Circulo: Saving Energy with Just-In-Time Hot Water Recirculation

The average home in the US flushes 1000's of gallons of water down the drain each year while standing at the fixture and waiting for hot water. Some households use a pump for hot water recirculation (HWR) to ensure that hot water is always immediately available, but these systems can incur more than $1000 per year in energy costs. In this paper, we explore the hypothesis that "just-in-time" HWR can reduce the energy footprint of HWR systems, without increasing hardware or installation costs and without increasing water waste or human annoyance. Our basic approach is to learn patterns of hot water usage in the home and to circulate hot water only when future hot water usage is highly likely. We call this approach Circulo. We evaluate Circulo by analyzing hot water usage patterns from 5 different homes over a period of 7--10 days each. Our results indicate that Circulo can reduce the energy needed for HWR by 30% while still providing households with hot water over 90% of the time.