Thermostat wiring advice needed: digital for heat only

The black wire is for the fan that circulates air into the basement. in my camper the fan is under the bathroom. take it off and that fan wont come on when furnace comes on anymore.

Im thinking the 3rd black wire is a dealer installed or connected option?
Similar thermostat thread (though faulty Tstat) showed the 3rd unknown black wire on an ArticFox camper.
The basement fan sounds plausible. If it were mine Id find what is being energized by the tstat throughput. Mostly out of curiosity but to see what kind of load is being placed on the tstat contacts?

An even older thread by deltabravo replacing same tstat with a programmable unit on an AF shows same crimped on 3rd wire connection


I dont know if the tsat he used is still available-

UPDATE:

Black wire: Still no clue what this is for. I wired the old thermostat back on, w/o the black wire, and the furnace performs the same as before. So I wired the new thermostat w/o the black wire for now.
Where is the basement fan located in an Arctic Fox? I'd like to see where its wires are directed.

Fan running after burner shuts off: My fan does not run after the burner shuts off. I realize it is typical with RV furnaces in general to run beyond the burner shut-down, but does anyone else's small Suburban furnace shut down the fan and the burner at the same time like mine?

Lux Thermostat Review:
I bought the LUX TX1500U digital programmable thermostat mainly for its swing setting and the ability to see the room and set temps simultaneously. I am going to return it for two reasons:
- Upon installation, one of the top plastic buds broke that holds the thermostat to its base. There are 4 plastic buds that snap onto the base, they are extremely small and not molded consistently----they all look different which makes the connection to the base unreliable. The face has to be opened each time you turn the furnace on/off, so maybe 4-6 times in a weekend. The face hinges downward and it takes so much force to unsnap the top of the face that it almost pulls the thermostat from the base, especially when one on the buds is broken. Good function, horrible physical design.

YES......when set point on t-stat has been reach, t-stat contacts open, burner shuts down and then fan continues to run until timed delay relay coil cools off shutting down fan power.

The black wire looks to be an added 12V DC power to '?????' that comes on when furnace fan gets power.
The gauge of wire and the blue crimp connector don't look OEM

My AF has a basement circulating fan. Wouldn't be surprised that's what the black wire is for.

BTPO1 wrote:
It sure would be nice to know where the added black wire is going since it is only powered when the heater is on. Maybe a 12V relay for an extra fan/blower motor?

I appreciate the responses. I did not have any responses as of last night so I went ahead and connected the white/black wires to the W1 terminal on the new thermostat. The furnace works. However, when I turn the temp down to cancel the call for heat, the furnace continues to blow the same hot air until the fan eventually cuts off. I sure thought I remembered this furnace cutting off the burner when the call for heat is cancelled and as the fan continues to run, the air would gradually cool down, then the fan would cut off. Isn't that normal?

The black wire is a bit of a mystery since everything appears to be stock on the camper (I'm the 3rd owner). I

It sure would be nice to know where the added black wire is going since it is only powered when the heater is on. Maybe a 12V relay for an extra fan/blower motor?

The white wire (to furnace) is 'power' not a ground.
12V DC comes thru Red then when contacts are made up 12V DC goes thru yellow to white. From t-stat power via white wire goes to timed delay relay.

So whatever that black wire (not OEM---somebody added it) goes to it is getting 12V DC when t-stat calls for heat. (Blue crimp with white wire and black together/size of black wire not OEM)