High-efficiency furnace cost

A high-efficiency gas furnace costs $6,000 to $12,000 installed, typically about $8,500. That buys a condensing furnace rated 96% AFUE or better, with new plastic venting through a wall or roof and a drain for the water it makes.

The thing that moves the price is not the furnace. It is how far the house is from being ready for one: where the vent can exit, where the condensate can drain, and what is left using the old chimney once the furnace stops using it.

What it costs

The gap between the two furnace rows is partly equipment and largely the venting and drain work a standard furnace never needs. The last row is there because of the water heater problem described further down.

What makes a furnace condensing

An 80% furnace burns gas, passes the hot gases through one heat exchanger, and sends what is left up a metal flue at a few hundred degrees. That leftover heat is what keeps the exhaust buoyant enough to rise up the chimney on its own, and it is the fifth of the fuel the furnace never delivers to the house.

A condensing furnace adds a second heat exchanger downstream of the first. It pulls enough heat out of the exhaust that the water vapor in it condenses back into liquid, releasing the heat it took to turn it into vapor. The exhaust leaves barely warm and wet, which a masonry chimney or a metal B-vent cannot safely carry, so a fan pushes it out through PVC or similar plastic pipe.

That is why there is no such thing as an 88% furnace on a chimney. Anything rated roughly 90% or higher is condensing and needs the plastic vent and the drain, whether it is labeled 92, 95 or 98. The price tiers inside the condensing range come from something else:

Single-stage. The burner is fully on or off. The bottom of the range.

Two-stage. A low fire for most winter days and a high fire for the coldest ones. Longer, quieter run cycles and steadier temperatures from room to room.

Modulating. The burner adjusts in small steps across its range, paired with a variable-speed blower. The top of the range, and worth it mainly in a house that is already well sealed and well ducted.

Venting: one pipe or two, and where it comes out

The new vent is the line that decides whether a quote is simple or not.

One pipe. The furnace draws combustion air from the room it sits in and exhausts through a single plastic pipe. Cheaper, but it pulls conditioned air out of the house and it needs a room with enough air volume, which a closet often is not.

Two pipes. One pipe brings outdoor air in for combustion and the other takes exhaust out. This is sealed combustion, and it is the better installation in most houses: the furnace no longer competes with the dryer and the range hood for indoor air. A concentric kit can run both through a single hole in the wall.

Where the pipes exit matters more than people expect. Manufacturers and most codes set minimum clearances from windows, doors, gas meters, and the ground or the snow line, and in winter the plume freezes on siding and drips onto whatever is below it. A basement furnace on an outside wall with nothing nearby is an afternoon. A furnace in the middle of a finished basement, or one whose only short route ends under a deck or beside a bedroom window, means a long run through framing or a roof termination, and that is the job at the top of the range.

Every model also has a maximum vent length and a limit on the number of elbows, and past a certain length the pipe has to step up a size. Ask the installer to show you the route before the quote is signed.

The condensate drain nobody budgets for

A condensing furnace makes several gallons of water on a cold day, and that water is mildly acidic. It has to go somewhere, every day, all winter.

A floor drain or laundry standpipe within reach is the easy case: a trap and a short plastic line by gravity.

No drain lower than the furnace means a condensate pump. It works, but it is a small motor in a wet location, and when it fails the furnace shuts itself off on a safety switch. Ask for one with an overflow switch wired to stop the furnace rather than flood the floor.

A furnace in an attic, garage or crawl space has a freeze problem. A line that ices over in January stops the furnace on the coldest night of the year. These installations need the line kept inside conditioned space or protected with heat tape, and some manufacturers restrict where their condensing models can go.

Many installers also fit a neutralizer, a small cartridge of limestone chips, so the acidic water does not eat at a cast iron drain line or a septic system over the years.

When the efficiency premium pays back

The savings are easy to estimate. Going from an 80% furnace to a 96% one cuts the gas burned for heat by about a sixth, because the same heat now takes 80 divided by 96 of the fuel. Take last winter's heating fuel cost, not the whole utility bill, and multiply it by 0.17. That is the yearly saving. Divide the price difference between your two quotes by it, and the answer is the payback in years.

How that comes out depends almost entirely on climate and fuel:

Cold northern climates. Long heating seasons, large fuel bills, and payback well inside the life of the furnace. Here the condensing furnace is usually the default, and local code may already require it.

Mixed climates. Payback is often close to the life of the furnace. A two-stage model can still earn its premium in comfort rather than in fuel.

Hot southern climates. The furnace runs a few dozen hours a year. The premium rarely returns, and the better question is whether a heat pump should replace both the furnace and the air conditioner.

Propane rather than natural gas. Propane costs more per unit of heat in most markets, so the same percentage saving is worth more and payback comes faster.

Retrofit gotchas

The orphaned water heater. If the old furnace shared a chimney with a gas water heater, the water heater is left alone in a flue sized for two appliances, which can condense inside the chimney and spill exhaust. The usual fixes, a liner or a new water heater, are covered in the furnace replacement guide. The one to add to that list is a tankless conversion, at $1,800 to $6,500 with its own venting, which retires the chimney for good.

An old furnace that was oversized. Furnaces put in decades ago were often far larger than the house needed. A replacement sized by a load calculation is frequently smaller, cheaper and quieter. Matching the old nameplate copies the old mistake.

Return air that cannot keep up. A two-stage or modulating furnace with a variable-speed blower is only as good as the duct it breathes through. Undersized returns make it noisy and keep it from reaching its rating. Enlarging a return trunk runs $10 to $35 per linear foot.

A worked example: an 80% furnace in a cold-climate basement

A two-story house in a northern state with a 20-year-old 80% furnace in the basement. The water heater is electric, so nothing else uses the chimney. The furnace sits near an outside wall, there is a floor drain beside it, and the single return duct is undersized.

• Condensing furnace with two-pipe venting and a condensate drain: $6,000 to $12,000
• Enlarged return trunk: 25 linear feet × $10 to $35 per foot = $250 to $875
• Total: $6,250 to $12,875

This is close to the easy case: a short vent route, a drain within reach, and no gas water heater left on the chimney. The return duct, at $250 to $875, is small next to what it does for a two-stage furnace's noise and airflow.

Against a standard furnace at $3,000 to $6,200, the premium is real money. In a northern winter with a large heating bill, the 0.17 rule usually puts payback well inside the furnace's life. In the same house in a mild climate, it would not.

Frequently asked questions

How much does a 96% efficient furnace cost installed?

$6,000 to $12,000, typically about $8,500, including the new PVC venting and the condensate drain. A modulating burner, a long vent route or an attic location push it to the top of the range.

Is a 95% furnace worth it over an 80%?

In a cold climate with a long heating season, usually yes. In a warm climate, usually not, because the furnace runs too few hours for a saving of about a sixth to repay the difference. Multiply your yearly heating fuel cost by 0.17 and divide the price gap by the result to see your own payback.

Can a high-efficiency furnace vent into my existing chimney?

Not directly. Its exhaust is cool and wet, and a masonry or metal chimney is not built to carry it. Some installers run the plastic vent pipe up through an unused chimney as a chase, where local code and the manufacturer allow it.

What happens if the condensate line freezes?

The furnace detects the backed-up water and shuts down until the line clears, often on the coldest night of the year. Lines in attics, garages and crawl spaces need to be insulated, kept inside the heated space, or protected with heat tape.

Related guides

• Furnace replacement cost — both efficiency tiers side by side
• Furnace repair cost — the faults to rule out before replacing
• Heat pump cost — the alternative where winters are short
• HVAC replacement cost — replacing the furnace and the cooling together