Wiring – How to replace an old dial thermostat on an oil furnace with Hydrostat 3250 Plus

I think you're out of luck, since the G wire passes through the ventilation control system. You have to leave the G wire as a G wire, or remove/bypass the ventilation system. The easiest solution, would be to run a new 5 wire cable. Or don't use a fancy thermostat.

If you follow the G wire through the system, you can get a better picture of what's going on.

When the thermostat energizes the G terminal, the Gt terminal on the ventilation control receives the signal. The ventilation control does whatever it does, and eventually energizes the Gf terminal. When the Gf terminal is energized on the ventilation control, the furnace receives the signal and starts the blower.

From what I can tell based on the images, this is what the whole system looks like.

_{On a side note. The installer that left that rats nest of wiring in the furnace, should be ashamed of himself.}

You should check the wiring in the furnace. There are no standards for thermostat wire colors, so you can't always trust the color of the wires. Seeing how the wires connect to the furnace, is a sure fire way to figure out what's what.

The O (cool mode) and B (heat mode) terminals are usually used for a reversing valve, in heat pump systems. I'm not familiar with oil systems, so I can't say for sure what they would be used for.

Some thermostats have separate O and B terminals, like this.

Others have an O/B terminal, and use a jumper to select which should be used.