Why are so many HVAC units on rooftops in Phoenix?

I moved to Phoenix back in the late 80s and when I moved, I noticed the oddest thing about some of the houses here.

They had their air conditioners installed on the rooftops.

I have seen this in commercial buildings before. In fact, prior to moving, the last company that I worked for had their HVAC unit on top of the roof. However, I have never seen it in residential homes until I lived in Phoenix. Well, after inquiring about it with some of the locals, I realized not many of them had a direct answer. Some said it was due to theft, some said it was to protect the unit from the elements. Others mentioned that the unit worked better on the roof. It wasn’t until I asked my HVAC technician when I got a solid answer. Turns out, it’s because the units are packaged HVACs that are found in older homes that were built in the 70s. The packaged HVAC is made up of 1 unit that provides both the heating and cooling. And these units were quite popular back when these older homes were being built. And having the unit on the roof was easier due to the positioning of the ductwork. Newer homes in Arizona have the split HVAC unit where there is an indoor air handler as well as the outdoor condenser. And typically, those condensers are not found on top of the roof but instead in the yard. So, whenever I see a house with the HVAC on top of the roof, I will know that the house was built around the 70s.

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