The industries leading oil-fired water heaters with capacities of 20 to 271 gallons and BTU/HR ratings of 60,000 to 1,500,000. Oil Fired Water Heaters 60,000-173,000 BTU. Oil-Fired Residential and Light-Commercial Water Heaters. Bock water heaters are the USA’s most popular oil-fired residential and commercial oil-fired appliances. Capacities range from 20 gallons to 113 gallons with inputs from 60,000 to 173,000 BTU.