Cooling in which no heat is added or removed from outside the airstream: sensible heat in the air is converted to latent heat as water evaporates, so the air gets cooler and more humid while its total heat (enthalpy) stays nearly constant. Direct evaporative cooling is an adiabatic process. See how evaporative coolers work.
Air changes per hour (ACH)
How many times per hour the full volume of air in a house is replaced. Calculated as CFM x 60 / house volume in cubic feet. Whole-house coolers commonly deliver about 15 to 30 ACH, far more than air conditioning, which mostly recirculates.
Aspen pad
A pad made of shredded aspen wood fibers (excelsior) in a plastic or fiber net. Inexpensive and absorbent, with roughly 60 to 80% saturation efficiency, but it usually needs replacing every one or two seasons. Compare with rigid media.
Barometric relief damper
A ceiling-mounted damper, also called an up-duct or up-ducted relief vent, that opens under the slight pressure a running cooler creates and lets air escape into a vented attic. It provides relief air without open windows and closes by gravity when the cooler is off. See venting and relief air.
Belt
The V-belt that connects the motor pulley to the blower pulley on belt-drive coolers. A worn or loose belt slips, squeals and reduces airflow; too tight a belt wears bearings. See belt and pulley adjustment.
Bleed-off
A small continuous discharge of sump water to a drain, usually through a tee and a thin tube on the pump outlet. It limits mineral buildup by replacing concentrated water with fresh supply water. See bleed-off and water quality.
Blower wheel
The squirrel-cage centrifugal fan that moves air through the pads and into the duct. Typically 15 to 20 in diameter on whole-house units, mounted on a shaft with two bearings or, on direct-drive units, on the motor shaft.
BTU
British thermal unit, the heat needed to raise 1 lb of water by 1 F. Cooling capacity is given in BTU per hour (BTU/h). For moving air, sensible cooling BTU/h = 1.08 x CFM x temperature drop.
CFM
Cubic feet per minute of airflow, the main capacity rating for coolers. Box ratings are often measured at low or no static pressure, so delivered airflow through ducts is lower. See CFM explained and the cooler size calculator.
Cooler cover
A fitted canvas or vinyl cover placed over a rooftop cooler for winter to keep out wind, snow and debris and reduce heat loss through the duct. Should be used with the cooler drained; see how to winterize a swamp cooler.
Cycles of concentration
The ratio of dissolved minerals in sump water to those in the supply water. Equals (evaporation rate + bleed rate) / bleed rate. Technicians commonly aim for about 3 to 6 cycles depending on water hardness.
Design conditions
Outdoor temperatures used for sizing equipment, such as the 1% design dry bulb and its mean coincident wet bulb, published by ASHRAE for weather stations. Values are exceeded only a small fraction of summer hours. Our climate index lists them for 50 cities.
Dew point
The temperature at which the air's existing moisture would begin to condense. It indicates absolute moisture content and changes slowly through the day, so it is a handy forecast number for cooler owners. Always at or below the wet bulb.
Direct evaporative cooling
Cooling in which the supply air passes through wet media and picks up moisture. All common residential swamp coolers are direct. See direct vs indirect evaporative cooling.
Distribution trough
The channel along the top of each pad frame that spreads water evenly over the pad. Holes or slots in the trough clog with scale and debris, causing dry streaks that let warm air bypass the pad.
Downdraft cooler
A cooler that discharges air straight down out of its base, typically into a roof-mounted duct. Common on homes with flat or low-slope roofs. See downdraft vs side-draft coolers.
Drain line
Tubing that carries bleed-off and overflow water from the cooler to a roof drain, gutter, landscaping or sewer connection allowed by local code. A blocked drain line is a common cause of roof leaks. See water supply and drain lines.
Dry bulb temperature
Ordinary air temperature, measured by a thermometer shielded from sun and moisture. The starting point for every evaporative cooling calculation.
Duct damper
A plate or blade in the cooler duct that can be closed to block airflow, typically for winter or when a separate AC system is running. A tight damper prevents heat loss and stops moist cooler air entering AC ducts.
Enthalpy
The total heat content of air, sensible plus latent, usually in BTU per pound of dry air. In direct evaporative cooling enthalpy stays nearly constant while temperature falls and moisture rises.
