We've got a rack room in a project where we dont have AC.

Ive got a room which we plan to setup as per the rough diagram (link below). The rack room has 3 external walls. : [Link: imgur.com] - (Apologies i couldn't work out how to post a image inline). The rack is an open frame rack with no sides / back and no fans on the rack itself other than the fans built into various equipment.

Ive vented rack rooms like this before and its worked, but id like to know if there is some way i can calculate the required fan / ventilation requirements.

I can get a total CFM for all of the hardware or a total BTU output, would it just be a question of making sure that the extract fan has a greater CFM output than the total output CFM output of the AV hardware ?

[Link: repnet.middleatlantic.com]

What happens on hot days? Or rainy days?

Hot days : The air temp in the UK rarely reaches above 30c and if it dose its for brief periods of time, the extract system is still running and circulating the air.

Rainy days : The extract system runs as expected, the vents are louvred so dont cause water / leak issues.

It would seem to me that pulling in humid outside air would be a bad idea long term