Can a Thermostat Common Wire (C-Wire) Be Added to a DNV40NB Heater?
Sure.  I've highlighted the C wire in the wiring diagram.



You'll have to bring a wire from the transformer in the heater, up to the thermostat location. This may require pulling a whole new 3 wire cable.

If there's an open terminal where the brown wire attaches to the transformer, you can connect the new C wire for the thermostat there.  Otherwise, you'll have to tap in somewhere else.

This image shows how the new thermostat should be connected to the system.




  WARNING: If you don't feel comfortable poking around yourself, don't hesitate to contact a local HVAC tech (or Electrician).