Efficiency and output answer different questions. AFUE describes fuel-use efficiency under a standard seasonal test; heating capacity describes how much heat the appliance can deliver. A higher efficiency rating does not establish the correct size for the house or resolve duct limitations.
The cabinet connects to a larger system. The supply plenum, return connection, filter arrangement and thermostat need to suit the selected model. If central air conditioning remains, the indoor cooling components and required airflow also need a compatibility review.
A different appliance can require a different exhaust route. Some replacements need new intake and exhaust piping or condensate drainage. Where other appliances share the old chimney or vent, their remaining venting arrangement needs assessment as part of the change.
Startup completes the installation. The final stage includes checks of operation, airflow and controls, with combustion and venting checks for fuel-burning equipment. A walkthrough should explain how to operate the thermostat and identify the documentation supplied with the new furnace.