In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
We all play the game of risk. A unit operating with o2 & CO2 in the groove can kill you if you block the chimney, rot out the HX, depressurize the CAZ or any number of things. However, as the potential goes up, society places greater constraints on us. Look at the building codes. As you place hazardous materials in buidlings, the construction and safety features such as sprinklers and forced ventilation escalate. Left alone in containers, these materials may not represent a direct hazard but the codes still require extra protection just in case. So too with combustion appliances...............to a degree.
Explain to me why a gas direct vent fireplace cannot exceed 25ppm air free flue gases while a ventfree gas oven gas hit 800ppm air free. The one more protected against negative forces such as depressurization and wind gusts is slapped with an unfair requirement. Meanwhile, you thought it was the tryptophan from the turkey making you sleepy at Thanksgiving. So, why not lockout the oven at 799ppm? It's all a game we play. The gas utilities have their own tariff with the State that empowers them to do what they want. They can come in and shutdown anything they feel like or conversely ignore whatever they feel like based upon the Just Cause principle, which states they can do what they want Just Cause they feel like it.
Which are you going to TagOut & Lock Out: a furnace that passes a Worst Case Depressurization Test with -5 Pascals in the CAZ and a stack air free CO of 250 or a Cat. I fan assisted furnace with a stack are free CO of 12 but it is common vented with a draft hood equipped water heater. Which poses the greater immediate threat to the homeowner if both vent into masonry tile 'lined' chimneys?