Installing hot water recirculating pump on a tankless water heater
I have added recirculating systems on a few occasions and even on a tankless. The pump , timer and cross over plumbing to use the cold water line as the recirculating line can be purchased in a kit usually under 200$ For the basic kit. With a tankless a pair of check valves are needed to create the pressure drop/ flow to fire the tankless. It can be done but in my opinion eliminates the advantage of a tankless. I usually suggest a point of use small tankless at the bathroom these smaller electric units are on demand like the main unit but will warm the water until the main supply of warm makes it Once the water makes it they can actually help to stabilize the water temp and there cost is about the same as a pump, timer and crossover kit. The difference is the point of use usually requires a 240v 30 amp circuit to be run for the unit. This would be my suggestion and it eliminates the warm water in the cold line. The advantage to the recirculating system is in cold areas with the timer adjusted properly the recirculating system keeps pipes from freezing as both the hot and cold are used.