Al Reynolds wrote:
Morning,

I want to put my PC in a cupboard to reduce the sound
impact of the fans. Before anyone suggests that I get
quiter fans, please bear in mind that this is a very small
form factor PC and I haven't got the time to go finding
exact match parts that will fit in its tiny case!

Anyway, it piles out a lot of hot air, and currently has
ventilation on both sides of the case. I'm conscious that
if I put it in a cupboard it may overheat quite quickly, so
I'm looking for options to ventilate the cupboard.

I did consider venting to the outside wall, but it's upstairs
and the wall is rendered, and I'd rather not disturb the
render unless absolutely necessary. I have a number of
other options:

(i) Vent to and from the room
I could just put in grills with quiet fans on the face of the
cupboard somewhere - one to suck air in and one to push
it out. My worries are that:
(a) it will look bad
(b) it will sound worse than before
(c) it'll just push warm air around the case

snip
What do people think?

I'll just mention what I did.
I started off with a PC in a knee-hole desks cupboard.
This was then lined with carpet, and two foam-mounted fans placed on
the back.

This was a good deal quieter.

Then I made an interior cupboard spaced some 1" in, also carpet-lined.
The fans were replaced with ones inside the inner cupboard that
connected to the outside of the inner cupboard (blowing in and sucking
out) through tunnels of carpet, and ducting (again of carpet) to the
original vent holes (which were in different places.

Alternatively, have you considered simply putting the PC in the attic?