Nest with AC and Boiler with no C wire

[Pdkarimi]

Hi. Can you provide a wiring diagram for a system with the following criteria

no C wire

separated a/c and boiler heating system

separate 24 v transformer

These wire are connected

Y1. G. Rc. W1. Rh

now I want to add transformer. One wire goes to C terminal. Where the other wire should l go. 

[CoolingWizard]

@Pdkarimi , you have likely have 2 transformers already. The Rh wire comes form the Boiler, and the Rc comes form the air handler.  If the thermostat cable coming form your air handler has an available conductor wire, it can be connected to the Nest C terminal.  In your air handler, find the thermostat cable entering the air handler and open that part of the cabinet.  Inside you might see a control board.  Look at it, find the C terminal and connect the wire from the thermostat cable you assigned to the Nest C terminal.  

AC Cooling Wizard

[Pdkarimi]

The problem is that there is no available free wire to be used for C. And I prefer not to use G wire as C wire because I use the fan circulation a lot. 
I have disconnected the Rh and connected he plug-in wires into C and Rh terminals as it is summer time and's I don’t use heating for a while. But still gives me N79 error and not working. 
I tried everything I could and couldn’t find a way yet. 
even tried the power connector by Google, but no luck

please advise me what to do about that