Most residential gas hot water furnaces range from 30 to 50 gallons. Smaller capacity furnaces are available for garages or workshops; such units often accommodate 2.5 to 19 gallons.
Homeowners can select the appropriate size furnace based on the size of their household. Thirty gallons is sufficient for one to two person households, while a 40-gallon furnace accommodates homes of two to four persons. A 50-gallon furnace is recommended for households of five or more people.
Another option for gas-fueled hot water furnace is a tankless heater. A tankless unit can provide about 3.5 gallons of heated water per minute. Such units are more energy efficient than the more common tank furnaces, but may not be practical for larger households.