You have a quote in hand for a gas furnace replacement. The equipment line is clear. The venting line and the gas line are not, and they are where many totals change. This is the walk-through for those two lines.
Start with what stays and what changes
Ask the bidder to say, in writing, whether the existing flue will be reused or replaced. A reused flue is a different job from a new one. Replacing a flue may mean a new pipe run, new fittings, and new roof or wall penetrations. Ask for the vent material named on the line, not just a note that venting is included.
Look for the reason behind each line. A new unit may have a different venting requirement than the old one. If the new furnace must vent through a different route or in a different material, the change belongs in the quote with its cause written beside it.
Check the liner and the condensate route
If the furnace is a high-efficiency model, ask whether a condensate drain is included and where it goes. Ask whether a liner is needed in the chimney, and who decides that. These are common lines that sometimes appear only after the crew opens the job.
Read the gas line the same way
Ask whether the existing gas pipe is being reused, and whether the bidder has checked it against the new unit's requirements. A line that is too small for the new equipment is a fix someone has to pay for. Ask who performs the pressure check and whether it is listed as a separate item.
Ask whether the gas connection and the leak test are in the trade's scope. If they are, they should be named. If they are not, ask who is responsible for them.
Ask these before you compare totals
Put the questions in a short list and send the same list to each bidder. Ask: Is the flue reused or replaced? What vent material is quoted? Is a liner included? Is the condensate route written? Is the gas line checked, and is that check priced? Two quotes with different answers are not yet comparable, even when the totals look close.
For the typical service life of the furnace itself and when to plan a replacement, see Gas Furnace: typical service life and when to plan. The published figures on this site live on those pages, not in individual quotes.