Who mis installed the furnace??? Or is this a recalled Exchanger if so Lennox should float the bill.. any reputable installer will fes up to his goof in mis-instalations and cut you a break. $700 bux for 3-5 hours of labor NO WAY!
mis-install problems that lead to premature exchanger failure. assuming a 90% effecient furnice.
1. gas valve pressure not set.
2. restriction in venting.
Contact Lennox ask for :
1. warrenty info
2. recalls on you unit.
3. the tech history of your unit and cause premature Heat Exchanger failure.
4. is the Service man a Lennox authorized service tech? just because they have a sticker dont mean they are trained!