The burners of older natural gas furnaces usually have a round plate that can be rotated to either open or closed positions, allowing a variable amount of combustion air to enter the burner along with the gas.
If the plate is fully open, the resulting flames seem shorter, faster, uniform height, and uniform color (blue).
If the plate is fully closed, the flames are longer, slower, variable height, and more red in color.
It seems that usually the plates are rotated fully open.
Is it true that back when natural gas was cheap, these intake plates are usually set fully open to create a faster-moving combustion flow to help increase exhaust temperatures that would help to prevent chimney condensation, and that by closing the plates you are increasing the efficiency of the furnace by slowing the combustion flow and allowing more of the heat to be transfered to the heat exchanger instead of escaping out the flue?