Evaporation rate
The amount of water a cooler turns into vapor, commonly 3 to 15 gallons per hour for whole-house units on hot days. Estimated as 1.08 x CFM x temperature drop / 1,060 / 8.34 gallons per hour. See the water usage calculator.
Evaporative cooler
The technical name for a swamp cooler: a device that cools air by evaporating water into it. Variants include whole-house rooftop units, window units, portable units and commercial systems. See types of swamp coolers.
Excelsior
Fine wood shavings, usually aspen, used to make traditional cooler pads. Also called wood wool.
Face velocity
The speed of air through the face of a pad, in feet per minute (fpm). Equals CFM divided by pad face area in square feet. Too high a face velocity lowers efficiency and can carry water droplets into the duct.
Fan and pad system
An evaporative cooling layout used in greenhouses and livestock and poultry houses, with wet pads in one wall and exhaust fans in the opposite wall pulling air through the building. See evaporative cooling for greenhouses.
Float valve
A valve operated by a floating ball or cup that opens to refill the sump as water evaporates and closes at the set level. A float set too high causes overflow; too low starves the pump. See float valve adjustment.
Humidity ratio
The mass of water vapor per mass of dry air, given in pounds per pound or grains per pound (7,000 grains = 1 lb). Unlike relative humidity, it does not change when air is heated or cooled without adding or removing moisture.
Indirect evaporative cooling
Cooling in which a secondary airstream is evaporatively cooled and then cools the supply air through a heat exchanger, so the supply air gains no moisture. Typical wet bulb effectiveness is about 50 to 75%.
Latent heat
Heat absorbed or released when water changes phase without a temperature change. Evaporating water absorbs about 1,060 BTU per pound, which is where a cooler's cooling comes from.
Makeup water
Fresh water added to the sump through the float valve to replace water lost to evaporation and bleed-off.
Mean coincident wet bulb
The average wet bulb temperature that occurs at the same time as a given design dry bulb. Used with the design dry bulb to predict cooler supply temperature on a typical hot afternoon.
Misting system
An outdoor cooling system that sprays fine water droplets into the air through nozzles, usually at high pressure. Effective in dry air on patios but different from pad-based coolers. See outdoor patio evaporative coolers.
Monsoon
The seasonal wind shift that brings moisture and thunderstorms to the US Southwest, typically from early July into September. The National Weather Service defines the Arizona monsoon season as June 15 through September 30. Rising dew points sharply reduce cooler performance; see swamp coolers during monsoon season.
Motor pulley
The small pulley on the motor shaft. On many coolers it is an adjustable sheave whose effective diameter can be changed to raise or lower blower speed. See belt and pulley adjustment.
Overflow
A standpipe in the sump connected to the drain line that carries away water above a set level, protecting the roof and ceiling if the float valve sticks open.
Pad frame
The removable side panel, usually louvered, that holds a pad in place and sheds rain. Also called a louver panel.
Pad retainer
Wire clips or rods that hold an aspen pad flat inside its pad frame so it does not sag and leave gaps for air to bypass.
Portable evaporative cooler
A self-contained cooler on wheels with a built-in tank, pump, pad and fan. Ranges from small room units to large units for shops and outdoor spaces. See the portable evaporative cooler buying guide.
Pre-wet
Running the pump alone for a few minutes before starting the blower, so the pads are fully wet and the first air delivered is cool rather than warm and dusty. Many thermostatic controls do this automatically.
Psychrometer
An instrument for measuring wet bulb and dry bulb temperatures. A sling psychrometer is spun by hand; digital versions calculate wet bulb from temperature and humidity sensors.
Psychrometric chart
A chart that relates dry bulb, wet bulb, dew point, relative humidity, humidity ratio and enthalpy. Direct evaporative cooling follows a line of constant wet bulb on the chart.
Purge pump
A second pump or a timed drain valve that dumps some or all of the sump water at intervals instead of bleeding continuously. Often better than continuous bleed-off in hard-water areas.
Recirculating pump
The small submersible pump that lifts water from the sump to the distribution system. The most commonly replaced cooler part; see how to replace a swamp cooler pump.
Relative humidity (RH)
The amount of water vapor in air as a percentage of the maximum it could hold at that temperature. RH falls as air warms even when moisture is unchanged, so it is a poor stand-alone guide to cooler performance. See do swamp coolers work in humidity.
