LIVE image
This online calculator allows you to calculate how much condensate can be captured from an air conditioning system. Click on image to enlarge.

Back in 2008 when I wrote a series of articles for Environmental Building News on water (all three can be accessed with this link), one of those articles, Alternative Water Sources: Supply-Side Solutions for Green Buildings, examined various ways of harvesting water and included an in-depth look at collecting air conditioner condensate.

Here's an excerpt from that article on how that condensate is generated:

Cooling systems rely on evaporator coils through which refrigerant fluid changes from liquid to vapor, cooling the coils in the process. Air blowing past the coils cools off as it goes by, and moisture from the air condenses on the coils. Condensate drains carry away the water, usually into the sewer. Instead of wasting it, more and more buildings, especially in parts of the country with hot, humid summers, are capturing that condensate for reuse. The city of San Antonio, Texas, has actively pursued this practice. It makes so much sense there because of the large cooling load and high humidity. The downtown Rivercenter Mall in San Antonio collects about 250 gallons of condensate per day, which is used to replenish the cooling tower losses, and the San Antonio Public Library collects about 1,400 gallons on condensate per day, which is used for irrigation. A six-month payback was calculated on the condensate-recovery system at the Rivercenter Mall.

When we ran that EBN article, we also provided an online calculator to assist designers or building owners estimate how much air-conditioner condensate could be recovered. Eddie Wilcut and Elliot Fry, of the San Antonio Water System (SAWS), developed the spreadsheet, and then Kelly Lucas on our staff "webified" it.

Users brought to our attention some problems with the condensate calculator, however, so we took it off our site.

I'm pleased to report that the BuildingGreen Condensate Calculator is back up, following some refinements by the SAWS team and Kelly's work to incorporate those changes into the online version.

Have a look, and take it for a test drive. Feedback will be very welcome; use the comments field.

Alex also writes the weekly blogs on Alex's Cool Product of the Week, which profiles an interesting new green building product each week, and Energy Solutions. You can sign up to receive notices of these blogs by e-mail--enter your e-mail address in the upper right corner of any blog page.

Alex is founder of BuildingGreen, LLC and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.

If you enjoyed this article, sign up for BuildingGreen email updates:



1 The calculator isn't somethin posted by Nadav on 10/03/2010 at 04:00 am

The calculator isn't something you download--it runs online. See the link towards the end of the story.

It estimates water collected off the cooling coils, regardless of what type of air handling unit they're in. An enthalpy wheel would change that calculation.

2 Appears to be a good tool. Wh posted by Bazeeth Ahamed on 09/28/2010 at 11:19 am

Appears to be a good tool. Where from I can download it? If the fresh air is treated by a centralized FAHU. Does the quantity gives condensated collected from FAHU? What about the condensate in FCUs? What if enthalpy wheels are used for heat recovery?


3 This is a very impressive too posted by Brian Koles on 09/27/2010 at 09:01 am

This is a very impressive tool. Those who find it useful may also want to check out the various ROI calculators available on the Resources page at While every project is different, and the metrics aren't perfect, they may be useful for estimating startup costs and payback for green projects. I hope this helps.

4 This is more a question than posted by Yumiko on 10/11/2010 at 01:13 pm

This is more a question than a comment: Is this tool applicable for residential use? If it is, how would I determine the indoor relative humidity? If it is not, where can I find a similar tool for residential projects?

5 RH calculator posted by Skip Steller on 07/13/2013 at 01:42 pm

I tried your new RH calculator, but the GPH condensate appeared to go DOWN, not UP, when I increased the inside RH, with all else remaining the same.  And when I  decreased the inside RH it resulted in an INCREASE GPH condensate. I could understand this if one is supposed to fill in the DESIRED inside RH, but not the actual RH.  Or, perhaps I'm just looking at something wrong.


Add comment

Login or register to post comments

— Share This Posting!

About the Authors


Recent Discussions

UltraTouch Natural Fiber Insulation
posted by hduncan
on Sep 28, 2015

From their website: UltraTouch meets the extremely stringent Environmental Specification 1350 Indoor Air Pollutant testing used for California...

UltraTouch Natural Fiber Insulation
posted by mhelou1
on Sep 23, 2015

does anyone know if the Ultra Touch batts meet CA01350 for IEQc4.6 in LEED for Schools?

Recent Comments

Getting to Know Spider Insulation

Robert Haverlock says, "Cynthia, All wood has formaldehyde. And as far as I know, Canada does not allow added formaldehyde in their building products, Roxul included. Ask..." More...

Alex Wilson says, "Cynthia, yes that's correct; there is formaldehyde in mineral wool (urea-extended phenol formaldehyde). I'm not as worried about that as many others..." More...

Cynthia Crawford says, "

Alex-Sorry- I meant to say sprayed- fiberglass. Thank you for your answer in any case, for both foam and fiberglass. It's the first time I've...

" More...

Alex Wilson says, "Cynthia, Spider is a spray-fiberglass product, not a spray-foam, but neither material could be considered rodent-proof. In our home, rodent entry at..." More...

Cynthia Crawford says, "Since sprayed foam doesn't require boric acid, how does it prevent rodents from entering a building or burrowing into the fiberglass? They certainly..." More...