I moved here 5 years ago. The house was built in 1984. The furnace is a Nordyne 90+ AFUE (I think).
I need to know how old it is and if it is considered efficiant by todays standards.
I just spend $300 to have a pressure switch replaced. 2 years ago I had trouble with condensed water backing up into the exhaust motor...about $400 before we got the trouble stopped for good.
The model # & serial # should be on the inside of the access panel, which you can use when you call Nordyne to find out what year & month it was made. I couldn't find anything on the web telling me how to decipher their numbering system.
While I don't know anything about Nordyne personally, if it is a 90+ as you believe, that tells me that it's 90% efficient, which is pretty good.
The only other recommendation I can make is to have regular servicing, which should keep it operating at peak efficiency.