... 20+ Macs in a room take the edge off the cold, in winter, very well.
de

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Last autumn I had 6 old Macs running all afternoon in a room and the temperature of the room got to 35 degrees Celcius (according to a thermometer on the wall), even though it was about 15 degrees outside. I also had my G4 running in that room with a 17" CRT, which both kick out a bit of heat when they've been on for a while.