Rinnai heaters are manufactured by the Rinnai Corp. of Japan, and are made in a variety of sizes to suit heating needs large and small. Rinnai makes heaters that operate on either electricity or natural gas, and when sold in the US market, their heating capacity is measured in British Thermal Units, the standard unit of measure for heat output in home heating products. The heating capacity of a Rinnai heater determines the size room it can effectively heat.