I found an interesting thing about tankless heaters - or at least about our Bosch tankless heater.
For the last few months, it was having problems with error codes, and would have to be reset. At about that same time, the sink in one of the bathrooms developed problems with water hammer and pulsing. Two different plumbers told me that it was a bad stem on the hot water faucet, and that it would be several hundred dollars to find and install a replacement. (It is a 50's era Crane sink)
Finally, the sink got to me, and I went out and got a new one. When I unhooked the hoses from the old sink, I decided to try an experiment, and switched hoses. When I did, the hot water was fine, and the cold water was messed up. It was a bad hose! (needless to say, I returned the sink I bought - the Crane has much more pizzazz.
Since I fixed it, the Bosch has worked flawlessly. I'm theorizing that the water hammer in the line tripped some sort of sensor in the Bosch, hence the resetting. Unfortunately, Bosch does not give you any sort of guide to what their error codes mean, or I might have been able to have figured it out sooner.