Guidance on proper way to caluclate domestic hot water usage

There is a calculator for this within the new Table 1.4 (Dec 2012) spreadsheet you can download from LEED Online or the USGBC web site - http://www.usgbc.org/resources/eap2-section-14-tables-new-all-bdampc-projects-regardless-registration-date Some of the assumptions used typically come from the ASHRAE HVAC Applications Handbook. It looks like Chapter 50 in the 2011 version. There is also an example in the Advanced Energy Modeling Guide for LEED but it contains an error (do not assume 100% hot water for most applications). Also make sure that your assumptions are in alignment with the calculations for WEp1 as the reviewer check that for consistency.

Sounds like you need the peak hot water demand for the project? When modeling you typically enter the peak demand, the hot water equipment and then schedule it. From those values comes the annual hot water energy usage. The annual water usage is something different and is estimated by the calculations for WEp1. For those fixtures using hot water a common assumption it that 50% of the total water use is hot water but this can vary depending on the system. This may not be of much help but as energy modelers for the project we will request the peak hot water demand from the project's plumbing engineer. They have to calculate it to size the equipment. Even if you calculate it yourself it must be coordinated to make sure it reflects the actual design value. Some guidance is in Chapter 50 that I referenced above. I am sure ASPE has some documents that outline the calculations - http://aspe.org/ Many engineers use rules of thumb and sizing charts provided by the hot water heating equipment manufacturers. The Advanced Energy Modeling Guide does not show water usage calculations.