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!
They make instant recirculation pumps that cycle the water through the tankless in a loop and stop when the hot water gets back to the pump. We have one. If you don't already have the return pipes for it, you'd need some additional plumbing work. (Either way, it should be professionally installed).
It's the same system you'd have for a tank, but it can't run all the time, or it burns up the heater. You have to trigger it. They make flow switches, but mine (new construction) was cheesed. I just set up a zigbee switch with a 1 minute timer to trigger the pump.