Why would home A/C units be on the roof

[Nukeman7]

Here is the explanation from a HVAC contractor located in Las Vegas:

Why are A/C Units on the Roof in LV?

 

[50yr Street Mech]

Personally, if I had to choose, I'd chose the indoor unit. Not worth damaging shingles from guys walking all over them. I know here in NY, asphalt shingles are most common. When they get old, they get brittle and crack easily. Putting a tech on an old asphalt roof is just looking to start a leak . Who needs that ? Then you spend more money on a roofing contractor to make repairs. No thanks.

 

[JHZR2]

Great thread and I can't figure it out.

I live in a subdivision of semi custom homes with weather that matches Las Vegas. The lots in our subdivision are 1/3 acres. The home next door to me was built by the same builder, the same year as our home, 2019.

Both Hines are very similar in design, ranches. We have about 400 more square feet than the home next door. We have the same roof material.

The home next door has a single AC unit in their roof. Out home has two ac units, both on the ground, at exact opposite sides of the home.

For the life of me I can't understand why the home next door has AC unit on the roof. Huge amount of space to mount the AC unit at ground level

Beats me.

I guess because it doesn’t matter.

It always struck me as a bad idea, primarily because the sun is pummeling it. Ideally I’d have them in the shade (as I do for the split units for my home).

The units are designed to condense/reject heat from the working fluid with X mass of air at y temperature. If the working fluid condenses and rejects heat in those conditions, and ensure phase change, then the system will work. Maybe not work the absolute best, maybe less efficient.

https://en.m.wikipedia.org/wiki/Heat_pump_and_refrigeration_cycle

 

[Fitter30]

Most likely there swamp coolers on older homes or they could be package unit (heat/ heat pump and cooling) houses that have no basements. Swamp cooler runs recycled water over panels that evaporate the water cools down the air because of the low humidity. Bottom of unit has a float type water level with a pump circulating water over the panels with a blower like your furnace blower

 

[ArrestMeRedZ]

For those suggesting they are swamp coolers that is incorrect. They are AC/heater units. When we were looking at houses in Vegas late last millennium, almost all the builders were going to split units. Only the cheap builders and older homes had the rooftop units. A rooftop unit was a deal breaker for us.
Also with the rooftop units here you get ugly roofs. Most homes had ceramic tile roofs. The ones with the rooftop units had mostly ceramic roofs with a big asphalt shingle section that surrounded the HVAC unit and ran from the peak to the eaves.