Hi folks,
I am a data center guy, so I look at the rigs from the 'how much heat do they generate' perspective. The short answer is that mining rigs are a MONSTER for a data center that is more than 3 years old. I did some math on bitcoin, and posted it on my blog
http://blunthammer.wordpress.com and the data points were picked up by the Wall Street Journal for an article about the heat problem - [Suspicious link removed]j.com/moneybeat/2014/04/29/bitbeat-for-bitcoin-miners-a-hot-problem-this-summer/ which is the #1 problem the rigs have.
The heat signature of a 4U rig is the equivalent of a 42U rack and most data centers are designed for the density spread out over 42U not 4. It's like trying to cool a heat gun in a shoebox. Very tough. The heat problem (at least here in the US) hasn't been much of an issue because it was winter and heat was a desirable benefit along with the bitcoin mined. There are the liquid cooled solutions that are available, and Martin Enclosures just released a new line of racks -
http://www.martinenclosures.com/product/bitrack/ that sound like they were developed to help deal with the heat problem.
Bottom line - It is expensive to mine where power is over $0.05 per Kwh. The cost doubles when it is warm outside because you're paying for electricity to run rigs AND to cool them. Heat is not a desired byproduct when it is 90F/30C outside. The data center contracts carry a cooling uplift of 30%-50% so you'll pay for the cooling no matter what, so the best defense is to go to a facility where power is inexpensive, or go big with the mining operation (400 + rigs).