[tnott]

The idea of boundary layer fans is to replace the chaotic boundary layer of warm air clinging to the face of the mirror that is ruining the view into a smoother, laminar one, reducing distortion. This phenomenon is the primary cause of what people used to call "tube currents".

If the mirror's temp. is within a degree of ambient then you won't have a substantial boundary layer to remove.

So, if you turn the mirror cooling fan/s on when you are setting up, collimating, waiting for it to to dark, deciding what objects to view for the night etc. like I usually do, then one good fan on the back should be enough according to the data and modeling I have seen.

Also, while the mirror is cooling the figure is distorted anyway.

Robert's mirror cooling calculator is approximate, but can give you a ballpark idea how long it takes to cool if you play around with the variables.

But, if you want to view things like planets and double stars in a hurry and don't want to wait for the mirror to cool down then boundary layer fans may help somewhat for this specific situation.