Capturing condensate by retrofitting AHUs.
Water reuse (Methods)
Air conditioning from central stations (Models)
Lawrence, Tom
Perry, Jason
Dempsey, Peter
Pub Date:
Name: ASHRAE Journal Publisher: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. Audience: Academic Format: Magazine/Journal Subject: Construction and materials industries Copyright: COPYRIGHT 2010 American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. ISSN: 0001-2491
Date: Jan, 2010 Source Volume: 52 Source Issue: 1
Product Code: 3585120 Central Station Air Conditioners NAICS Code: 333415 Air-Conditioning and Warm Air Heating Equipment and Commercial and Industrial Refrigeration Equipment Manufacturing SIC Code: 3585 Refrigeration and heating equipment
Organization: University of Georgia
Geographic Scope: United States Geographic Code: 1USA United States

Accession Number:
Full Text:
A discrepancy seems to exist between the cost of water to the average consumer and the actual value of that water to the society and environment. Because it is such a vital component to life and functions in society, municipal water systems strive to provide water to customers at the lowest costs. This natural resource is available for such a low cost that we purify the water to make it suitable for drinking, pump it through distribution networks over considerable distances, and then flush it down the toilet.

Guz (1) in 2005 brought attention to successful condensate collection in San Antonio that led to requirements for new construction to incorporate condensate collection. That article provided suggestions for how to build condensate collection into new project designs. This article focuses on the more difficult task of retrofitting condensate collection into existing buildings. Since existing buildings comprise approximately 98% of the building stock (the other 2% being new construction), they represent a significantly greater immediate benefit to society in terms of energy or water consumption savings. In the right situation, retrofitting existing buildings can potentially provide significant water consumption savings.

ASHRAE has recognized the importance of water to society and the consumption of water done by buildings and building systems. ASHRAE Standard 189.1-2009, Standard for the Design of High Performance Green Buildings Except Low-Rise Residential Buildings, and proposed Standard 191, Standard for the Efficient Use of Water in Building, Site and Mechanical Systems, provide requirements for water-using systems and condensate collection.

Some consider water concerns to be as important as energy. For example, in an article published in 2009 concerning a survey by McGraw-Hill, 69% of those surveyed indicated that water was a critical issue for a sustainable building and that number is anticipated to rise to 85% by 2013. (2)

It takes energy to produce potable water, and water is used to produce energy. For example, in California it has been estimated that water-related energy use consumes 19% of the total electricity, 30% of the natural gas and 88 billion gallons (333 billion L) of diesel fuel annually. (3) While that may be an extreme, we don't see the hidden cost in terms of energy needed to obtain, cleanse and distribute water. Nationwide, the number is about 4% of total electrical consumption, (4) with some estimates higher. Conversely, we often forget that a large amount of water is used generating electricity in thermoelectric and hydroelectric power plants (some proportion is lost to evaporation in the process while some is lost due to additional evaporation due to higher downstream temperatures in the waterways). Studies are being done on this issue. (5)

Condensate Collection

Factors that determine whether condensate collection should be considered include location (climate), building type (particularly relating to amount of outdoor air required), the size, number, and accessibility of air handlers that condition outdoor air, location of potential uses for the condensate, etc. Location determines the potential to collect a significant amount of condensate, as well as the value of the water to the local community. In periods of drought, the actual value of a unit of water to the local society and economy may be worth more than the rate paid to the local utility. A similar situation can occur in areas where water cost is artificially kept low through subsidies or other means. As water demand continues to grow due to population pressures and lifestyle changes, even regions that might have been considered moist or water rich will experience tight supplies of fresh, clean water.


Building or space occupancy type determines the amount of outdoor air required and the amount of moisture in the incoming air. As the air passes across cold cooling coils, if the coil surface temperature is less than the dew point of the airstream passing by, then the potential exists for water condensation on the coils. (See sidebar on How Much Water Will Collect at Design Conditions?) Water that condenses on the coils collects and drops to the drain pan below. The actual amount of water collected depends on parameters such as the absolute humidity level, total airflow and coil bypass ratio. A major source of this condensed moisture is from outdoor air brought in through outdoor air intakes or through infiltration exchange with the outdoors (other sources include the people in the building, cooking, etc.).

A building or space that requires a large quantity of outdoor air on an ongoing basis (e.g., laboratories) is suitable for condensate collection. Other obvious candidates include spaces with indoor water features, natatoriums, gymnasiums and shower rooms, although these may face special challenges. Any building that has a cooling tower located on site is another prime candidate as this offers an easy means for reuse of the collected water. Dedicated outdoor air systems are another candidate.

