Instantaneous or Storage Hot Water Heaters, Nonelectric (N.E.S.)

The instantaneous gas water heater heading covers tankless units specifically fueled by gas. This residual heading captures nonelectric water heaters of other fuel types (oil, solar, biomass) and storage-type configurations not classified elsewhere, making the fuel source and tank configuration the primary classification differentiators.

Solar thermal storage tanks that function as hot water heaters — storing water heated by solar collectors — generally fall under this residual nonelectric water heater heading. However, bare storage tanks without heating elements may be classified as pressure vessels or tanks depending on their construction, so the functional role of the unit at import is the deciding factor.