.Langley, B. C. (1991). High-Efficiency gas furnace troubleshooting handbook. Prentice-hall, Inc.
Cita Chicago (17th ed.).Langley, Billy C. High-Efficiency Gas Furnace Troubleshooting Handbook. New jersey: Prentice-hall, Inc, 1991.
Cita MLA (9th ed.).Langley, Billy C. High-Efficiency Gas Furnace Troubleshooting Handbook. Prentice-hall, Inc, 1991.
Atenció: Aquestes cites poden no estar 100% correctes.