The amount of water collected at cooling design condition may not fully represent the total amount of condensate collected over the course of a year. This depends on the range of conditions that exist throughout the course of the year. Similar calculations could be done for any location using a more detailed, hour-by-hour analysis. We have taken hourly weather data (6) and constructed a spreadsheet for doing such analysis on our university campus buildings for use when evaluating a situation for potential retrofit. The spreadsheet estimate is based on the amount of outdoor air conditioned by a particular air-handling unit, and can be adjusted to account for situations when economizing cooling is expected, as well as variable outdoor air requirements, and operation schedules, etc.

The simple spreadsheet model predicts condensate to occur when the outdoor air absolute humidity levels are above those for the assumed supply air conditions. Based on typical supply air conditions, this spreadsheet model estimates approximately 12.5 gallons per cfm (100 L per L/s) of outdoor air if supplied around the clock throughout the entire year for a unit located in Athens, Ga.

We designed and installed a condensate collection system on an AHU that conditions 100% outdoor air and supplies one portion of a research laboratory building. Although there is a variable-speed drive installed, the actual fan speed varies by only [+ or -]5%, with a typical outdoor air intake of about 19,400 cfm (9150 L/s). Since this unit conditions 100% outdoor air and runs 24/7/365, there is no need to make adjustments in the condensate collection predictions for items such as economizer operation, unit operating schedule or adjustment of outdoor ventilation air through demand-controlled ventilation. Based on typical supply air conditions, an estimated 240,000 gallons (908 500 L) of annual condensate could theoretically be collected for this unit.

We collected daily condensate production data from this air-handling unit during the 2009 cooling season while using dataloggers to monitor fan speed, as well as temperature and humidity of the outdoor and supply air. We will use this to help validate and refine the model for future use and application to other climate regions. How detailed a model and prediction of water collected is needed depends on the situation. Certainly, if a project requires justification strictly based on short-term economic analysis, then an accurate prediction is probably necessary. In other cases, such as with our university during a time of extreme drought, it is sufficient to know within a general range what the collected water amount would be.

Predicted Annual Collection At Five Locations

The same spreadsheet model was modified to estimate the condensate collected at the other representative cities:

* Athens, Ga. =12.5 gallons per cfm or 100.3 L/(L x [s.sup.-1]) of continuous annual outdoor airflow

* Houston = 22.4 gallons per cfm or 180.5 L/(L x [s.sup.-1]) of continuous annual outdoor airflow

* Boston = 4.5 gallons per cfm or 35.7 L/(L x [s.sup.-1]) of continuous annual outdoor airflow

* Sacramento, Calif. = 1.3 gallons per cfm 10.4 L/(L x [s.sup.-1]) of continuous annual outdoor airflow

* Denver = 0.5 gallons per cfm or 4 L/(L x [s.sup.-1]) of continuous annual outdoor airflow


The total annual rainfall is only a partial indicator of how much condensate might be collected, as shown in Table 1. Some U.S. cities have similar rainfall totals, but vary in the total amount of condensate collection potential.

What to Do With Collected Water

The best end use for the collected water depends on the particular circumstances of the location. Drought in Athens, Ga., led to the realization that the primary use of city water at the University of Georgia (UGA) is for makeup water in campus cooling towers. In this situation, the most logical choice was to route a building's collected condensate to its cooling tower sump. In most cases, peak condensate production will occur at the same times as peak makeup water demands, creating an elegant feedback loop. This is also the simplest retrofit, involving reasonably inexpensive equipment and piping. Water can be routed directly to the tower with no need for treatment.

Complications arise when dealing with district cooling systems with satellite chillers, because it is possible to produce condensate in an air-handling unit while the chiller and cooling tower for that particular building are idle. While it is no tragedy that condensate sent to the cooling tower will simply overflow to the sewer, where it would have gone prior to retrofit, there is the risk that treatment chemicals in the sump will be diluted and needlessly washed away. In this scenario care should be taken to prioritize condensate retrofits in buildings with baseline chiller plants.

Condensate collection also can be integrated into a rainwater collection system, a scheme sometimes referred to as "rainwater plus." This will usually involve a storage tank or cistern, and can require considerably more expense and engineering than using the condensate in a cooling tower. Depending on the intended use, such as irrigation, fountains, toilet flushing, or potable water, different amounts of further treatment will be required. In all cases, local building codes must be followed.

Condensate Collection Retrofit Program

The second severe drought in 10 years brought down water levels in local lakes and rivers to critical levels. The university and the greater Athens community worked together to prevent water shortages by imposing watering bans and shutting off fountains. However, it didn't take long to realize that better, long-term solutions were needed.

