I have a 35 yrs old furnace and I would like to replace it. I have 2045 sqft living space with 11 vents. How many BTU is required. The contractor suggested to input 80K, and an output of 74K BTU, the furnace AFUE 92% return. Any suggestions please.

Yes have the contractor do a manual J for your house. It will tell you exactly what you need instead of guessing.

If you can, get three contractors to price out the job. And don't tell them what you need, let them tell you. Many times I have seen a homeowner TELL the contractor what they want rather than letting the contractor determine their needs and wind up dissapointed with the outcome. Get references, ask around, make sure you know who you are dealing with. Most HVAC contractors are reputable, but there are some who will sell you a bill of goods rather than what you need.

Most reputable contractors will listen to what you want, and then tell you what they think you need. Prices will vary from contractor to contractor depending on what brand furnace they carry. I would listen mostly to the BTU suggestions and go from there. All BTU suggestions should be very close. Also if you ever plan on adding an addition, this information may be helpful to the contractor so they can oversize accordingly.