Denon, Onkyo/Integra, Pioneer, Harman/Kardon all recommend a minimum of 4-6"
free-air clearance for the L/R sides and top cover....
Some AVR brands such as Onkyo/Integra and Harman/Kardon bias their output stages on the higher side to eliminate X-over notch distortion which simply means that the output devices are always partially ON so they give off more heat even
when idling or playing @ lower levels.
Unfortunately many users overlook this crucial spec of free-air clearance when installing their AVR, plus many of the home entertainment furniture racks lack adequate clearance as well...
Keeping the AVR @ a lower temperature will assure longer and better reliability in the long run...
That being said some have mentioned @ what temperature is the max..
The AVR's internal temperature protection circuitry is usually set @ about 85 degrees C which is 185 degrees F...
Keep in mind that this setting is inside
the chassis so actual temperature measured outside on the top cover can/will be significantly less..
Other factors influencing the AVR's operating temperature are:a.
Loudspeaker specs for impedance and sensitivityc.
Source material specs, for compression and dynamic ranged.
Average volume level setting and listening levelse.
Price class of AVR and amount of component overdesign
Just my $0.02..