The Paul D. Coverdell Center for Biomedical and Health Sciences was constructed in 2006 on the UGA campus. It represented the first large-scale implementation of water conservation practices in a newly constructed building at the university. The design included condensate and rainwater collection systems, as well as sumps around the perimeter of the building. The successful implementation of this project provided some direction for retrofitting existing buildings on campus.

The UGA Main Library was the prototype building for the retrofit effort. The library has two penthouses, each with its own AHU, on either side of a central, roof-mounted cooling tower. For each unit, the original drain pipe from the cooling coil's condensate collection pan ran to a floor drain (in this case the AHUs are so large that the floor drain is inside the unit), which ultimately runs to the city sewer system. A sump basin was installed so that the drain pipe runs into it, with an emergency overflow completing the original pipe route to the floor drain. A sump pump with a diaphragm-type float switch pumps the condensate out of the basin and up about 8 ft (2 m), where it then runs horizontally through the exterior wall and drains into the cooling tower sump. The prototype was successful with one major lesson learned: the water meter for recording the total amount of collected condensate should be installed either in the vertical section of pipe above the pump, or in a U-trap in the horizontal run, because, otherwise, the pipe is never totally full of water, and an impeller-type meter could misread the flow volume.


With the knowledge and confidence gained from the project at the Main Library, UGA was able to move forward and expand the retrofit program into other buildings on campus. The focus was on buildings with laboratories and buildings that operated on a 24/7 schedule due to the high condensation potential mentioned previously. Highly visible facilities on campus also were targeted for campus education efforts, such as the Miller Student Learning Center and a new building for the Lamar Dodd School of Art.

The basic system design has not changed much since the prototype, but we have continued to evolve the design with improvements such as a cover on the basin to prevent debris accumulation and insulation on the basin and on indoor pipe runs to prevent sweating. In most applications, so far at UGA, the condensate is ultimately delivered to a cooling tower sump or a collection cistern that is used for cooling tower makeup water or building internal use. However, one system has been built and two more have been designed in which the condensate will be routed to rainwater collection cisterns for irrigation purposes.

Cost Savings Analysis

As discussed earlier, dire conditions may lead to less emphasis on economic payback when it comes to water. However, it is possible to demonstrate good economic performance with conden sate collection. A system designed and installed in UGA's College of Veterinary Medicine is predicted to produce 450,000 gallons (1.7 million L) of condensate per year. Using the combined supply and sewer utility rate from the local utility of $0.0075/gallon ($0.002/L) as the cost of water, installing this system will save UGA $3,375 per year. The system cost was $3,200 installed, and the simple payback is expected to be less than a year. This admittedly was one of the more favorable installations we have done so far, but it indicates the potential that exists.

Another AHU retrofit that we did during the winter of 2008-09 was monitored daily for condensate collection through the 2009 cooling season. The installation had a similar installed total cost of approximately $3,000 and about 200,000 gallons (750 000 L) were collected during the 2009 cooling season. This translates to $ 1,500 in water savings and a simple payback period of two years. Drought conditions that existed during June through August probably reduced the amount of condensate for this cooling season, possibly making this retrofit even more economically viable.

Lessons Learned

The near future may bring a surge of interest in retrofitting buildings with condensate collection systems, particularly in the eastern United States where water concerns continue to build. We are confident and hopeful that these type projects will continue to become more effective as more is learned about the possibilities. For our part in furthering the development, we'd like to share some of the lessons we learned and suggestions for the future.

A significant concern in these projects is the cleanliness of the water and the system components. The external collection pan should be covered to prevent any foreign particles from getting into the system. In these pans, as with condensate pans within air handlers, there may be a potential for biological growth and contamination. Also, there may be an increase in corrosion potential in the cooling tower loop, if that is where the condensate is sent. We are closely monitoring the particular cooling towers where this is being done to verify our thinking that this is not a major concern.

Sweating on the outside of the condensate piping can be an issue, particularly in semi-conditioned mechanical rooms, so all lines (as well as the collection basin itself) should be insulated. If the condensate line is tied into rain downspouts, sweating on the downspouts can cause potential problems such as discoloration and damage to the exterior of the building. One possible solution is to run a smaller pipe or tube inside of the downspout.

The dimensions of the U-trap in the existing condensate drain pipe between the AHU and the floor drain should be maintained when connecting the drain pipe to the external collection pan. It is also highly recommended that a meter be installed, and that it be located to facilitate easy reading. The additional cost of the meter is worthwhile because of the feedback on functionality and the education potential it provides, as well as a means to verify water and cost savings.

We recommend also that future study be done on improving methods to predict the amount of condensate collected, as well as the potential impacts that might exist with routing this water to cooling tower condenser water systems.



