Does anyone know what the CO ppm limit is when a contractor should tell the homeowner to shut the furnace down? I'm from the mid-west and was told to call the local fire dept and they told me to call the gas company. The gas company didn't get me anywhere. I have a CO tester and I know that 100 ppm is where it starts getting dangerous but is there some rule to go by that a local gas company will red tag a furnace???