A furnace quote covers considerably more than a furnace. Understanding the parts is how two numbers become comparable.
The components
The furnace itself, by input BTU, AFUE and staging.
Labour — removal, disposal, setting, connection, startup.
Venting. On a like-for-like 80% replacement this is minor. On a conversion to a condensing furnace it is a new PVC run, a termination, and often chimney work for the orphaned water heater.
Gas line. Sometimes reused, sometimes resized if the input changes materially.
Electrical. A dedicated circuit and a service switch, per code.
Condensate. Drain, trap, neutraliser, and a pump where gravity does not reach a drain.
Duct transitions. A new furnace rarely matches the old cabinet dimensions exactly, and the plenum connections are fabricated.
Permit and inspection.
Thermostat, if the staging changes and the existing one cannot call for it.
Where two quotes legitimately diverge
80% versus 96%. Not just the equipment price — the venting conversion, the condensate provision and the chimney remediation are all consequences of that choice.
Single-stage versus two-stage versus modulating.
Whether the air conditioning coil is being replaced at the same time. It sits on the furnace and it has to come off regardless; replacing an old coil while it is accessible is much cheaper than doing it separately later.
Duct correction. A quote that includes fixing a return that has always been undersized is not more expensive for the same job.
Chimney relining, which appears on one quote and not the other and is a real requirement rather than an upsell when the furnace leaves a shared flue.
What a low quote is usually leaving out
The permit. The chimney work. The condensate neutraliser. The duct transitions done properly rather than taped. And, frequently, a load calculation — with the replacement sized by matching the old unit.
None of those makes the low quote dishonest, and all of them make it a different job.
The item worth adding rather than removing
Duct sealing. Leaky ducts in an unconditioned basement, crawl or attic waste a larger fraction of the fuel than the difference between two efficiency tiers.
Done at the same visit, with the system already open, it is far cheaper than as a separate job, and it improves the performance of whatever furnace is installed.
Emergency versus planned
A furnace that fails in January is replaced in a day, from whatever is in the warehouse, at whatever the winter rate is, with no time for a load calculation or a second quote.
A planned replacement in spring or autumn gets the right size, the right equipment and a better price. For a furnace that is old but working, that difference is the strongest argument for not waiting.
What to insist on
A Manual J load calculation rather than a match to the old unit — heating loads are straightforward to calculate and the existing furnace is very likely oversized.
Sizing against output BTU, not input.
A permit, because the inspection is an independent check.
Startup readings on paper: gas pressure, temperature rise across the heat exchanger compared to the nameplate range, and static pressure.
That last set is what tells you whether the furnace was set up correctly, and it is the part that determines whether the equipment delivers what was paid for.
Financing and what it hides
Contractors commonly offer financing, and a monthly payment is a poor way to compare two quotes.
A longer term makes a larger system look affordable, and the interest is real money on a purchase that will be paid off over a substantial part of the equipment's life. Compare the cash prices, then decide separately how to pay.
Utility rebates and federal credits apply to the purchase regardless of how it is financed, and they are worth confirming before signing rather than after.
The warranty registration
Most manufacturers extend the parts warranty substantially — often from five years to ten — when the equipment is registered within a set window, commonly 60 or 90 days after installation.
Registration is normally the contractor's task and it is frequently forgotten. Confirming it costs one email and it is worth several years of coverage on the most expensive component in the furnace.