Tankless water heater - one shower got cold
A few years ago we installed a tankless hot water heater and we love it. However, we have had several house guests get ice cold showers, even after tutorials on how to adjust the temp. Several claim they didn't even touch the temp., and that the shower started out hot, then turned ice cold after about 10 min. and never got hot again. This happened to at least 4 different people. What could be causing this problem? It's embarrassing to have this happen to guests!