furnace started blowing air that is colder than it should be. On my normal setting(23.5 degrees C) it blows VERY cold air but when I crank it up to 28 degrees or so its blows my normal air. When I looked at the furnace the flame looked normal, and the pipes leading out of the unit were hot. The outside temperature right now is around -26 degrees + windchill. Any ideas why this is happening?