To calculate how much heat every 6990 in the setup will produce, you have to consider how much energy the computers containing them will draw from the wall. PSUs aren't 100% efficient, and all the "lost" energy gets converted into heat as well.
Calculating 65 W power consumption outside the video cards, we get a total of 1565 W for a computer holding four 6990s. If the PSU operates at 85% efficiency, the whole setup will draw approximately 1840 W from the wall, giving 460 W per video card.
By DeathAndTaxes view per 4 6990's we'll need 1565W from the wall. Producing 1565W heat output.
No. The 85% efficient PSU draws 1840 W from the wall. 15% get converted directly into heat, the remaining 85% get converted into heat by the rig's remaining components.
If we are on a duty cycle they won't run 24/7 so hopefully they don't get abused to hard..
By far too wasteful. Since the rigs will be mining coins only while heating, you should run them 24/7. An 18000 W rig running 8 hours per day will produce the same heat as a 6000 W rig running continuously, but it will cost thrice as much and produce only one third of the coins.