Southwest Environment

Stories written by University of Arizona students

Department of Soil, Water and Environmental Science
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Air conditioners can water plants while cooling air

By Vanessa Lentini

Figure 1
Photo by Vanessa Lentini
Three air handling units sit on the roof of the CAPLA building on the UA campus.
From the roof of the College of Architecture, Planning and Landscape Architecture building on the University of Arizona campus, there’s a view of the treetops in the garden below and Tucson’s Catalina Mountains in the distance. Beneath the sound of cars going by on Speedway, there’s a mild hum coming from three air handling units on the CAPLA roof.

The size of an SUV, each unit contributes to cooling the three-story building constructed between 2005-2006 and the gallery of the old part of the building. Each floor of the new building is “the equivalent of about four Tucson houses,” said Ron Stoltz, professor of landscape architecture and associate dean for Facilities and Operations for CAPLA.

What is less known is that these units also help to water the plants and pond in the Underwood Garden below. Like all air conditioners, these units pull water out of the air, creating condensate that can be used to water the landscape.

“The condensate is our biggest source of water for supplementing irrigation,” Stoltz said. The supplemental water, a mixture of rooftop runoff, condensate, and drinking fountain water, provides 83 percent of the water needs for the building and garden.

Figure 2Photo by Vanessa Lentini
One of the copper pipes from the air handling units collects the condensate and drains into the eaves trough on the CAPLA building rooftop.
Rooftop collection of water from these units is a quick process. Water trickles through the condensate drain, into the copper pipes, down the eaves trough, and finally into the cistern. The cistern can hold 11,600 gallons of water, which Stoltz noted is about half the size of an average swimming pool.

In this case, the condensate goes into a cistern that feeds a pond. When the tank is full, it overflows into the pond at the Underwood Garden, which has created habitat for animals such as the lowland leopard frog. On a normal day there is a trickle of water pouring over the edge of the spout. During the summer monsoon season, the cistern overflows more frequently, producing a large waterfall.

During the humid summer months, Stoltz said, the combined air handling units are capable of producing 45 gallons of water an hour.

In comparison, a typical Tucson home can produce on average 1.5 to 3 gallons an hour during the monsoon season, according to senior water policy and economics consultant Gary Woodard, a retired University of Arizona faculty member who now works for Montgomery and Associates.

About 99 percent of new homes in Tucson have AC units, Woodard said, and two-thirds of older homes now have units or have converted over to central AC. As they cool a home, air conditioners are simultaneously condensing humidity into water. Figure 4

Graphic courtesy of WeatherSpark
The dew point in Tucson Az. for the past year is shown to be the highest during monsoon season (July-August), which is also when high temperatures cause more AC condensate to form.
Stoltz said it takes a few steps to create condensate from humidity. First, the unit pulls hot outside air through a chamber and across a chilled metal pipe.

As the air cools down, it reaches dew point – the temperature at which it is saturated at 100 percent absolute humidity. This leads to condensation. In other words, water comes out of the air because it has become colder and unable to hold as much moisture as the warmer air did. After condensation occurs, the air is then heated back up to a desired temperature. 

In most homes, this condensate goes into the sewage system, drains off the roof, or is funneled out through pipes and discarded outside. Sometimes the water pulled out of the outside air drains into a pan and is left to evaporate.

“The pan catches it. There has to be a drain, and it has to go somewhere.” said Woodard. “You just need to divert it into a big bucket or cistern.”

The condensate is essentially distilled water. It is often treated as a waste product, not because it is harmful to human health but because it is corrosive. This is why the air handling units on the CAPLA building release their water into copper pipes instead of steel.

Figure 3Photo by Vanessa Lentini
Chilled water is moved through tubing such as this into the air handling unit to help cool the air for the CAPLA building.
Homeowners can collect the condensate from their air conditioners using supplies they can pick up at any hardware store. Woodard said it could be an afternoon project of rerouting the drain pipe with copper or PEX tubing, a flexible polyethylene that can replace copper.

“You do have to be careful if you put it directly on some plants,” Woodard said. “It’s so cold and has so little minerals in it, it can strip the minerals out of the soil.”

Woodard recommends mixing the condensate with rainwater, preferably in a cistern that is also collecting rainwater.

That’s what CAPLA does. Rainwater provides about 85,000 gallons a year to the cistern and pond, while the condensate yields about 95,000 gallons a year, Stoltz said.
Both Woodard and Stoltz have set up collection systems using these techniques at their homes. Woodard diverts his water into a cistern which helps to water a large mesquite tree.


Vanessa Lentini is a senior at the University of Arizona studying environmental Science. She plans to graduate in December 2013.


The College of Architecture, Planning and Landscape Architecture

Weather Spark

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