Hot water distribution efficiency is now included in HERS ratings, under an amendment to ANSI/RESNET Standard 301-2014. This is a Good Thing. To date, all of our efforts to represent, and then minimize, hot water usage have been skewed by distribution systems (ie, by the vagaries of plumbing). According to David Butler (Optimal Building Systems LLC), the amendment not only incorporates distribution efficiency metrics, but also makes signification revisions to basic DHW efficiency calculations (consumption and waste rates, inlet/mains water temperature). Here's the amendment in full.
In some discussion groups, there's been a few posts about drainwater heat recovery (DWHR) units. I like these beasties. They are passive collectors of waste heat from the drainwater from showers. With the right plumbing set up, they can be very effective. They became less attractive in the last few years with the price of copper going up, but when operating costs don't go down (and where inexpensive natural gas isn't the go-to source for hot water), the case can still be made for using DWHR units. This is especially true in new houses in cold climates, where the average inlet or mains water temperature hovers around 44 - 70°F (with most of the year staying on the nasty cold side of that range).
I wrote up a Research Highlight on DWHR units for Canada Mortgage and Housing Corp (CMHC) a few years ago based on two studies run in the Canadian Construction Test Houses in Ottawa. It's based on cold-climate housing and cold water feeds (2005 study range 49-67°F, 2006 study regulated at 46°F), so not applicable further south, but interesting read on the performance levels of the various configurations. Here's the pdf link.
And here's a groovy little calculator based on the findings from those studies (and others). Dial in the water usage, temp settings, plumbing configurations, fuel type and price, DWHR type and size. Again, only in Canada, I'm afraid ;-).
Add to this calculator some ways of achieving efficiency in hot water distribution (ie, best practices for plumbing runs in general, requiring DWHR units to be tied into one vertical drain for all the showers in the house, take toilets and other all-cold waste water producers off that shower drain) and you may have a strong case for improved water efficiency.
The problem: DWHR savings are very hard to map. In part, because occupant behavior changes. When there is more hot water, more hot water gets used. I've not seen a conclusive behavioral study, but have lots of anecdotal evidence. Can't waste that recovered hot water! Just like with solar thermal -- it looks like I get whopping savings out of my 2 panel system (especially in the summer), but I also know we use way more hot water in the summer and should seasons now that we have the solar thermal system because otherwise we're wasting all that lovely 'free' heat. It shouldn't come as a particular surprise, give then number of articles and papers documenting bigger footprint houses fitted with any number of gadgets and electronics leading to occupant behavior that trumps any energy savings improvements in the building envelope and mechanical systems.