For some reason the thermostat / heater in the middle of the circuit are not working correctly. Here is an image of the wiring diagram.
These are three small heaters (500w), each thermostat is rated for 2000W. I can't figure out why I'm only getting 120 V in my second thermostat (black wire). The other two thermostats, when I measure the load ENTERING thermostat, and LEAVING it says (240).
Also, if I disconnect thermostat in BOX 3, the heater completely stops. However, in BOX 2, the heater keeps working. I think it's running on just 120 V, instead of 240, but that should tell me something where the problem is, I just don't know what it is.
Essentially in all 3 boxes, all RED wires, are just wired into one nut together, the BLACK is passed through, with one PIGTAIL out, into the thermostat. Then, the wire going out of the thermostat, is going into the BLACK wire of the baseboard heater.
NOTE: All grounds are wired together in all boxes, going to the heaters etc. I didn't put them in the diagram to avoid confusion.
EDIT: I tested resistance between RED > GROUND(120.4O) and BLACK GROUND (0.746MO).
Best Answer
Heater 2 has an unintended connection (short) between black and ground, based on what you have reported. Specifically, somewhere between the black at the heater and the black at the thermostat. The black to the thermostat is not implicated.
Go hunting for defects in the wiring (circuit off, of course.) Those will almost always be in junction boxes (possibly in the one built into the heater, typically, here) though things like a nail driven the wrong place is possible, but uncommon, unless the problem started right after you did some nailing in the vicinity.
With power off check the resistance between that black wire and ground. Anything other than infinite is a sure sign of the problem.