The height of the Isis One's two nozzles relative to one another must be perfectly set for the printer to work correctly. The printer ships with both in alignment, but if you have to replace a nozzle or otherwise remove a hotend, you'll need to re-level.

In its default configuration, used for printing with PVA support, the support nozzle should be slightly (1/4 turn of the captive nut) higher than the material nozzle. If, however, you are using the second extruder to print in a second color, the nozzles should be perfectly level.

1. After making sure that both extruders are cool, use the LCD console to select Prepare > Auto Home, and then turn the printer off.
2. Move the axes manually such that the nozzles are over the bed. Turn the vertical leadscrews manually such that the lower of the two nozzles is just touching the bed. Turn them by their shaft couplers so as not to get your fingers greasy, and turn both simultaneously, so as not to throw the gantry out of level.
3. See which nozzle is touching the bed. This is the one you'll adjust. Use a wrench to loosen the locking nut (nut on the barrel just below the bracket) slightly.
4. Turn the thumbwheel until the other nozzle touches the bed.
5. If you want the nozzles perfectly level, you're done. Otherwise, set the support extruder (the right one) 1/4 turn higher than the material extruder. Tighten the locking nut, while holding the heater block to make sure it doesn't rotate.