Watch out if you own a ranch style home with a packaged heat pump or gas pack on the roof. A common HVAC contractor mistake was to install the wrong type of elbow on the roof. An elbow is the curved piece of sheet metal between the heating and cooling system and the ductwork in the attic.
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.
2. Down draft. Down draft elbows are better than over-under elbows because the limited amount of exposed metal, especially in the hot Phoenix sun. However, the down sides outweigh the benefit of the reduced heat gain. The main downside is that the down draft elbow is also still restrictive to airflow (are you beginning to see a pattern here?). Having those restrictions will make a 5 ton heat pump only give 4 tons of air! During our energy audits we measure the amount of airflow restrictions in the duct system and time and time again we find these types of elbows highly restrictive.
3. Side-by-side. Even though side by side elbows have more area exposed to the sun and have more heat gain (yes they are insulated), the restriction is far less than the down draft elbow so the AC unit doesn't have to work as hard. That also means that a 5 ton heat pump will deliver 5 tons of air through the elbow! HVAC contractors need to be a little more experienced when installing side by side elbows because there are a few more measurements needed but it is worth the extra effort.
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