Hi,
I installed a W/R 1f85-277 t stat for my trane xl90 furnace. The problem is that there is a switch on the t stat for either electric or gas heat. My furnace is natural gas. Do I need to set the switch for Gas or can I leave it on Electric? The furnace does work on the Electric setting but, my concern is that the blower comes on when the burners light up. With my old t-stat( another W/R) the burners lit and then the blower came on.

The other thing is that in order to not to have to keep replacing the batteries every year there is a terminal for the Common. I have only four wires coming through the wall and I have connected them all correctly to the right terminals. How do I connect to the Common terminal to give the t-stat power all the time?

Thanks

Do I need to set the switch for Gas

Yes switch to gas if you have a gas furnce.

How do I connect to the Common terminal to give the t-stat power all the time?

Run a extra wire from the common furnace transformer wire to the C terminal of the thermostat.
Read below

http://www.white-rodgers.com/wrdhom/pdfs/06_Cat_pages/Cat_06_pg0180.pdf


http://www.white-rodgers.com/wrdhom/pdfs/instruction_sheets/0037-6560.pdf