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
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?