Hi,
I think you need a little hatch on the back to shovel in the dry ice!!
Seriously though, is your ambient temp really 29C or is your machine in an enclosed space where it is perhaps breathing in its own exhaust? As you are probably aware the iMac takes in cool air at the bottom and circulates it partially by convection and pertially by fan up and out at the slot on the back cover. If you have the machine boxed in (not uncommon) then the hot air might be circulating back underneath and into the inlet again.
Check if ambient really is 29C using a thermometer if possible and/or try siting the machine on a table in the middle of the room and see if temps drop.
Failing that I would try to get a statement from apple as to what the maximum ambient temperature should be for the iMac to ensure that you are not in breach of your warranty. Most computer server equipment that I have come across usually has a max ambient temp of around 30C but even then most servers are kept in an air conditioned room. I've never seen an ambient limit for domestic equipment.
If they say that 29C ambient is okay then you can ask them if the other temps are acceptable.
Hope this helps,
Craig.
