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!
You'll have to relocate the heater closer to the shower. You can have someone run the gas line to the new location. That's about all you can do.
You could insulate the hot water pipe, but that will only help when the water in the pipe is already hot, like if someone showered earlier. It only extends the time the water already in the pipe remains hot.
They could install a small - around 2 gallon - electric water heater near the shower. I have a similar problem in my kitchen and it was solved quite cheaply by putting one of those under the sink.