Can I turn my gas furnace blower *

[efaraldo]

I have a 45 year old, or so, Dunham-Bush gas furnace model GUFD-755 in my house with no cooling provisions.
I purchased a Honeywell Programmable Thermostat RTH230B some time ago which would allow me to turn the
blower on by itself, IF I HAVE a three wire heating system, the third wire being to connect to the blower relay. This would be nice in the summer since I have no A/C.
The problem I have in trying to enable this feature, is that I have four wires coming from the furnace to the thermostat, but
only two are connected at the furnace terminal board. There is connection at the terminal board that is not connected, but
I don't know what it is for, and I'm afraid to connect it to the thermostat.
I have searched the Internet for my gas furnace, but it is too old to find any information on it.
Can anyone tell me if my gas furnace IS a three wire heating system, and if so, can I connect a wire from the
unused terminal without blowing something *

Thanks,

[nomadpeo]

how are the terminals identified on your existing thermostat * you should have 'r' (input), 'w' (heat) and possibly 'g' (fan) and if it is a heat/cool thermostat, it would have 'y' (cool). is this also the original thermostat * if so, no telling. with four wires, it's possible the third wire connected at the furnace is connected to the 'g' terminal. if that is the case, you should be able to connect to 'g' at your new thermostat and get fan control. is it possible for you to take a photo of the connections at the control board. also, what terminal was/is the third wire connected to at the old thermostat *