Recently, Ryan H commented on this project. He asked why not simply use a hot water recirculation system. As I had never heard of such a thing, I started doing a little research. These systems are basically comprised of a pump (installed at the water heater) and a loop/check valve installed between the hot and cold supply lines at the furthest faucet. Multiple loops and valves are needed for each branch in the plumbing. The pump forces the water through the hot water supply line and back into the heater tank through the cold water side. The user gets instant hot water. No wait time, no water waste. So at first glance, it appears that this system does the things I was trying to do, and probably better.
I was seriously considering stopping the project, but as I thought about recirc systems, a thought occured to me. I takes a lot of energy to heat water. These systems have to reheat the water everytime they cycle water back into the tank. After some more research, it turns out that the better recirc systems have built in timers. This way, the user can set the times that the pump operates. This helps a lot, but can we do it better? What has a larger environmental impact: reheating the water, or just pouring it down the drain? How do you compare the impact of water vs power?
I found this really good resource about the carbon footprint of water: http://www.csu.edu/cerc/researchreports/documents/CarbonFootprintofWater-RiverNetwork-2009.pdf
It's worth the read. One interesting piece of information was that the average power to treat and pump water to a residence ranges from 1050-36200 Kwh/MG. With common values ranging from 1250-6500 Kwh/MG. In my area, we use ground water, and since I have a septic system, my water energy cost (not including heating) is probably around 2000 Kwh/MG. The average power used for heating water (for a shower specifically) is 148,832 Kwh/MG. That's 74.4 times more power to heat the water than to treat it and pump it to my house. Another staggering number is that 58% of all water related carbon emmisions come from residential water heating.
I'm sure someone could make the case that if you set the timers perfectly on a recirc system, and showered at exactly the same time everyday, you would not waste very much energy. My reality is that I have four young kids. The messes they make are rarely on any kind of schedule, and impromtu showers/baths are a common occurance. If I used a recirc system, I would probably want to have it on at least 12 hours a day. That is a lot of wasted power. Maybe someone could figure out a way to only turn on the recirc pump whenever hot water is needed. I can't figure out a way that would be cheap or practical enough (maybe an app where you could manually activate the pump a few minutes before using hot water?)
Taking all this into consideration, I think my idea still has merit. I think I can save water and tip the carbon scales in the right direction.