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Unlisted water heaters
shall be installed with a clearance of 12 inches (305 mm)
on all sides and rear. Combustible floors under unlisted
water heaters shall be protected in an approved manner.
[NFPA 54:10.28.2.2]
Unlisted appliances for use with undiluted
liquefied petroleum gases and installed indoors, except attended laboratory equipment, shall be equipped ...