Yes, you can use the Noritz tankless water heaters for domestic hot water in conjunction with a heating system but not solely for a heating system (closed-loop1 system).
See installation manual located at http://support.noritz.com/technical-literature.php
and plumbing diagrams located at http://www.noritz.com/resource-center/manual-documents/
or call technical support at 866-766-7489.
We also manufacture a number of Combination Units:
NRCB180-DV, NRCB199-DV, CB180-DV and CB199-DV.
Water contained in the system is reused over and over again. Water contained in the system is non-potable
- Models: ALL MODELS