It takes a few minutes for my tankless water heater to warm up, so we end up wasting a lot of water in our shower. Is there a way to avoid this? A friend mentioned a “comfort valve” or something? What is it and how does it work? Or is there another solution? Thx!
Look into retro fitting recirculation pumps. You don't necessarily need access to the plumbing behind a wall and can add a loop under a sink in your bathroom. Ask This Old House has quite a few videos on the subject: https://youtu.be/KdA_gfau1s4