When I add an additional fan to a case, should it blow in or out?
Depends on what your case is like, where you're putting the
fan, and why are you adding an additional fan. Presumably
you feel it needs more airflow as something is too hot?
Focus on what that overheating thing is, where the placement
and orientation will benefit that part while not negatively
impacting the existing airflow/path through the system.
Sometimes, another fan isn't even necesssary as the problem
is a poorly designed case that uses fan grills stamped out
of the metal wall which impede airflow too much, or if the
case is fairly well sealed and practically all intake is
through the front bezel, some bezels don't have large enough
intake area (holes/slits/etc).
If it blows out, does it stand the chance of lowering the internal pressure
of the case enough to cause the PS fan to not move enough past it's own
components?
Yes, with each exhaust fan you add to an otherwise unaltered
case, you will reduce the exhaust rate out of the PSU.
However, most decent PSU these days have a fan control with
thermal sensing so if it were running hotter it would ramp
up the fan speed which again improves PSU exhaust rate.
Problem is, some PSU are better than others at a good and
appropriate fan control circuit that increases fan speed
proportionally, instead of either getting too fast too soon
(at too low a temp, creating excessive noise unnecessarily)
or not fast enough soon enough (PSU always runs hotter than
what would be best for long life).
Basically "competing" with the PS fan, doesn't seem too prudent
since that thing really needs air flow.
True, BUT there's enough margin built in that normally it is
still better to have the first added fan in a case as an
exhaust fan. IF merely having this first chassis fan is a
problem, the solution is not to reverse it's airflow, it is
to improve the air intake areas, since with ample (infinite)
air intake area the added fan would have no effect on the
PSU and practically speaking there is a middle ground far
from an infinite intake area but still sufficient.
Also, with the larger intake area, this too would result in
higher exhaust rate out of the PSU, and with more airflow in
the system you can use a slower additional fan which also
mitigates the effect on the PSU.
If it blows in, do I need to position it as far from the PS as possible?
(PS fan blows out of course).
On average, this will have the best effect on other parts.
It is typically placed in front of the HDD rack as any other
placement would decrease the passive air intake through the
rack.
In other cases/system-configurations, there might be a
particular hot spot where you must place a fan, like in the
side panel across from a high-end video card. In that
situation, ideally you would add that fan there and also add
a second fan in front of the HDD rack so you preserve the
intake there as well.
Are in-blowing fans problematic?
Yes, no, maybe. It is easier to design or redesign a system
where all intake is passive and exhaust is fans. That
doesn't make it hard to do it another way, but it can be a
lot of work to carve up a case to make other methods work
well. Above all when you significantly change airflow be
sure you have again checked all parts' temps.
