Rain Water Tank Base Building

Hi Luis,  Can you ask the base building for the documentation they provided as part of their CS certification? Having that information as supporting documentation for your project should help tremendously, and should also give you insight as to how to make the calculations specific to your space.  I would advise that once you think you have determined a way to calculate your project's savings, you can contact GBCI and ask if your calculation method is appropriate. http://www.gbci.org/contact 
They usually cannot confirm if your calculations will be accepted (for that, you would likely need to submit a CIR), but they should be able to advise if you're going about the calculations in the correct manner. Hope that helps.

Ok, it's unfortunate that they're not more helpful. In the FAQ section for this credit, there are some general guidelines in how to go about calculating the system. It states the following: "Once the annual design case water is calculated, nonpotable water can be subtracted to determine total water savings." That said, I would probably fill out the water use calculator provided for the credit as a normal project. Once completed, I would take the annual total from the design case (what you're installing), and subtract out the water used for the flush fixtures only that is shown in your calculator. Do this in a separate document that also includes a narrative describing what you did. The end total after you subtract the flush fixture water should give you the new design case total you should use for your calculation. You can then use that number to compare to your baseline water use number shown in the calculator, and determine your percentage of savings.

Hi Luis - I have a similar project. Were you able to achieve the additional reduction? If so, how did you show the calculations for alternative water use?