Yes. It is best that the preheat system has a mixing valve to eliminate temperature fluctuations and insure that the water going into the Noritz heater does not exceed 120°F.
Click Here for an example plumbing diagram showing solar preheat. The model shown in the diagram is just an example as all the plumbing outside of the unit will be the same regardless of the tankless unit used.
Please be aware: For residential models the heat exchanger warranty is reduced to 3 years when used with pre-heated water. See warranty terms here:
http://www.noritz.com/about-noritz/warranty/
Applicable Models
- Models: ALL MODELS