Heating and Cooling
Ive been looking at the heating angle. The new CPU is cooler but its the GPU that puts out the most heat, and the new one puts out more. But the heat news is not all bad. Just below the pedestal mount theres a square grill to let the air flow out. There are air inlets at the bottom. The CPU fan looks to exit near the vent. The GPU fan is concealed; the unit is slightly above the top of the vent line, to the left looking with the screen removed. The HHD is to the right, a bit farther up. Its fan is not obvious. This presents balanced heat-sources: left, center, and right, tending to concentrate about half way up. The fact that the vent is square suggests to me that air flow is more complex than in my old iMac (more below).
The fans do not draw air in through the bottom in an important way, but mostly create local circulation patterns. The heat of operation creates rising convection currents, drawing air in at the bottom. The expanding heated air creates a slight pressure rise with a flow out of the vent as a result. The fact that the vents are not along the top, as in my 2006 iMac, is at first glance surprising. In my iMac convection draws like a chimney. But the rear of the new model bulges in the middle, slightly farther out than the 2011 iMac. I think this creates a secondary convection flow from each side tending toward the center. Hot air accumulating at the top would contribute to the heating of the aluminum back generally (which cools by radiation and enhances the convection flow). Now Apple could put the vents on the back anywhere they chose. I think that they chose to maximize the cooling effect when they put them where they did. And I think a good deal of the room for the thinness is created by the new mounting of the display. I would guess that volumetrically the space behind the screens is comparable in the two models.
