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.
Calculate the area of rooms you need to heat by multiplying the length by the width of each room in feet, and adding the total.
Determine the number of BTU per square foot that you need depending on your local climate by consulting a heating guide such as the one listed in the resources section.
Multiply the square footage by the BTU per square foot to arrive at the total BTU rating you'll need from your Rinnai heater. All Rinnai heaters are rated for a particular number of BTU, ranging from 8,000 up to 40,000 or more, giving you a lot of flexibility in the model you can buy.