Rainwater Harvesting for Drylands and Beyond by Brad Lancaster

Condensate Harvesting

Top of the Austin, Texas City Hall air-conditioner-condensate waterfall

Bottom of the Austin, Texas City Hall waterfall. The water is harvested condensate from the building’s air conditioners.

Condensate Harvesting is the harvest of water vapor in the air that freely condenses on cold surfaces at your site.

When water vapor in the air (humidity) comes into contact with a colder surface, the water changes from a gas to a liquid and collects on that colder surface. This water vapor in the air that becomes liquid is referred to as condensate. Common examples are the drops of condensate that form on the outside of a cold glass of iced tea on a hot, humid day; condensate collecting on the cold coil of an air conditioner or refrigerator unit; or the condensate that forms on a metal roof cooled by the night air in early humid morning hours.

Passive condensation does not require the consumption of energy generated at a power plant to create the cool surface on which the moisture condenses.
Examples include condensate forming on a night-cooled metal roof in the early hours of a humid morning, or the ancient practice of creating a small pile of cool rocks (with ample air space between the rocks – so they can cool off at night) on which condensate will form in the early humid morning hours to passively irrigate an adjoining plant. An air well is a condensate-harvesting structure that works similar to the pile of rocks.

Active condensation does require the consumption of energy generated at a power plant to create the cool surface on which the moisture condenses.
Examples include moisture condensing on outside of a glass of an iced drink (energy had to be generated and consumed to make the ice), and condensation on an air conditioning unit (lots of energy has to be generated and consumed to run the air conditioner).

Ample condensate typically collects on the cold coil of air conditioner and refrigerator units, which is often wastefully discarded via a drain or pipe (see photo below). The hotter and more humid the climate, and/or the more moisture (such as from respiring and perspiring people) in an air-conditioned building; the more condensate air conditioners, refrigerator units, ice machines, and freezers will discharge.

Air-conditioning condensate wastefully drained to sidewalk and street. Living air conditioners in the form of street trees could be planted here to reduce the need of mechanical air conditioning, while attaining free irrigation from both the HVAC condensate and runoff from the adjoining street. Hatch, New Mexico. Photo: Brad Lancaster


How much condensate does an air conditioner discharge:
In a dry climate or season:
• a home air conditioner can generate 0.25 gallons (1 liter) of condensate/day

• a large commercial air conditioner can generate 500 gallons (1,900 liters)/day

In a humid climate or season:
• a home air conditioner can generate 18 gallons (68 liters) of condensate/day

• a large commercial air conditioner can generate 1,000 gallons (3,750 liters) or more/day

Verdant, living-air conditioner-like tree, irrigated by the air conditioning condensate drained from the air conditioners it shades and cools. Truth or Consequences, New Mexico.


White drainpipe drains condensate to the root zone of native desert willow tree. Truth or Consequences, New Mexico

Quality of condensate
This condensate is basically distilled water, containing low amounts of minerals such as salt, but may contain bacteria. Condensate is great for watering plants, but don’t drink it unless you adequately treat the water. Comparatively, municipal or well water is often relatively high in salt — which can be toxic to plants and soil life — thus condensate can help dilute those salts, making for a healthier growing environment (though salt-free rainwater does an even better job of this).

How to harvest and use condensate
To irrigate with condensate simply direct the condensate (ideally with free gravity—no pumps) to vegetation. It typically should not puddle on the surface, but rather quickly infiltrate into the root zone of the plants to reduce the chance of mosquitoes, bacteria growth in the water, or water loss to evaporation. More organic matter, diverse soil life, and vegetation in the soil increase the rate of infiltration into the soil.

I like to plant a relative oasis of plants (local natives are the easiest to succeed with) at the “spring” where condensate discharge pipes outlet into the landscape. And I like to first route the pipes and outlet where such an oasis can provide the maximum benefits (such as passive cooling, a verdant entryway, etc.)

For more information

See the book Rainwater Harvesting for Drylands and Beyond, Volume 1 3rd Edition to estimate how much condensate your air conditioner may discharge, the water needs (expenses) of potential plants for your condensate “spring” to grow a sustainable oasis in balance with your free on-site waters’ budget (income), and how to situate vegetation for free summer cooling and winter heating of associated buildings and outdoor spaces.

Other free on-site waters can include rainwater, stormwater, snow, fog, greywater, and dark greywater. The new, full-color editions of Rainwater Harvesting for Drylands and Beyond Volumes 1 and 2 show you how to harvest all of these to great effect, along with case studies of success.


