About 2 years ago I had a Bosch Model 2400 E NG tankless water heater installed by a contractor in a renovated and expanded older wood frame home. Since the beginning it has been impossible to adjust the water temperature in the sinks: water eventually gets very hot; any attempt to adjust the temp. causes the water to run very cold; no matter what is tried, the only way to get hot water again is to turn everything off and start over. Up until recently the water temperature in the shower has been fine. However, the shower now acts just as the sinks do; some days it is almost impossible to use - you are either scalded or frozen. Also, recently I began to hear a "singing" noise when I am using the hot water. I found out that when this is happening there is a noise, rather high-pitched coming from the vent - outside of the house the noise is really LOUD; the neighbors have noticed! Given that this unit is powered with gas, the inconvenicence has now become a real worry. I seem to live in an area where not many people know about these units. Any suggestions?!