Reznor UDP-60 - Need C Wire
Pick your poison
You have two options for solving this, but both of them are going to be a bit more inelegant than pulling a new thermostat cable.  Either you can use a gizmo that &quot;multiplexes&quot; two signals over the same wire to free up the third wire for C-wire duty (there are several out there, with different installation instructions, so you'll need to follow the directions for what you purchase), or you can supply 24VAC from an additional class 2 transformer either plugged into a nearby receptacle or hardwired to the faceplate of a nearby junction box, removing the jumper between Rh and Rc then landing the new 24VAC source on Rc and C when you do this.