Case study harvesting active condensation

Pearl Brewery, San Antonio, Texas
29? N Latitude. Elevation: 650 feet (198 meters)
Average humidity 54.9%
Average annual rainfall 30 inches (762 mm)

Water feature at Pearl Brewery in San Antonio, Texas supplied solely by roof runoff and air conditioning condensate from the brewery’s roof. Overflow water is directed to planted acequias (water channels for irrigation) that cool and beautify the landscape (see next photo). Design and photo by Ten Eyck Landscape Architects.


Planted acequias receiving roof runoff and air conditioning condensate just before they overflow to San Antonio River (seen in upper right quarter of photo). Design and photo by Ten Eyck Landscape Architects. Pearl Brewery, San Antonio, Texas.

The first active condensate recovery systems from air conditioners and other refrigeration appliances in San Antonio have worked so well that San Antonio became the first city to require  new commercial buildings design drain lines so that condensate can be captured and reused on site (OSA 34-274.1 Condensate Collection).

More info on the Pearl Brewery site’s condensate and rainwater harvesting at https://1703broadway.com/sustainability-design/water-conservation/

See Rainwater Harvesting for Drylands and Beyond, Volume 2, 2nd Edition for more case studies of condensate harvesting.

Case study harvesting passive condensation

Dew-harvesting roof in West Texas
In her book, “Water In Plain Sight,” Judith Schwartz reports how Katherine and Markus Ottmers of Ottmers Agricultural Technologies in West Texas are able to harvest up to 60 gallons per day of moisture passively condensing on, then running off, a 50’ by 50’ foot (15 by 15 m) metal roof.1

In Tucson, Arizona on humid mornings I too find dew passively condenses on my metal roof, much like water condenses on the outside of a glass of iced tea. Just before, and as, the sun rises and warms the air, the metal roof (that cooled off at night) is cooler than the air temperature, so moisture in the warmer air then condenses on the cooler roof. Sections of roof sloping away from the morning sun, stay shaded and cooler longer, thereby collecting more passive condensation than the sections of roof facing the morning sun.
Though I have never seen anywhere near the volume of condensation that the Ottmers reportedly receive (my roof is a third the size of the Ottmer’s roof; and has a single layer, rather than the two layers the Ottmers have; and my climate may be less humid – average annual humidity in Tucson is 29%).

1. The New Water Alchemists, The Desert’s Secret Water Falls by Judith D. Schwartz, https://craftsmanship.net/the-new-water-alchemists/

Energy costs of condensate and other waters

Check out the energy costs of various sources of water, such as air conditioning (AC) condensate, in the chart below, excerpted from Rainwater Harvesting for Drylands and Beyond, Volume 1, 3rd Edition.

Note the typically free on-site waters highlighted within the red borders — especially AC condensate—, and how their energy costs can range from 0 kWh per gallon/liter to 360 kWh per gallon (95 kWh per liter). A kWh is a kilowatt hour of energy.



The zero energy costs for various water sources reflected in the chart above typically result from harvesting those waters with no energy-consuming pumps or water treatment. Energy costs are incurred once pumps or mechanical water treatment is used. Or, in the case of AC condensate, the high energy cost does not reflect the use of pumps, but rather the large amount of energy the air conditioning unit itself consumes when on.

Due to the high-energy cost of the manufacture and use of air conditioning units (and their harmful chemical refrigerants – see next section), I don’t consider their condensate to be a truly sustainable on-site water source (even if renewable energy is the power source). But if air conditioners are already on site and used, you ought to at least harvest and use their condensate in beneficial ways, rather than just wastefully drain it away.

See appendix 9 of Rainwater Harvesting for Drylands and Beyond, Volume 1, 3rd Edition for charts and examples that show the water costs of different energy sources and the carbon costs of different energy sources in order to make more beneficial and informed choices in your life.


The refrigerant in air conditioners, refrigeration units, freezers, and ice machines is a major contributor to climate change

According the book Drawdown, the number one cause of climate changing gases in our atmosphere is the chemical refrigerants used in refrigerators and air conditioners.1

As reported in the book, Losing Our Cool: Uncomfortable Truths About Our Air-Conditioned World (and Finding New Ways to Get Through the Summer) by Stan Cox, “The air-conditioning of buildings in America is responsible for a quantity of carbon dioxide equivalent to what would be produced if every household in the country bought an additional vehicle and drove it an average 7,000 miles (11,200 km) per year.”2

This is horrifying due to the massive use and ever-expanding growth of the use of such appliances. But the main problem is not the appliances, but the chemical refrigerants used within them. And as history shows there is a path that could remedy this.

