a 150 watt hps would prob not fenerate much more heat than the required amount of cfl,.. i have a grow cab just a smidge over 2 ft by 3 ft and 4.5 ft high, with 4- 4ft flouros in the corners and 2 65 watt cfls hanging down and it got to 94 deg. in there,.. i have a 4.5 inch duct with a small fan...