I'm about to purchase a replacement thermostat The thermostat currently installed contains a mercury sw, temp sel slide lever at the bottom, heat / off / cool sw on rt side and auto /on fan sw on lft side. The existing connection is.. blue wire to grn, wh to wh, red to red and blk to yel. The wires at the furnace terminal connect the same way except blk goes to C. The system is a carrier weather maker 8000. My question is what replacement to buy without all the bells and whistles but with day night settings.