Furnace Replacement Cost Calculator

Furnaces are sold by input and sized by output

A "100,000 BTU furnace" is 100,000 BTU of gas going in, not 100,000 BTU of heat coming out. At 80 percent AFUE it delivers 80,000 to the ducts; at 96 percent it delivers 96,000. The number that matters for sizing is the output, because that is what has to match the house's heating load, and the number that appears in the model name is the input. A quote that lists "100K furnace" without saying which one it means is ambiguous by a fifth.

The practical consequence is that the same house needs a smaller input as efficiency rises. A house that needs 60,000 BTU of output takes a 75,000 BTU input at 80 percent, a 67,000 at 90, and a 63,000 at 96. Furnaces only come in round sizes, so the arithmetic usually lands on the same cabinet anyway — but it explains why a high-efficiency quote can look like it is proposing "a smaller furnace" when it is proposing the same heat.

The 80 versus 90 decision is a venting decision

An 80 percent furnace vents into a metal flue, and its exhaust is hot enough to rise on its own. A 90-plus condensing furnace extracts so much heat that the exhaust is lukewarm steam, which cannot rise up a chimney — it has to be pushed out through PVC pipe with a draft inducer, and the condensate it produces has to be drained somewhere, usually a floor drain, a condensate pump, or a neutralising cartridge because the water is acidic.

That is where the money goes. If the house already has a masonry chimney in good condition, an 80 percent furnace slots in for a few hundred dollars of flue work. If it needs a liner, that is $700 to $1,500 on its own, and if the chimney is on an exterior wall it may be too cold to work properly at all. Conversely, if the old furnace was already condensing and there is PVC in place, the 90-plus unit is the cheaper install as well as the more efficient one.

So the honest comparison is not "10 percent more efficiency for $800". It is: what does the flue route cost in this house, and does the condensate have somewhere to go. In a house with a good interior chimney and a basement floor drain, 90-plus wins on both counts. In a house with an exterior chimney, no drain, and a furnace in a closet, the venting bill can exceed the entire efficiency benefit.

What a furnace replacement actually includes

The furnace itself — $900 to $1,800 at 80 percent depending on size, plus $700 to $1,100 for condensing versions.
Venting — a metal flue connection, a PVC kit, or a liner, from $180 to $1,500.
Gas piping and shutoff — usually minor, occasionally a new sediment trap and connector.
Electrical — the existing circuit is usually reused; a new 120V circuit adds $300 to $500.
Plenum transitions — the sheet metal that connects the new cabinet to the existing ducts, which almost never lines up exactly.
Commissioning — gas pressure, temperature rise against the rating plate, and a combustion check. This is the step that catches an oversized furnace before it overheats the exchanger.

Do not let the tonnage be upsized "for safety"

The most common furnace mistake is installing the next size up because the old one was that size. An oversized furnace heats the house quickly and shuts off, which creates temperature swings and — more seriously — cycles the heat exchanger through more thermal stress than it was designed for. It also runs at a lower temperature rise than the rating plate specifies, which is a commissioning failure, not a comfort preference.

Two-stage and modulating furnaces absorb oversizing much better than single-stage, because they can run at 60 percent or less on mild days. If the load calculation lands between sizes, that is the argument for paying for two-stage rather than for going up a size.

Cost per year, not cost up front

At $1.10 a therm, a 60,000 BTU output furnace running 1,800 equivalent full-load hours a year burns about 675 therms at 80 percent AFUE and about 600 at 90. That is 75 therms, or roughly $82 a year. The condensing premium is $700 to $1,100, so the efficiency alone pays back in nine to thirteen years — around the life of the equipment, which means the decision should be made on venting, comfort and the house rather than on a payback that barely closes.

In a cold climate with 3,000 hours and expensive gas the arithmetic improves; in a mild climate it does not. Run your own numbers on the heating cost page before accepting a claim that a condensing furnace "pays for itself".

What this estimate leaves out

Ductwork, air conditioning, and any electrical service upgrade. It also assumes the existing plenum can be adapted, which is true most of the time and false often enough to matter in a house with a strange duct layout. And it does not include the federal 25C credit, because that credit ended for equipment placed in service after December 31, 2025 — a 2026 furnace gets nothing from the IRS, whatever a contractor's paperwork may still say.

Frequently asked questions

How much does a furnace replacement cost in 2026?

Most installed prices run $4,000 to $8,000 for a standard 80 percent furnace, and $6,000 to $11,000 for a 90-plus condensing unit, depending on size, venting and region. The venting route is the single biggest swing factor between two quotes for the same furnace.

Is a 90% furnace worth the extra money?

It depends on the flue and the climate. In a cold climate with an interior chimney, a usable condensate drain and a $900 premium, the efficiency pays back in about a decade and the venting is easier. In a mild climate, or where the chimney needs a $1,200 liner, it often does not.

Can I vent a 90% furnace through an existing chimney?

No. Condensing furnaces produce lukewarm, wet exhaust that will not rise, and the acidic condensate attacks masonry. They vent through PVC, either out a side wall or up a dedicated chase, and they need a drain for the condensate.

What size furnace do I need for 2,000 square feet?

Usually 60,000 to 80,000 BTU of output in a mixed climate, which means a 75,000 to 100,000 BTU input at 80 percent AFUE. Square footage is a weak guide; the load calculation is the answer, and oversizing is the common error.

Why is my new furnace louder than the old one?

Often because it is moving more air through the same ducts, or because the blower speed was set for cooling rather than heating. A temperature rise outside the rating plate range is the diagnostic, and the fix is usually a blower tap rather than a new furnace.

Should I replace the AC at the same time?

If the AC is more than 12 years old, yes — the labour overlaps and the coil match matters. If it is newer, replacing only the furnace and reusing the coil is normal, provided the coil is compatible with the new cabinet.

Is there a tax credit for a new furnace?

Not for equipment placed in service after December 31, 2025. Section 25C ended on that date. Utility rebates for high-AFUE furnaces still exist in some states and are worth checking.

Disclaimer: This tool prices a furnace replacement line by line — equipment, labour hours, venting, and permit — from published ranges. It is not a quote, and it excludes air conditioning, ductwork, electrical service upgrades beyond the furnace circuit, and any rebate you may qualify for.

Last reviewed: October 30, 2026 · Sources: published 2026 contractor price surveys, DOE furnace AFUE standards (10 CFR 430), ANSI Z21.47 venting requirements

Furnace Replacement Cost Calculator