I have run a 400 GPU farm with absolutely no A/C. If you need help, please contact me on skype (Kotarius). I can save you a lot of money.
I'm quite curious how this is done and what climate you're in that no AC is used. I live in the Midwest, United States and in the summer, the ambient temp will far exceed what you want a room full of computers to be at.
I'm having my HVAC company come over next week to consult on adding a supplemental AC/system to only cool the server room(s). If I can do this with mostly moving air and a little cooling (in the hot summer days), that'd be ideal. If I had to choose between shutting down the farm on those hot days and paying for electricity to cool it, that's an easy choice.
The "pressure" aspect is interesting, I'll definitely do some additional reading on that.
Good stuff guys.
I'm in southeast USA an in the summer, i use fans over AC other then maybe AC to keep the house cool and us in it . The fans run 24/7 or when ever i mine, i don't always mine 24/7 can't stand it but love it, if that makes sense . You have to move that hot Air and AC alone just won't cut it . it's misleading to think that AC will some how cool everything with out moving that cool air over something hot to cool it . TO me AC was invented mostly for health and comfort in the Summer times but led to other uses over time .. an AC still can't replace a good cheap fan for fast instant cooling when it's needed the must..
You can run your farm and still keep it cool with fans over AC an use AC to keep your self cool in the same house ... but the Power bill is still gonna go up even with just normal AC use and no farm running.
With the farm running full force and AC, it's actually won't be that much more in cost if you do it right .