When should a furnace be replaced?

My gas furnace/AC was 26 years old and both still working fine when I replaced them. I am not suggesting you do the same, but I am in the biz and therefore would not be held hostage should either of them have failed during peak season. That said, since you are asking you must have some concerns regarding your own system so I will say dont wait for a catastrophic failure and be forced to go with the only contractor who will give you immediate service, because you will be living with that purchase for the next 15-20 years and youll probably over pay for whatever that guy wants to sell you. If you have doubts, have your system looked at by someone you trust(ask around town or work for referrals). There is currently a 30% tax credit for the total installed cost of replacement of your equipment up to a maximum of $1500. If your new system meets the energy guidelines which arent that strict. Do a google search on AHRI tax credit. Anyway, youll know when it needs to be replaced, just be smart about it and do it before you need to.