With any household the safety of the residents is of huge importance. A reg tagged furnace indicates that the current furnace installed in the home is unsafe for operation. It gets its name from the colour of the tag placed on the furnace by the diagnosing technician indicating that the heater is no longer usable with either a replacement or repair. To be clear, the HVAC technician who is examining the furnace does not have a choice but to place a red tag on the furnace if it is indeed dangerous in any way to operate. If a technician did not place a red tag on a unit he or she knew was dangerous he or she would likely lose their license.