A 12-VDC supply is to be preferred instead of a rotating magnetic field near the CRT, as many a step-down transformer or AC adapter near the rear of my iMacs has demonstrated to me. Much preferable to a nearby fan of any voltage, however, is a duct above the iMac's outlet running at least a few feet to a box containing the fan and remote from the Mac.
That said, I have had the rear feet of my CRT iMacs (five of them) elevated at least 18mm (a handy commercial size of timber) in addition to the lift of the hinged front foot, and in one case 40mm (to make the display face vertical in a sound-editing rack), for some years. Always-on Summer 2000 iMacs are still going strong with the freer passive ventilation.
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That said, I have had the rear feet of my CRT iMacs (five of them) elevated at least 18mm (a handy commercial size of timber) in addition to the lift of the hinged front foot, and in one case 40mm (to make the display face vertical in a sound-editing rack), for some years. Always-on Summer 2000 iMacs are still going strong with the freer passive ventilation.
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