Reasons to upgrade furnace : Most times a furnace upgrade becomes a necessity as old furnace either becomes too costly to repair compared to new or parts are unattainable. Catastrophic failure of heat exchanger is a main cause of furnace failure. Lack of regular service is another culprit.The upside of a furnace replacement is the minimum efficiency required by building code is 92 %. An old furnace may run half of this so an upgrade will save operating costs. Not to mention the benefits to the environment.