Supplementary Boiler : A boiler designed to burn one type of fuel (gas, oil, or solid) that is intended for supplementing a boiler burning another type of fuel (gas, oil, or solid) by means of a common heat transfer medium
NFPA 97 Standard Glossary of Terms Relating to Chimneys, Vents, and Heat-Producing Appliances, 2003 ed. http://travelwidpinx.info/nfpa-97-standard-glossar y-of-terms-relating-to-chimneys-vents-and-heat-pro ducing-appliances-2003-edition.html, (Std: Glossary of Terms Relating to Chimneys, Vent, and Heat-Producing Appl - Amer) {241 NFC}