This paper describes the design and environmental improvements that can be achieved using drain water heat recovery systems (DWHR) to reduce the energy consumption associated with residential showering. DWHR systems transfer heat from hot drain water to the shower’s incoming cold water stream, thus reducing the demand on the hot water heater. There are various DWHR systems available that differ in heat exchanger type, cost, and performance. This article focuses on designing a flat plate and gravity fed heat exchangers for a range of residential showering conditions. This is useful since there currently is no peer-reviewed published data on the effectiveness of DWHR, nor is there published research considering the emissions reductions that can be achieved with realistic DWHR systems. The governing equations for heat exchangers are used to model empirical data and to derive implementation recommendations for DWHR design. The model is validated using a prototype flat plate heat exchanger and test stand under varying flow rates and temperatures. A Monte Carlo simulation of the results showed that DWHR could save an average $74 a year for homes with natural gas water heaters and $160 a year for homes with electric water heaters. This corresponds to 0.3 metric tons and 1.5 metric tons of CO 2 offset per home per year for natural gas and electric water heaters, respectively. The results are compiled and organized into a software program that allows consumers to input their household showering habits and location to get an estimate of their CO 2, energy, and cost savings to determine if they should install a DWHR system.