Having built and rebuilt computers and other electronics for more than the last two decades, I know this inside and out and have also learned it the hard way!
Do NOT get overly confident when putting hardware together and go on auto-pilot (with a slight beer-buzz). It can easily bite you in the butt and make you feel like the dumb-ass you are!
So I took my three S7's apart to brush out the dust and also use an electronics duster/vacuum to get all of the crap, dust, cat hair, etc. from the nooks and crannies.
After putting them back together all nice and clean, I decided I would fire them up and run them along with the S9's for the weekend. After all, with the coin price what it is, the S7's should theoretically pull in ~$6.00 a day. ~$18.00 extra a day is always nice.
As the S7's were starting up, I monitored them in Awesome Miner.... Man, those boards were getting hot!!! Too hot!
Time to shut them down and investigate... Can you guess?? I knew what I had done right off the bat. I put one of the fans in each of the three S7's in backwards.
What a dumb-ass I am.
So an hour later, all is as it should be. Definitely time for another beer! LOL
Cheers! 
~Goodnight!