I had a Generation 1 tankless water heater from Girard and had problems with it for 2 years. In Girards defense they tried to make it work, but never any such luck. I finally contacted them again recently and informed them I was still having issues with the unit and they decided to replace it with a Generation 2 unit.
I wish the gen 1 had worked as well as the gen 2. I have used it on both city water and on tank and have had no problems. The temperature control has been great and with the gen 2 you can now use cold water blending if temp is to hot. Since it has been hot here I have turned down the temp control some to get cooler water. I was running at the halfway point. I imagine as temps cool down I will have to start adjusting upwards again, but I don't think there will be any problem keeping temperature hot enough. But, I live in Southern California.
I applaud Girard for standing behind there product and doing the right thing.