What's funny is that I never actually felt it was hot in a datacenter that hosted a mainframe, I think because they were water cooled and also not packed in that tight.

Circa 2005 I would regularly go to the data center on the 7th floor of Rhodes hall [1] and it had a mainframe (cool, but it might have been CMOS instead of bipolar) and huge amounts of rack space devoted to RS-6000, SGI, SPARC and other legacy RISC servers and just a few racks of x86 servers which felt a lot hotter both from radiant heat in the front and hot air blowing out the back because they were packed in tight -- three racks of modern servers would outpower that old CMOS mainframe although of course they do a lot more.

[1] I work on the 6th now but never go to the data center