Honeywell has a video that shows how to use the fan wire ("G") of the cooling system as the "C" wire for the thermostat. It is in the video - "Honeywell DIY wifi...substitute g-wire for c-wire" on youtube.
They say to move the g-wire in the thermostat and and move it to the c-wire terminal.
As shown in the picture below this part can be done. (I have a boiler and an a/c connected to this thermostat. The "R" and "W" wires are for the boiler. The "Rc" "G" and "W" wires are for the a/c.)
The problem is in the a/c unit. In the youtube video they show wires attached to the "Y" "G" and "R" terminals. (I am ignoring the "W" because I think that is for the heat, and I have a separate boiler.)
As you can see in the picture below, mine has wires connected to the "R" "C" and "G" terminals. The a/c works fine and I am able to turn on the fan separately from the a/c.
How would I change the wiring to use the "C" wire on my thermostat?