Relief air
The path by which air supplied by a cooler leaves the house, through open windows, doors or barometric relief dampers. A common guideline is 1 to 2 sq ft of opening per 1,000 CFM.
Rigid media
Cross-fluted cellulose or synthetic sheets bonded into thick blocks, often called by brand names such as CELdek. Typically 8 in or 12 in thick with about 85 to 95% saturation efficiency and a service life of several seasons. See types of evaporative cooler pads.
Roof jack
The sheet metal transition that carries the cooler's discharge through the roof into the duct system, sealed with flashing. Leaks at the roof jack are common in older installations.
Saturation efficiency
The fraction of the wet bulb depression a direct cooler achieves: (dry bulb - supply temp) / (dry bulb - wet bulb). About 60 to 80% for aspen, 85 to 90% for 8 in rigid media and up to about 90 to 95% for 12 in rigid media.
Scale
Hard mineral deposits, mainly calcium carbonate, left behind as water evaporates. Scale stiffens pads, blocks distribution holes and coats sumps. Controlled with bleed-off, purge systems and regular cleaning; see how to clean a swamp cooler.
Sensible heat
Heat that changes air temperature and can be read on a thermometer. Sensible cooling BTU/h = 1.08 x CFM x temperature drop.
Sheave
Another name for a pulley, especially an adjustable (variable-pitch) motor pulley whose two halves can be screwed closer or farther apart to change belt position and blower speed.
Side-draft cooler
A cooler that discharges air horizontally out of one side, used on pitched roofs, ground-level pads and wall or window installations. Also called a horizontal or side-discharge cooler.
Spider
A water distributor at the top of the cooler with several tubes that carry water from the pump to each pad's trough. Clogged spider outlets leave a pad dry.
Static pressure
The resistance the blower must push against from ducts, registers, pads and a closed house, measured in inches of water column. Higher static pressure reduces delivered CFM.
Sump
The water reservoir pan in the bottom of the cooler, typically holding 2 to 5 gallons on residential units. Needs periodic draining and scrubbing to prevent scale, algae and odors.
Supply air temperature
The temperature of the air leaving the cooler. For a direct cooler: dry bulb - saturation efficiency x (dry bulb - wet bulb). Calculate it with the cooling temperature calculator.
Swamp cooler
The common nickname for a direct evaporative cooler, used mainly in the US Southwest and Mountain West. See why is it called a swamp cooler.
Thermostat (evaporative cooler)
A control made for evaporative coolers that switches pump and fan speeds separately, and may offer pre-wet, purge timing and humidity readouts. Standard AC thermostats usually cannot control a cooler correctly. See thermostats and controls.
Ton of cooling
12,000 BTU per hour, a unit from refrigerated air conditioning. Coolers are rated in CFM instead, but their capacity can be converted using 1.08 x CFM x temperature difference.
Two-speed motor
The typical whole-house cooler motor, with high and low speed windings selected by the switch or thermostat. Commonly 1/3 to 1 hp. See the swamp cooler motor guide.
Two-stage evaporative cooling
An indirect stage followed by a direct stage (also called indirect-direct, or IDEC). Can deliver supply air a few degrees below the outdoor wet bulb with less added moisture than direct cooling alone.
Water hardness
The concentration of dissolved calcium and magnesium in supply water, usually stated in grains per gallon or parts per million. Harder water needs more bleed-off and causes faster pad scaling.
Wet bulb depression
The difference between dry bulb and wet bulb temperature. It is the maximum possible temperature drop for a direct cooler; multiplying it by saturation efficiency gives the actual drop.
Wet bulb effectiveness
The indirect-cooling equivalent of saturation efficiency: the share of the wet bulb depression achieved in the supply air by an indirect or two-stage unit. Two-stage units can exceed 100%.
Wet bulb temperature
The lowest temperature air can reach by evaporating water into it, and the floor for direct evaporative cooling. See wet bulb temperature explained.
Window cooler
A side-draft evaporative cooler mounted in or beside a window, cooling one room or a small house. See window evaporative coolers.
Winterizing
Preparing a cooler for freezing weather: shutting off and draining the water supply, draining the sump, disconnecting the pump, closing or covering the duct and covering the unit. See how to winterize a swamp cooler.