High-Current fans are not recommended. The reason is their inductive load and the noise injected to the power plane of the
board. The protection diode placed before the FAN header can withstand max 5A of current. So this sets the upper limit to 5A,
which translates into 60W on 12 Volts rails. This amount of power, along with the power consumption of the board itself can
exceed the Barrel Jack current rating, which is why we advise not to take any risks in this regard.
Regards,
BF Labs Inc.
Are there 2 different diodes, one for each header? Or just one that covers both of them?board. The protection diode placed before the FAN header can withstand max 5A of current. So this sets the upper limit to 5A,
which translates into 60W on 12 Volts rails. This amount of power, along with the power consumption of the board itself can
exceed the Barrel Jack current rating, which is why we advise not to take any risks in this regard.
Regards,
BF Labs Inc.
Also, if the barrel jack was replaced with something else, would the board itself support that current draw without issues? I'm guessing the warranty would be void in this case.
There is enough copper on the board to support maybe 20A on 12V rails. However, doing so will void the warranty,
and should be done at users own risk. Each fan header has a protection diode and a small stabilizing/filter capacitor.
EDIT:
I would also think that if someone is willing to take an extreme measure for cooling (speaking about minimum several units),
then it would be best to buy a split Air-Conditioner to get the job done. It will consume about 480W (samsung series),
and if the target is 20 singles for instance, then it would be equivalent to an additional 24W per single.
Regards,
BF Labs Inc.
Their CFM rating is lower than the stock Single one. I am thinking if one is going to put these somewhere far away a higher CFM fan would be better. I found 68 CFM fans with a noise signature of 43 dBA which is a tad bit higher than the stock Rev 3 fan (32). 
