A drain water heat recovery (DWHR) device is a heat exchanger that recovers heat from warm grey water in buildings. This thesis examines the transient performance of DWHR devices through experimental and analytical investigations. Thermal performance analyses were conducted, and two transient mathematical models were developed. The first model predicts overall system transient performance based on the DWHR device's operating conditions, while the second model focuses on the DWHR device itself as a counter-flow heat exchanger, taking into account the thermal inertia of the solid material, and hot and cold fluids. A novel DWHR device was designed, manufactured, and tested identical to a conventional device. The novel device exhibited lower thermal resistances compared to the conventional one, leading to improved thermal performance. The experimental data validate both mathematical models. The models and experimental results reveal that a substantial reduction in actual effectiveness occurs during short and frequent hot water usage.