Photo of an evaporative cooler or swamp cooler in Tucson (C) Daniel Friedman Swamp Coolers & Evaporative Coolers for Air Cooling: Design & Usage Guide

  • EVAPORATIVE COOLING SYSTEMS - CONTENTS: Guide to Swamp coolers or Evaporative Coolers for building cooling. Evaporative cooling systems for homes. Design specifications for cooling systems with under-floor air ducts. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
  • POST a QUESTION or READ FAQs about using a swamp cooler to cool a building interior - when do swamp coolers work better than an air conditioner?
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Swamp coolers:

This article explains the use of an evaporative cooling system or "swamp cooler" including cool air delivery design considerations when using under-floor ducts for cool air delivery in buildings. Swamp coolers or evaporative cooling systems work well in dry climates - usually, as we explain here. The discussion of cool air duct location and routing includes general air conditioning duct design concepts that also apply to all air conditioning and cooling designs.

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Evaporative Cooling Systems or "Swamp Coolers" - How They Work

Evaporative cooling system types (C) Carson Dunlop Associates

Evaporative coolers, also called "swamp coolers" rely on the evaporation of water to cool building air, rather than the movement of a refrigerant through cooling coils. Cooling towers, swamp coolers, and even a simple window fan blowing air across a pan of water and into a room are types of evaporative cooling systems.

[Click to enlarge any image]

Swamp coolers systems may use less energy than a refrigerant-gas and compressor type air conditioner but they'll only work where the ambient humidity is low enough to make it easy to evaporate water, such as in Arizona and other areas of the Southwest and Mexico.

Our sketch (left), courtesy of Carson Dunlop, shows three types of evaporative cooling systems or "swamp coolers".

All three evaporative cooling system approaches are similar: dry outdoor air is blown across an evaporative cooling device that uses water evaporation to cool the incoming air.

The differences among these swamp cooler designs are principally how the water to be evaporated is delivered to the evaporative cooling device (drum or pad bottom is immersed in a water reservoir pan, or a pump delivers water to the top of the evaporative pad), and the type of evaporative device itself: a rotating drum or an evaporative pad.

Delivery of Cool Air Using an Evaporative Cooling System

Sketch of under floor cool air ducts (C) Steve Bliss, Solar Age, Daniel Friedman

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

Topic: Under-floor ducts for cool air delivery with (for example) an evaporative cooling system

Question: We plan to build a pilot energy-efficient home in our city. We have a long and somewhat severe cooling season, with a humidity level that is relatively low (10-15 % RH). An evaporative cooling system or swamp cooler should suffice, but there are several drawbacks: dust, open windows, safety, noise, and the like.

I would like to duct the cool air delivery system under a wood-framed floor, supplying cool air through floor registers, and exhausting it out through a roof cupola. (See sketch above, courtesy of Solar Age Magazine). I am sure that an attic fan will be necessary to drive the warm air exhaust out of the building.

Do you see any obvious problems with the scheme I have described? -- Jimmy Moore, Odessa, TX


Single-stage direct evaporative coolers, also called "swamp coolers" do provide a cost-effective means of cooling houses during hot, dry periods. Their main drawback is moister indoor air - the cost for a lower dry-bulb temperature.

Due to low energy use (for a small fan and water pump), swamp coolers are popular in the U.S. in parts of the West and Southwest. For example, our photograph at page top shows a swamp cooler system whose operating components are mounted on a Tucson Arizona rooftop.

Delivering Cool Air Through Floor Ducts?

Warm and cool air delivery duct design (C) Carson Dunlop Associates

Delivering cooled air through the floor and exhausting it through the roof runs counter to the tendency of warm air to rise and cool air to fall in a building. But an exhaust fan in the roof should pull air upwards through the entire house.

Watch out: Two warnings:

Make sure that the exhaust fan capacity in cubic feet per minute (CFM) is matched to the volume of air that is being supplied from the cooling system upwards from the floor ducts and air registers.

Carson Dunlop's sketch (left) shows a different approach for a closed air duct system that changes its duct operation depending on whether warm air or cool air is being delivered into the building.

You would probably not use this duct design in an evaporative cooling system because the evaporative cooling approach depends on continuous introduction of dry outside air into and through the building during the cooling system, but the sketch makes clear a duct system intended to deliver cool air in a building needs to operate differently than ducts intended to deliver warm air during the heating system if airflow is to be optimized.

Using a Rooftop Cupola to Exhaust Swamp Cooler-Delivered Cool Air?

Whole house fan and building exhaust requirements (C) Carson Dunlop Associates

Provide an air duct between the top floor ceiling grille and the roof exhaust cupola. This will keep moist (and possibly cooler) air out of the attic, where it might condense at night and cause moisture problems in the structure and in the attic insulation.

(See Attic Moisture or Mold Sources and see Mold in Fiberglass Insulation.

Readers ducting cool air through a crawl space should also see CRAWLSPACE MOLD ADVICE.)

