Grand Hall. Both the Eternal GU20 and Eternal GU26 hybrid water heater models meet required standards for federal tax credits. The Eternal hybrid water heating system combines benefits of tank and tankless systems by utilizing thermal activation (similar to a tank) and flow activation (similar to a tankless water heater). Using a patented counter-flow design and multipass heat-exchanger technology, Eternal water heaters are measured to have 86% thermal efficiency with constant pressure. The units are PVC-vented for easy installation. 866.946.1096.