The IDEAL set up is OTR, 4 Roof airs, and Driver/dash air. Should your OTR fail, start the genset, run the dash air (especially if headed west in the summer) to keep the driver and co-pilot cool. Sort of like wearing a belt and suspenders! Trust me, headed west in the afternoon sun, even 4 roof airs will not keep the driver and co-pilot cool without dash air! You hardly ever see this set up...but it would be ideal. Next best set up is OTR with 4 roof airs for back up.