framer,
Without getting my books out I think that the equipment cooling fans do a good enough job if it's less than 38 degrees C. After that, it looks like they need a bit of help from the left recirc fan. Sound about right?
Sorry, to do this to you. Have a look at the Hot Weather supp proc. After eng start, eng bleeds off, APU bleed on, packs to high.
According to FCOM 2 packs to high maximizes the airflow with the APU. According to a later paragraph this turns off the left recirc fan.
You will notice with the APU and packs high, there is a reduction in FD noise that corresponds to the recirc being swtiched off.
Note the conditions for recirc fans are dependent on air/ground logic. In flight both recirc fans will be off with both packs in high. On the ground just the left recirc fan. So I figure the left fan is not essential for equip cooling. The description of the forward cargo compartment in Chap 2 and Equip Cooling in Chap 2 suggest the recirc fans have nothing to do with equip cooling. But given at least one recirc fan will be operating on the ground I guess it does that mean it is more difficult to cool people on the ground.