A two stage forced air gas furnace is almost like having two furnaces in one. It has the ability to run at a low BTU output and a high BTU output depending on the needs of the home. This can save a considerable amount of energy, especially in the spring and fall. A two stage furnace works like this: your thermostat calls for heat and the furnace kicks on low. After a predetermined amount of time, if it has not satisfied the temperature, it kicks up to high until the temperature is satisfied. For this extra stage, you need to connect one additional wire to your furnace and your thermostat.
Things You'll Need
Access the control board on your furnace. Remove the access panels in front of the furnace to do this.
Strip the brown thermostat wire back one-half inch and connect it to the "W2" terminal on the furnace. This controls the second stage of heating. Verify that the red wire is connected to the "R" terminal, green wire to the "G" terminal and white wire to the "W" terminal. If not, connect them in this manner.
Put the access panels back into place and go to where your thermostat is located.
Pull the thermostat off the wall. Strip the brown wire back one-half inch and connect it to the "W2" terminal on the thermostat. Verify that the red wire is connected to the "R" terminal, green wire to the "G" terminal and white wire to the "W" terminal. If not, connect these also.
Push the thermostat back onto the wall plate. It should now be fully functional with two stage heating capability.
If you also have air conditioning, ensure the yellow wire is connected to the "Y" terminal on both the thermostat and the furnace. If you have two stage cooling, the light blue wire should be connected to the "Y2" terminal on both the thermostat and the furnace.