<P> The first category of furnaces are natural draft , atmospheric burner furnaces . These furnaces consisted of cast - iron or riveted - steel heat exchangers built within an outer shell of brick , masonry , or steel . The heat exchangers were vented through brick or masonry chimneys . Air circulation depended on large , upwardly pitched pipes constructed of wood or metal The pipes would channel the warm air into floor or wall vents inside the home . This method of heating worked because warm air rises . </P>
<P> The first category of furnaces are natural draft , atmospheric burner furnaces . These furnaces consisted of cast - iron or riveted - steel heat exchangers built within an outer shell of brick , masonry , or steel . The heat exchangers were vented through brick or masonry chimneys . Air circulation depended on large , upwardly pitched pipes constructed of wood or metal The pipes would channel the warm air into floor or wall vents inside the home . This method of heating worked because warm air rises . </P>
heat exchangers in gas fired furnaces are made of
heat exchangers in gas fired furnaces are made of
cast - iron or riveted - steel
cast - iron or riveted - steel