A gas furnace swap touches more of the house than the unit itself. The venting, the gas line, the air supply, and the drain all have to match the new equipment. Settle these questions before you agree to the scope of work.
What happens to the vent?
Ask whether the existing flue or vent pipe will be reused, lined, or replaced. The new unit may have different venting needs from the old one. Ask about the pipe material, the slope, the length of the run, and where it terminates outside. Ask the installer to write the vent path into the scope.
Can the gas line carry the new unit?
Ask for the gas load the new unit needs and whether the existing supply line and meter can carry it. Ask where the shutoff sits and who checks the connection. If the line needs work, ask who does it and whether that work appears on the same scope.
Is there enough combustion air?
Ask how the installer will confirm combustion air for the space where the unit sits. The room may need an opening, a duct, or a clear path for air. Ask whether the closet or utility room has changed since the old unit went in.
Where does the condensate go?
If the new unit is a high-efficiency model, ask where the condensate drain will run and what it connects to. Ask whether a pump is needed if the drain sits above the outlet. Ask whether the trap and drain line will be checked before the crew leaves.
What does the duct and filter path look like?
Ask whether the existing ducts suit the airflow of the new unit, and whether the filter rack and return air path will change. Ask who confirms the air balance when the job is done.
What goes in writing?
Ask for a written scope that lists the vent work, the gas work, the electrical work, the thermostat wiring, the permit, and the removal of the old unit. A verbal promise does not tell you what was done. Compare the written scope against the questions above. Any gap is something to raise before the work starts. For typical service life and planning, see Gas Furnace: typical service life and when to plan.