Gecko 3-73-5000 in.yj2 Heat Recovery System without Heater. Gecko 3-73-5000 Heat Recovery System without Heater. Gecko’s new Y series boasts impressive features and the most advanced technology in convivial, reliable, safe, and long-lasting control systems that fit all spa models and benefit all spa users. Available in 3 versions, the control systems of Gecko’s new Y series are adaptive, optimized, and expandable making them the perfect choice to power any model of spa. Our new in.yj is the perfect system for any small footprint spa.