Around early August we started having issues with the water pressure in our master bath shower. Along with that we were having intermittant hot water outages. I called our service manager who felt like it was the water saver in the shower head but arranged for a plumber to come out just to be sure. And that’s exactly what it turned out to be.
Inside the shower head is a little rubber ring that when it functions correctly is supposed to save water. It’s prone to clogging however and when it does it causes the water pressure to drop which in turn causes the water flow rate to drop to the point where the tankless hot water heat shuts off. The simplest fix is to unscrew the shower head and remove the small rubber ring inside. I didn’t get a picture of the exact one from our shower head but I found a picture via Google below.
Lastly, it’s worth mentioning that the removal of the ring results in far harder water pressure coming out of the shower head. (It’s noticable in the faucet but not nearly as much.) We were slightly concerned this would lead to a higher water bill but in the months since we really haven’t noticed much, if any, of a difference.