I have installed an add on corn furnace to an existing oil fired forced air furnace. I would
Like to control the fan on the oil furnace with the thermostat of the corn furnace. Oil furnace is 24volt 4 wire. The corn furnace is millivolt (I believe) 2 wire. What I'm looking for is to have the oil furnace fan run when the corn furnace is calling for heat. Also not disrupting the normal operation of the oil furnace. The oil furnace uses a honeywell fan
Limit/control on it if this helps.
Thanks

If your furnace is an older one that uses a limit switch to activate the coil of the fan relay, all you need to do is buy another limit switch, and instal it on the corn furnace. Run a wire from the W terminal of the furnace to the limit switch, and another to the coil of the relay. The limit switch in the furnace will keep the power from backfeeding into it.

If your furnace is only 10 years old, it may be more complicated. You need to start with the R terminal. The W is only hot when the thermostat calls for heat. You need to have the 24 volts from the transformer flow from the R terminal, usually a red wire, to a limit switch open when cold, NOC, mounted near the fire box where it will warm up and close soon after the corn furnace thermostat calls for heat. From the switch, it needs to flow to a coil of a relay controlling the fan. If that relay is on a circuit board, and power to the coil switched by logic, not the limit switch, it will be messy enough that you may do better to install another relay.

Check the name plate on the motor for its amps, maybe 5-10. You need a relay rated for that amperage or higher and 120 volts with a 24 volt coil. You may have to hit an electrical or HVAC supply. Or Radio shack may have them. The nice thing about Radio shack is that they won't hassle you about the furnace model number or anything else except do you want a cell phone. The relay has 4 terminals, line, load, and 2 coil terminals. The wire from the limit switch goes to the one coil terminal, and the other one goes to the other terminal of the transformer. Connect power from the house to the line terminal, and the lead to the blower to the load terminal. These connections will be in addition to existing connections. If either the limit switch or relay confuses you with a NCC terminal, tape over it and ignore it. It will be hot when the switch is cool. If the Honeywell Fan and Limit Control, part #L4064B, you have, has 3-4 terminals, it already has the relay you need. If so, check the coil voltage, it could be 120.

To protect you from any mistake I may make, I am also posting this to the H&C forum. The other members and your common sense are your only protection from bad advice. Nobody passes a test to post here. If I am right, it can help other too.