Following the sale of his furnace business, Dave Lennox continued to manage the Lennox Machine Company in Marshalltown, employing more than 100 people locally in the manufacture of portable gasoline engines, boilermakers' tools, wagon scales and pressured pipe taps. Lennox sold the Lennox Machine Shop in 1912 to the Ryerson Brothers of Chicago for $110,000, but continued to work in a small machine shop behind his Marshalltown home during his subsequent retirement.
Lennox died at his home in Marshalltown on February 15, 1947, at the age of 91. Since 1972, two actors have portrayed Dave Lennox in advertising campaigns for Lennox-branded air conditioners, furnaces, and indoor air quality products. From 1972 until his death in 1986, Lennox was portrayed by Hollywood actor Bill Tracy, whose photo still appears on many Lennox dealer trucks and advertising. Tracy was originally hired because he had hands that looked worn like a real working man. Lennox is currently portrayed by Bob Tibbets, a St. Louis-based actor and radio personality.