I have recently installed a wood add-on furnace to my oil furnace. The way it works is a trunk line comes from oil furnace plenum(heat) to bottom of wood add-on, then all duct work for house is connected to plenum on wood add-on. The problem I'm having is getting the wiring for wood add-on correct. The wood add-on has it's own temperature control (I think that's what it's called-unit for turning fan on when furnace reaches a certain temp), it has a manual door -for putting in wood, has an electronic draft control-which is operated by a little one way motor-2 wires coming out of it. What's happening is the thermostat works in reverse. When the thermostat calls for heat the draft goes closed and when it reaches temp the draft opens. The temperature control system works perfect. When the furnace reaches temp it turns on furnace fan and when it reaches to hot of a temp the limiting switch shuts down the draft. Here's the way I have wired it up (I have no owner's manual as wood add-on was given to me), hot wire is connected to hot side of controller, which powers fan control and limiting switch. Neutral to neutral. Fan wire leads back to oil furnace-no problems here, it works fine. Limiting switch goes to black wire on 24v transformer and white wire from transformer goes to neutral. No problem with this, it works fine. The step down tranformer and unit for thermostat are all one-no separate transformer. The unit has a "c", "o", "d", and two "t" marked on it. I have thermostat white on "c", thermostat red on "o", draft control on "c" and the other draft control on "d". When limiting switch kicks in it shuts draft down, I just can't figure out why thermostat works in reverse? The terminals marked "t" never seem to have any power on them no matter which way the magnetic switch inside the unit is on. I purchased a cheap heat only thermostat and transformer unit is a White Rogers(no number). Any help would be greatly appreciated.

I am having trouble sorting out all the parts of the system. Are you saying when the thermostat calls for heat, it should start the draft blower? The limit switch then shuts the draft off and the blower continues as long as thermostat calls for heat? Is it the thermostat that has the C, O, D, and T terminals? If so, you must go through the directions that should have come with it, and determine what they mean. Those are not the standard terminal markings for thermostats. A simple heat only thermostat should only have 2 terminals. One is power from the transformer, and the other is switched to it when the temperature is below the set point, and returns to the source of heat, activating it. I may be able to help you figure it out, but I need more information.

Yes, when the thermostat calls for heat the draft should open(it's not a blower it's just a little one way motor with 2 wires coming out of it. When it's powered it moves one way(open draft) when power is taken off it closes with gravity), instead it closes. The limit switch overrides the thermostat and should shut the draft if the plenum heat exceeds the set temp-which it does. It's not the thermostat that has the c,o,d and t terminals, it's the transformer unit that has these. The thermostat is just a cheap heat only with two terminals (white and red). I don't have the furnace instructions-so I need to know where to connect draft wires and thermostat wires in order to get the two to work together. Right now I have white thermostat wire connected to "c", red thermo wire to "o", one wire for draft to "c" and other draft wire to "d".

Labman, I should also add that I have a programmable thermostat that runs oil furnace and the heat only thermostat I purchased to run add-on. I didn't change anything with oil furnace I just added wiring from add-on's fan wiring over to oil furnace so fan would come on when add-on's temperature switch kicks in. Inside white rodgers control unit: "c" terminal has a blue wire coming from transformer, "o" terminal has a red wire coming from one side of magnetic switch, "d" terminal has a yellow wire coming from other side of manetic switch and the two "t" terminals have a red wire coming from bottom of magnetic switch but they are connected to a point system (points that open and close). Also other side of point system has a yellow wire coming from transformer. Hope this makes sense?

Can you tell what any of the terminals on the transformer unit connect to inside it? Usually you have one terminal of the transformer connected to the thermostat by a red wire. When it calls for heat, it switches power to the white wire which connects to whatever needs power to produce heat, in this case the draft motor, which then will have a second wire going back to the other terminal of the transformer. Identify the terminals connected to the transformer. Route one through the thermostat to the motor. Route the other through the limit switch to the other connection to the motor. The motor should then keep the draft open anytime both the thermostat and the limit switch is closed. One of the mystery terminals may already be connected to the limit switch.

Labman, I don't know how to thank you enough. I connected red thermo wire to red transformer wire, white thermo wire to one wire on draft motor, other draft motor wire to transformer and everything works like it should. Thanks again for taking the time to help!