I have a strange issue with my Rinnai RL94iN tankless water heater I had installed about 5 years ago. It runs fine, provides hot water and runs quietly - in fact we love it and have loved it for years.
But now there's this thing - it'll be working fine until I turn *OFF* the hot water. At that time I get a loud fog horn sound reverberating through the house.
To make things even more curious - it appears to coincide with the weather turning cooler - i.e., it is dead quiet all Summer. In fact, it was doing this noise for the first time last Winter and then stopped as soon as we had our first real heat spell (California central valley). I was hoping the problem just "went away" - but sadly not. Just today it's started doing it again with full vigor - interestingly, just as the temps have come down. To add to the mystery, this noise will emanate now sporadically with or without turning on the tap. In fact, since the first episode early this morning, it has blown it's horn maybe 10 times today and is driving me and my wife nuts!
Any clues? Do I need to replace it?
Thanks a lot for any insights!
Wild guess, you have not kept up with descaling and the pressure relief is releasing.