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 could look into a booster pump. It would be installed as the first thing on your incoming water supply and can boost the pressure to something reasonable inside the house.
That assumes you have good piping throughout the house. If you had old galvanized iron or something similar, the problem might just be that you need to replace your pipes.