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!
If wiring and plumbing allow, install another tankless heater closer to the shower. I just put one into my one bedroom apartment and it was reasonably simple and small.
Unfortunately, the shower has no exposed plumbing and the gas line is back near the ground floor heater.
I don't think I've ever seen a shower in the US (assuming you're in the US) without access to the plumbing somewhere, I'm sure they probably exist somewhere, but it's not common in my experience, usually there's a little access panel on the other side of the wall somewhere, maybe hidden in a closet or behind a piece of furniture or something. If there's not, I'd consider adding one anyway, at some point if you're there long enough you're probably going to want access to it for some issue or project that comes up down the line.
They make small tankless electric water heaters that run off of regular 110V outlets for heating a single sink or shower, if there's convenient electrical nearby you may be able to just hook one of those up.
Living in the US I've never seen what you're describing
What part? Most houses I've seen in the Northeast either have some kind of access panel, or the shower is backed by a closet in another room where you could cut in if you needed to.
I'm in Jersey and I do not have that on my home. The only home I can think of that had it was my childhood home. Current home is 1920s, childhood home was probably 50s, teen home was 80s, and lived in various other places after that, and really never saw the access panel, though I agree it makes sense to have.
Form over function, can't have people judging me for my prudence.