I have an older furnace with only 2 wires at the thermostat "white and red". No issues at all with my system works as should. Thermostat is new Honeywell basic model not smart. I want to be able to turn it on fan mode only. I do not have an ac unit attached. I only want this option to help circulate air in my house. Cooler in the summer and in the winter we run a gas fireplace that roast us out of the living room but everywhere else is cooler.i have an electrical background but Zero funace knowledge. I know ill have to run a new wire 3 or 5 doesn't really matter have easy access. My question is how i would hook this up ive seen plenty of post "c" wire G wire and what I'm looking at doesn't look that straight forward or im over thinking this.
Your furnace has fan terminals, thankfully
The good news is that your furnace has a fan (G) terminal and corresponding Rc and C terminals, found on the relay module depicted in your second photo. The bad news is that the combustion controller is totally separate from the fan relay in your setup, so you'll need to wire this a bit differently from a standard furnace. First off, your thermostat will need to be set up for two-transformer operation by moving a slider, removing a jumper link, or flipping a switch; the instructions should document how this is done. Once that is done, you can wire yellow to Rc, green to G, and blue to C at each end, with the white wire going to W/W1 on the thermostat and one T terminal on the combustion controller, and the red wire going to Rh on the thermostat and the other T terminal on the combustion controller.
Note that while a 5-wire cable will get the job done here, it may be a better choice to put a fatter thermostat cable, such as an 8-wire or even a 10-wire, in now so that the wires are available for a future HVAC upgrade, such as fitting a heat pump. Also note that if you're having trouble getting the heat to work, you may need to swap the red and white wires at the combustion control end (one of them is Rh and the other W, but your combustion controller doesn't differentiate between the two, while your thermostat just might).