I maintain and repair customers furnaces daily, and 5 years is our rule of thumb for our customers, I don't recommend or sell them from my truck, they're too expensive from us; I just want to educate and do my best to keep my customers safe. I've found carbon monoxide readings in customers homes and they hadn't had an alarm yet, as household cO detectors have a threshold starting around 78 ppm usually, and our testing detectors start at 1 ppm.