Many customers are unsure what type of furnace they have and how efficient it should run. The basic categories of forced air furnaces are: High (90+) efficiency, medium (80%) efficiency, and standard efficiency. Any furnace that has PVC plastic vent piping is at least 90% efficient and therefore belongs in the most efficient category. If a furnace is vented by metal vent piping, it is at best a medium (80%)efficient furnace, however depending on it's age, it may also belong to the standard efficiency category. One easy way to determine whether your metal vented furnace is a standard efficiency or medium (80%) efficiency furnace is to find out whether the furnace has a forced vent motor. All furnaces that utilize a forced venting system are at least 80% efficient. When calling for furnace repair it will help the service technician to know what type of furnace you have. Some problems may even be able to be diagnosed over the phone.