There are specially made versions of on demand hot water heaters for this purpose which will adjust the amount of heat provided to the water based upon its input temperature. I suspect that in most locations these will be more of a specialty item and will not be carried in the general stock of a diy center, but maybe in some of the more sunny areas. If you use one that does not have this feature, you may put out extremely hot water capable of scalding and burning users, or trip the over temperature sensor on the unit, shutting it off, or open up the over tempurature/pressure relief valve (or if both of those were to fail blow the thing up) Most basic units, like the Bosch 125NG units I have been using, provide the same amount of heat regardless of the input water temperature. I had one where the valve that turns on the heater based upon the flow rate got stuck. The unit turned off based upon the over temperature, and cleaning the unit was pretty straight forward. Rich