Chagrin:
Just to verify, from the Arduino to the relay board you're only connecting VCC and the relay IO pins (no GND)?
Hi Chagrin, that is absolutely correct.
Chagrin:
FYI, the diodes on the relay board are there to protect from the EMF spike caused by de-energizing the relay coil. They do nothing with respect to EMF spikes from, in your case, the fan. Hopefully someone else will comment on that.
Oh ok, appreciate the enlightenment, hopefully more help follows.