The bed heater works but immediately resets the duet, as the power supply cannot keep up with the 500 Watts request (it is a 350 Watts PSU). I ordered a 200 Watts bed heater now. I assume the circuit itself is ok, as if it was burn't I wouldn't be seeing any increase in the temperature.
As a side note, I removed the fan connectors, removed the crimps, connected them directly to the PSU to individually test them and they work, so it is probably some bad wiring by me creating the 24 volts fans not working problem.
I read the fan page on the wiki. As I understand, it is possible to either use all 5 volts fans (I don't have any), use all 24 volts fans (in that case I need 1 more 24 volts blower and a 24 volts large fan to cool the Duet board), use all 12 volts fans (in that case I'll need a buck converter, that I already have, and connect the +and - inputs of the converter to my 24 Volts PSU, and connect the positive output to V-fan jumper. I that case, all of the fans (PWM and always on) will be 12 volts. Or in my case, if I want to use 2 24 volts and 2 12 volts fans, I need to apply 24 volts to V-fan jumper (which means I can leave the jumper in place, as my PSU is 24 volts and V in will provide that 24 volts), however for the 12 volt fans I need to apply 12 volts from a buck converter, but the negative ends will go to the Fan - connectors. I assume I can use a single buck converter for supplying 2 12 Volts fans (the buck coverter can output 3 A, so it should be enough). My best option, until I have a all 24 Volts setup, seems to use the last option, which will require a minimal chang in wiring only.
Anything wrong with my understanding?