A salesperson comes to the door selling a service designed to check “chimney rot”. He explains that if the CO2 content of the gases leaving the chimney rises above 15%, it is dangerous to your health,...

A salesperson comes to the door selling a service designed to check “chimney rot”. He explains that if the CO2 content of the gases leaving the chimney rises above 15%, it is dangerous to your health, is against the city code, and causes your chimney to rot. On checking the flue gas from the furnace, he finds it is 30% CO2. Suppose that you are burning natural gas which is about 100% CH4 and that the air supply is adjusted to provide 130% excess air. Do you need his service?