An interesting note... a gas furnace can be "tuned" in much the same way that an oil furnace is tuned with a Bacharrach tester. The test fluid for a gas furnace is green, and checks for oxygen (as opposed to CO2 in oil furnaces). Perhaps one in every 100 HVAC companies owns the "green Bacharrach gas kit", and maybe someone that owns one actually knows how to use it. Almost everyone (HVAC wise) owns and uses the red kit to tune an oil furnace.
"Nothing is foolproof to a sufficiently talented fool"