Final answer:
The minimum clearance for an unlisted water heater used to be 5 inches side-to-side and 7-8 inches deep, but recent suggestions propose as much as 8 diameters side-to-side and 15 diameters deep for the safety and functionality of the water heater.
Step-by-step explanation:
When installing an unlisted water heater, it is essential to maintain adequate clearance on all sides and the rear to ensure safety and maintenance accessibility. The older "rule of thumb" for clearance was approximately 5 inches side-to-side and 7-8 inches deep. However, newer guidelines suggest that the clearance may need to be as much as 8 diameters of the water heater side-to-side and 15 diameters deep. These measurements provide enough space for proper operation, airflow, and service technician access.