Furnace replacement cost

A gas furnace replacement costs $3,000 to $6,200 at standard efficiency and $6,000 to $12,000 at high efficiency. Most homeowners replacing like for like pay around $4,500.

The gap between those two tiers is wider than the equipment prices suggest, and the reason is venting. It is the thing to understand before you compare quotes.

Standard against high efficiency

Standard efficiency, 80 to 89% AFUE. Eighty cents of every dollar of gas becomes heat. The remaining hot exhaust goes up a metal flue, usually the same chimney the water heater uses. Installed, $3,000 to $6,200.

High efficiency, 96% AFUE and above. These are condensing furnaces. They pull so much heat out of the exhaust that the flue gas turns cool and wet, which metal flues cannot handle. So the furnace vents through PVC pipe out a side wall, and the condensate it produces needs a drain and often a small pump. Installed, $6,000 to $12,000.

That venting work is why the installed gap is $3,000 or more when the equipment gap is closer to $1,500. There is a middle tier at 90 to 95% AFUE, installed around $5,200 to $8,800, which is also condensing and carries the same venting requirement.

The orphaned water heater

This catches people out. If your furnace and water heater share a chimney, and you replace only the furnace with a condensing model, the water heater is left venting into a flue sized for two appliances. Too large a flue for one appliance means the exhaust cools before it reaches the top, condenses inside the chimney and corrodes it, and can spill combustion gases back into the house.

The fix is either a chimney liner sized for the water heater, $600 to $1,800, or replacing the water heater with a power-vented or electric model at the same time. A good contractor raises this before you ask. It should appear on the quote.

Does high efficiency pay for itself

Sometimes, and the arithmetic is easy enough to do yourself.

Going from 80% to 96% AFUE cuts gas use for heating by roughly 17%. If you spend $1,200 a year on heating gas, that is about $200 a year. Against a $3,000 premium, payback is fifteen years, which is most of the furnace's life.

If you spend $2,500 a year on heating, as many northern households do, the saving is $400 a year and payback is under eight years. That is a straightforwardly good decision.

So the honest answer is that high efficiency is worth it in cold climates with long heating seasons and expensive gas, and hard to justify in mild ones. Ask what you actually spend on gas in January before deciding.

What else lands on the bill

• Chimney liner where the water heater is orphaned: $600 to $1,800
• New PVC venting through a side wall: usually included, $300 to $900 when quoted separately
• Condensate pump where there is no gravity drain: $150 to $400
• New gas shutoff and sediment trap: $100 to $250
• Return air modification where the old furnace was undersized for airflow: $300 to $1,200
• Permit and inspection: $100 to $400

When to replace rather than repair

Furnaces run 15 to 20 years, sometimes longer. The signals that point to replacement rather than another repair:

A cracked heat exchanger. This is the one non-negotiable. A cracked exchanger can put combustion gases into the air you breathe, and the part alone on an out-of-warranty furnace usually costs more than most of a new one. A technician who finds one will red-tag the furnace, which means shutting off the gas.

Age past fifteen with a major part failing. An inducer motor or control board on a 17 year old furnace is $500 to $900 spent on a machine likely to need the next thing within two years.

Short cycling that survives a service call. A furnace that starts and stops every few minutes is either badly oversized or failing. If a technician has already checked the airflow and the flame sensor, the answer is usually the equipment.

Yellow rather than blue burner flame, or soot. Both indicate incomplete combustion. Have it looked at immediately rather than waiting.

Frequently asked questions

How long does a furnace replacement take?

A straight swap in an accessible basement or closet is four to eight hours. Add a day when a condensing furnace changes the venting, when a chimney liner is installed, or when the furnace is in a tight attic or crawl space.

Should I replace the air conditioner at the same time?

If it is within a few years of the furnace's age, usually yes. They share the indoor coil, and doing both at once avoids paying twice for access and for the refrigerant work. Doing them together typically saves 10% to 15% against two separate jobs.

Is an electric furnace cheaper?

The equipment is cheaper and the installation is simpler, because there is no gas line and no flue. Running it is usually far more expensive: electric resistance heat converts electricity to heat at 100% efficiency, but electricity generally costs three to four times more per unit of heat than natural gas. Where there is no gas service, a heat pump is almost always the better answer than electric resistance.

What does AFUE actually mean?

Annual Fuel Utilisation Efficiency: the share of the fuel's energy that becomes useful heat over a heating season. An 80% AFUE furnace turns 80 cents of every gas dollar into heat in your house and sends the rest up the flue.

Do I have to use the same brand?

No. Furnaces are not tied to the outdoor unit the way a coil is, though matching brands can simplify warranty claims when both halves are replaced together. Pick on installation quality first: a well-installed mid-tier furnace outlasts a badly installed premium one.