When inspecting a new home, the professional home inspector might have to call for service at a new furnace -- even if the furnace is only a few months old. The problem has to do with wet weather. At least in the wet climate of Bellingham and Whatcom County, builders cut wood and drywall in the garage during the fall and winter. They do so, logically so, to get out of the cold and wet. But guess what is located in the garage at many houses. Yep, the furnace.
And all of that sawdust, shavings and drywall dust ends up inside the furnace and/or the heating ducts and system. The photo below is a classic example of this scenario at a furnace that is only six months of age.
Manufacturers' state that a furnace should be serviced annually but, when a new house is being sold, I usually recommend HVAC service at that time if the furnace is in obvious need of cleaning. Sure, the service might be three our four months shy of one year since installation, but better to get it taken care of sooner rather than later since the device is obviously crusty inside. Estimated design life of a furnace is 15 to 20 years.