at 5:33 he states that the 1955 lennox furnaces was fairly efficient, I beg to differ, it's 2016, those are atmospheric gas burners, the units may measure an AFUE of @70-80% but technically those are probably much lower because AFUE is not realistic, add that given the age many of them probably have heat exchanger issues and could be leaking CO into the living spaces and those should have been changed out 30 years ago, at best they should have had new units in 75, 95, and 2015. 20 years is a good run on residential HVAC equipment, over 50 is dangerous.