secondary fuel type

(1) The source of energy for space heating to supplement that provided by the central heating system of a household residence.

(2) If the residence is centrally heated by a form of energy other than that set forth above, the source of energy that is needed to operate the central heating system. (This includes water for steam-heating systems, such as radiators.)

(3) If the residence is not centrally heated, a source of energy that is used for home heat to a lesser degree than the main fuel type.