In certain cases, two gas furnaces on the same floor within a house can share a common flue, as can a gas furnace and a gas hot water heater on the same level. Some wood furnaces are designed to share a flue with an oil furnace, if at the same floor level. Some flues are unlined in that there is masonry exposed on the inside of the flue. Unlined chimney flues are most common in houses built before the Second World War. These unlined masonry flues have performed reasonably well for fireplaces and oil-fired furnaces, however most have deteriorated beyond a safe condition.
Gas appliances require a suitable liner.
Flue can be lined with clay tiles or metal liner. Skylights Skylights are windows installed into a roof, where the ceiling is vaulted with the roof, or where a well has been constructed thru the attic area, to provide a passage from the ceiling to the roof window. Ideally, skylights should be installed on curbs, projecting at least 6 inches above the roof surface, in order that proper flashings can be applied. Some skylights are manufactured with built-in curb and flashing assemblies. Sometimes skylights are installed flush to the roof.