The different types of elbows, in order of worst to best, are outlined below.
1. Over-under elbow. Over-under elbows are worst type of elbow HVAC contractors can install. These were originally installed on ranch style homes and are still sitting on roofs today even with brand new AC units! The problem with these sheet metal elbows is that they reduce in size when going into the roof and choke the air off to and from the HVAC system. This causes major restrictions to the airflow, making the heat pump work harder and shortening it's useful life. The good news is that over-under elbows can be changed without having to replace the heating and cooling system.