In the past, the chemical refrigerants (specifically chlorofluorocarbons [CFCs] and hydrochlorofluorocarbons [HCFCs] used to absorb and release heat) in refrigerators and air conditioners used to deplete the ozone layer of our atmosphere. That ozone layer is essential for healthy life on our planet as it absorbs, and protects us from, the sun’s ultraviolet radiation.

Thanks to countries around the world agreeing to and signing, the 1987 Montreal Protocol on Substances That Deplete the Ozone Layer; CFCs, HCFCs, and other ozone-depleting chemicals such as those that used to be used in aerosol cans and dry cleaning; have been phased out of use. As stated in the book Drawdown, “It took two short years from discovery of the gaping hole (in the ozone layer) over the Antarctic for the global community to adopt a legally mandated course of action. Now, three decades later, the ozone layer is beginning to heal.”3

The replacement chemical refrigerant to CFCs and HCFCs, is primarily hydrofluorocarbons (HFCs), which does not destroy the ozone layer, but HFCs’ capacity to warm the atmosphere is one thousand to nine thousand times greater than that of carbon dioxide!4

As Drawdown states, “In October 2016, officials from more than 170 countries gathered in Kigali, Rwanda, to negotiate a deal to address the problem of HFCs. Despite challenging global politics, they reached a remarkable agreement. Through an amendment to the Montreal Protocol, the world will begin phasing HFCs out of use, starting with high-income countries in 2019 and then expanding to low-income countries — some in 2024, others in 2028. HFC substitutes are already on the market, including natural refrigerants such as propane and ammonium.”5

The Kigali Amendment took effect January 2019, and over 65 countries have ratified the agreement,6 but as of this writing the U.S. is not one of them.7 Though on January 27, 2021 President Biden took the first steps for U.S. ratification.8


Take action
Communicate to your politicians and policy makers the importance of ratifying the Kigali Amendment; and make changes in your own lives, by reducing or eliminating your reliance on, and use of, appliances using HFC refrigerant (Rainwater Harvesting for Drylands and Beyond, Volume 1, 3rd Edition gives you many strategies to do this), and dispose of the refrigerant in a way that will keep it out of the atmosphere (Construction and Demolition: How to Properly Dispose of Refrigeration and Air-Conditioning Equipment, https://www.epa.gov/sites/production/files/documents/ConstrAndDemo_EquipDisposal.pdf, U.S. Environmental Protection Agency, accessed 11-1-2019).

1. Paul Hawken – editor, Drawdown: The Most Comprehensive Plan Ever Proposed to Reverse Global Warming, Penguin Books, 2017)

2. Stan Cox, Losing Our Cool: Uncomfortable Truths About Our Air-Conditioned World (and Finding New Ways to Get Through the Summer), (The New Press, 2010), page 41.

3. Paul Hawken – editor, Drawdown: The Most Comprehensive Plan Ever Proposed to Reverse Global Warming, Penguin Books, 2017)

4. Paul Hawken – editor, Drawdown: The Most Comprehensive Plan Ever Proposed to Reverse Global Warming, Penguin Books, 2017)

5. Paul Hawken – editor, Drawdown: The Most Comprehensive Plan Ever Proposed to Reverse Global Warming, Penguin Books, 2017)

6. “Kigali Amendment Enters into Force, Bringing Promise of Reduced Global Warming,” by Faye Leone, SDG Knowledge Hub, January 9, 2018, https://sdg.iisd.org/news/kigali-amendment-enters-into-force-bringing-promise-of-reduced-global-warming/, accessed 11-1-2019

7. “What’s Keeping Trump from Ratifying a Climate Treaty Even Republicans Support?,” by Phil McKenna, Inside Climate News, February 12, 2019, https://insideclimatenews.org/news/12022019/kigali-amendment-trump-ratify-hfcs-short-lived-climate-pollutant-republican-business-support-montreal-protocol, accessed 11-1-2019

8. “Biden Announces Move to Ratify Kigali Amendment on HFCs,” by


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THE UMBRELLA: A catch-all of resources, events, media, and more from Brad Lancaster In this time of Covid-19 and spending more time at home to be safe, I’ve been grateful for the solace, inspiration, and bountiful sustenance my water-harvesting gardens, landscape, and neighborhood forest has provided me, my family, friends, and neighbors. Record summer heat […]

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