Carson Dunlop's sketch (left) warns of another safety consideration: a whole house fan or an attic exhaust fan (such as we describe above), can cause back-drafting at fireplaces and atmospheric-vented gas heating appliances - backdrafting can in turn cause dangerous, potentially fatal carbon monoxide hazards in buildings.

Watch out: The hazards are described at COMBUSTION GASES & PARTICLE HAZARDS

and the remedies are described at COMBUSTION AIR for TIGHT BUILDINGS.

Finally, remember to provide a means of closing off the rooftop exhaust during the heating season. Otherwise your warm house air will simply escape by natural convection currents carrying it up and outdoors through your rooftop-mounted exhaust system even if the fan is not running.

See WATER & ICE IN DUCT WORK for a case history where just this problem occurred in a building.

When is Evaporative Cooling Effective

Swamp Cooler (C) D Friedman Andy Gieseke

See Solar Age Magazine "Notebook" 2/84, for the conditions where evaporative cooling is effective.

In that issue, author Jerry Germer reviews use of the psychrometric chart that relates moisture, temperature, and indoor relative humidity to discuss the conditions that make people comfortable indoors, and how to relate indoor comfort to the outdoor climate, particularly to temperature and humidity in those two environments.

Our photo (left) shows an evaporative cooler or "swamp cooler" in use blowing cool air into an outdoor mechanic's work area at Canyon Auto Sales in Tucson, AZ. Photo by the author and Andy Gieseke, Canyon Auto.

Swamp coolers or other evaporative cooling systems can cool a building interior using less energy than a conventional air conditioner or heat pump system except during times of very wet or humid weather such as the "monsoon" season in southern Arizona.

During wet and very humid weather evaporative cooling simply cannot evaporate enough water to produce good cooling. Some building owners install both evaporative cooling or swamp coolers and a conventional refrigerant-based air conditioner or heat pump, running the latter only when necessary due to humid conditions.

Watch out: leaky swamp coolers or just about any mechanical equipment that regularly spills or overflows water onto a roof surface is likely to cause stains on the roof surface and might lead to roof wear as well.

Details are at WHITE STAINS on ROOFS.

8 Indoor Air Cooling & Heating Strategies

Mr. Germer mapped eight heating and cooling "design zones" onto areas of the psychrometric chart and then described eight indoor air conditioning approaches and related them to the outdoor climate. For areas or times of the year when temperature and humidity outdoors, and thus also relative humidity, fall within multiple zones on the Germer chart, multiple of the 8 zones and 8 indoor air conditioning strategies below might be usable.

  1. Active solar or mechanical heating: this zone has outdoor temperatures too low for passive building environment conditioning strategies.
  2. Passive solar heating: direct solar gain, indirect solar heat gain, hybrid systems, combined with air infiltration and temperature conduction control - we describe passive solar heating
    and further
  3. Dehumidification: high outdoor (or indoor) humidity requires using some method to remove the excess humidity in order for building occupants to be comfortable. In the 1984 article Germer pointed out that passive dehumidification had not been well developed. See HUMIDITY LEVEL TARGET.
  4. Mechanical air conditioning: temperature & humidity combinations in this climate zone cannot be managed by passive means and so mechanical air conditioning
    See AIR CONDITIONING & HEAT PUMP SYSTEMS or active solar cooling will be needed.
  5. Ventilation: although temperatures and humidifies may be high, occupants may be made comfortable by direct evaporation of sweat from the skin if enough air velocity is available - in other words, fans can do the trick.
  6. Thermal mass: these air conditioning systems are usually found in hot-dry climates with dry cool nights, such as southwestern United States and parts of Mexico, particularly at higher elevations (see PASSIVE SOLAR DESIGN METHOD and see THERMAL MASS in BUILDINGS
    well as THERMAL MASS in UPSTAIRS). Radiant cooling and night-flushing of internal mass with cool outside air stores coolness (the ability to later absorb heat) for the next day, and similarly, during the warmer day, the same thermal mass can store warmth to even out cooler nighttime temperatures indoors.
  7. Evaporative cooling: low energy "swamp coolers" (discussed in this article) can be used under conditions of low humidity such as in Arizona and higher elevations of Mexico.
  8. Humidification: moisture needs to be added to very dry air for indoor comfort.
    See HUMIDITY LEVEL TARGET. We prefer local or room humidifiers after seeing duct, mold, concerns with some centralized humidifier add-ons to heating and air conditioning systems.

The link to the original Q&A article in PDF form immediately below was preceded above by an expanded/updated online version of this article.

The articles at this website describe the basic components of an air conditioning system and then we discuss how to estimate the rated cooling capacity of an air conditioning system by examining various data tags and components. The limitations of visual inspection of A/C systems are described.

Evaporative Cooling Resources for Products, Manuals, Installation Guides


Continue reading at ROOFTOP A/C / HEAT PUMP - the air conditioning system seems to be "running" but not enough cool air, or no cool air at all is being delivered to the occupied space, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see AIR CONDITIONING & HEAT PUMP SYSTEMS - HVAC inspection, installation, diagnosis & repair procedures provided free

Or see HEAT PUMPS - home


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