Instant Hot Water Recirculating System Sensor Valve Kit. That hot water inlet (Navien fluff pic) is for the dedicated recirculation line. Since the Navien has a built-in recirculation pump, the valve kits work perfectly. So, my longest runs (master bath and kids bath) had the kits installed. Since now I know what it is, I might install another at the kitchen sink. It's the longest wait time of 15s.