As the value to society of fresh water supply is recognized, the role of condensate collection in new designs, as well as retrofitting existing buildings will continue to grow. The experience we have seen with several cases of air-handling unit retrofits within our university campus indicates that these retrofits can be done with manageable up-front costs and favorable simple payback periods.


(1.) Guz, K. 2005. "Condensate water recovery." ASHRAE Journal 47(6):54-56.

(2.) Greener Buildings. 2009. "Water to Rival Energy Efficiency as Key Concern in Greening Buildings: Report." www. green erbuil dings. com/news/2009/06/04/water-efficiency.

(3.) California Energy Commission, 2005. "California's Water-Energy Relationship." Final Staff Report CEC-700-2005-011-SF.

(4.) EPRI. 2002. "Water & Sustainability (Volume 4): U.S. Electricity Consumption for Water Supply & Treatment--The Next Half Century." Electric Power Research Institute Report 1006787.

(5.) Torcellini, P.A., M. Long, and R. Judkoff. 2004. "Consumptive water use for U.S. power production." ASHRAE Transactions 110(1):96-100.

(6.) National Solar Radiation Data Base, 1961-1990. Typical Meteorological Year 2, National Renewable Energy Laboratory,

By Tom Lawrence, Ph.D., P.E., Member ASHRAE; Jason Perry, Associate Member ASHRAE; Peter Dempsey, Student Member ASHRAE

About the Authors

Tom Lawrence, Ph.D., P.E., is public service associate, Faculty of Engineering, Jason Perry is a research engineer at Engineering Outreach Services, and Peter Dempsey is a student at the University of Georgia in Athens, Ga.
How Much Water Collects At Design Conditions?


For simplicity, consider the process of a unit conditioning 100%
outdoor air (such as with a dedicated outdoor air system or DOAS).
The psychrometric chart shown in Figure 1 represents a path of
outdoor air as it passes across the cooling coil for the 0.4%
cooling design condition in Athens, Ga. Assuming a supply air
condition of 55[degrees]F (12.8[degrees]C) and 85% relative
humidity (wet bulb temperature = 52.5[degrees]F [1 1.4[degrees]C]),
the humidity ratio changes across the coil from 0.0141 to 0.0078
[lb.sub.water]/[lb.sub.dryair] (kg/[kg.sub.air]). The difference in
absolute humidity ([omega]) between the incoming outdoor air and
supply air leaving the unit represents the amount of condensation
that occurs. Thus, for every pound of air supplied by the unit,
0.0141-0.0078 or 0.0063 lbs (kg) of water are condensed.

The total amount of condensate expected is determined by the
equation below:

condensate collected = airflow x density x 60 min/h x

Assuming 1,000 cfm (472 L/s) of outdoor air is being conditioned,
the total amount of condensate expected would be:

condensate = 1,000 [ft.sup.3]/min x lb/13.133[ft.sup.3] x 60 min/h

x (0.0141- 0.0078) [lb.sub.water]/ [lb.sub.dry air] = 28.8 lb/h
(13.1 kg/h)

This is approximately 3.5 gallons (13.1 L) per hour at the cooling
design condition.

Similar calculations can be run for any locality, and the result
can vary widely depending on the climate. For example, when the
calculation is run for other representative cities that range from
hot and humid to dry climate types, the condensate yields are:

* Houston (95.1 [degrees]F [33[degrees]C] dry bulb/77.6[degrees]F
[25[degrees]C] MCWB)

= 7.0 gallons (26.5 L) per hour

* Boston (90.8[degrees]F [33[degrees]C] dry bulb/73.3[degrees]F
[23[degrees]C] MCWB)

= 3.2 gallons (12 L) per hour

* Sacramento, Calif. (I00.4[degrees]F [38[degrees]C] dry

[22[degrees]C] MCWB) = 0.8 gallons (3 L) per hour

* Denver (94.3[degrees]F [35[degrees]C] dry bulb/60.3[degrees]F
[16[degrees]C] MCWB)

= no condensate collected at design cooling load

Table 1: Annual condensate collection compared to total annual

                            Annual Condensate           Average Annual
                     For 1 cfm of Continuous Outdoor       Rainfall
                     Air gal/cfm [L/(L x [s.sup.-1])]      in. [m]

Athens, Ga.                    12.5 (100.3)              47.8 (1.21)
Houston                        22.4 (189.5)              54.0 (1.37)
Boston                          4.5 (35.7)               42.5 (1.08)
Sacramento, Calif.              1.3 (10.4)               17.9 (0.45)
Denver                          0.5 (4)                  15.4 (0.39)
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