubricated annually and typically last 5-10 years.
N
Negative Pressure
The slight vacuum created inside a home when an evaporative cooler is running without adequate exhaust. Signs include doors that are hard to open, whistling around windows, or reduced cooling. Solve by opening windows or installing dedicated exhaust vents. Proper exhaust area should equal 1-2 square feet per 1,000 CFM.
O
Overflow
Water spilling over the edges of the cooler reservoir, typically caused by a stuck or misadjusted float valve. Can cause water damage to roofs, walls, or foundations. Check float valve operation at the start of each season and adjust water level to about 1 inch below the overflow outlet.
Oscillating Louvers
Motorized louvers on some portable or window evaporative coolers that sweep back and forth to distribute cool air more evenly across a room. Similar to oscillating fans. Not common on whole-house systems, which rely on ductwork for distribution.
P
Pump
The submersible pump that circulates water from the reservoir to the distribution system, keeping the pads wet. Measured in gallons per minute (GPM). A failed pump means no water on the pads and no cooling. Pumps typically last 2-5 years.
Purge Pump (Drain Pump)
A timer-controlled pump that periodically drains and refills the cooler reservoir to flush mineral deposits. An alternative to continuous bleed-off systems. Reduces water waste while managing hard water buildup.
Pre-Wetting
Running the water pump for several minutes before turning on the fan to fully saturate the pads. Ensures maximum cooling from the first moment the fan starts. Many modern coolers have an automatic pre-wet cycle.
R
Reservoir (Sump)
The water tank at the bottom of the cooler that holds water for the pump to circulate. Should be drained and cleaned regularly to prevent algae, scale, and sediment buildup. Also called the "pan" or "basin."
Rigid Media Pads (Cellulose Pads)
High-efficiency cooling pads made from treated cellulose paper in a honeycomb structure. More expensive than aspen pads but last 3-5 years, provide better cooling efficiency (up to 90% vs. 70% for aspen), and resist clogging. Standard on premium brands like MasterCool.
S
Scale (Mineral Scale)
Hard mineral deposits that accumulate on cooler pads, pumps, and water lines from hard water evaporation. Scale reduces pad efficiency, clogs water distribution, and shortens component life. Regular descaling and bleed-off systems help prevent buildup.
Side-Draft Cooler
An evaporative cooler mounted on the side of a building (often a window) that blows air horizontally. Common for single rooms or small spaces. Easier to install than downdraft units but provides less whole-home coverage.
Spider (Water Distribution Spider)
A fitting at the top of the cooler that distributes water from the pump to multiple tubes, each feeding a different pad. Named for its spider-like appearance with tubes radiating outward.
Swamp Cooler
The common nickname for an evaporative cooler, derived from the slightly musty smell that poorly maintained units can produce. Despite the name, properly maintained swamp coolers produce fresh, clean air. See "Evaporative Cooler."
T
Thermostat
A temperature-sensing device that controls the cooler. Basic coolers use manual switches, while advanced models have thermostats that automatically cycle the cooler to maintain a set temperature. Some smart thermostats can control evaporative coolers alongside HVAC systems.
Two-Stage Cooling
An advanced evaporative cooling system that combines indirect and direct cooling. Air is first pre-cooled by an indirect heat exchanger (no humidity added), then passed through direct evaporative media. Provides greater temperature drops and works in slightly higher humidity than standard systems.
V
Variable Speed
A motor or control system that allows the blower to run at multiple speeds rather than just high/low. Variable speed motors provide better comfort control, quieter operation at lower speeds, and can extend pad life by reducing airflow during mild conditions. More expensive than single-speed units but offers energy savings and improved comfort.
Vent (Exhaust Vent)
An opening that allows humid air to escape from the home when the evaporative cooler is running. Can be open windows, dedicated roof vents, or ceiling barometric dampers that open automatically when the cooler pressurizes the home. Proper venting is essential—without it, humidity builds up and cooling decreases.
W
Water Distribution System
The network of tubes, troughs, and fittings that carry water from the pump to the top of the pads. Water flows down through the pads by gravity, keeping them wet. Clogs in the distribution system cause dry spots and uneven cooling.
Wet Bulb Temperature
The lowest temperature that can be achieved through evaporation alone at a given humidity level. Evaporative coolers can theoretically cool air down to the wet bulb temperature (though real-world efficiency is 70-90% of this). The lower the humidity, the lower the wet bulb temperature and greater the potential cooling.
Winterization
The process of preparing an evaporative cooler for winter storage. Includes draining all water, disconnecting the water supply, covering the unit to prevent debris and weather damage, and closing dampers. Proper winterization prevents freeze damage to pumps and water lines.
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