Furnace Cost Calculator

What a furnace costs over its whole life

A furnace bill has two halves. One is the check you write on installation day: equipment, labor, a permit, and sometimes new venting. The other is the money that leaves your account every winter for fuel and service. Most shoppers compare only the first number, which is how a cheap unit ends up costing more by year eight than an efficient one.

The calculator on this page puts both halves together. You enter the heat your home needs, the fuel you are pricing, the AFUE rating of the unit, local fuel prices, and the quote you were given. It returns fuel use per year, annual cost, installed cost, and a total for the years you plan to stay in the house.

Two cautions before you read the output. The heat figure is the heat that reaches the rooms, not the input rating on the furnace label. And every price in the tool is an input, so replace the defaults with your own quote and your own fuel bill before you trust a comparison.

The formulas behind the numbers

Fuel is sold by unit, and each unit holds a known amount of energy. Natural gas is billed in therms, propane and heating oil in gallons, electricity in kilowatt hours. AFUE is the share of that energy that becomes heat inside the house instead of going out the flue.

Fuel units per year = heat delivered in BTU / (BTU per unit x AFUE). Annual fuel cost = fuel units x price per unit. Cost per MMBtu = annual fuel cost / heat delivered in MMBtu.

The BTU values match the conversion tables published by the US Energy Information Administration. The AFUE definition, useful heat output divided by fuel energy input over a full season, comes from the test procedure described in Department of Energy guidance on residential furnaces.

Work one line by hand to see the shape of it. For 60 MMBtu of delivered heat with a 90 percent furnace: 60,000,000 / (100,000 x 0.90) = 667 therms. At $1.50 per therm that is 667 x $1.50 = $1,000 for the heating season.

What goes into an installation quote

A furnace swap is not a box that drops into place. The quote usually covers the unit, the labor to remove and reconnect it, a permit and inspection, and any changes to the flue, gas line, electrical supply, or duct connections. Ask for those lines separately, because two bids that differ by $2,000 are often differing on venting or duct work rather than on the furnace itself.

• Equipment. A single-stage 80 percent unit and a two-stage 96 percent unit are different products with different price tags. Match the model number to the AFUE you type in.
• Venting. Condensing furnaces vent through PVC pipe and need a drain for condensate. An 80 percent furnace uses a metal flue. If your house has no route for PVC, that is extra labor.
• Gas line and electrical. A larger input rating can need a bigger supply line. A new circuit may be needed for the blower and controls.
• Permit and inspection. Local fees vary, and skipping the permit can complicate a future sale.
• Duct work. Replacing a furnace is the moment to fix crushed or disconnected runs, since the system is already open.

Installation quality also shows up in the fuel bill. A unit that is not commissioned, with the wrong gas pressure or a blower speed set for the wrong duct system, will not deliver its rated AFUE. Ask the contractor for the commissioning numbers, not just the model number.

How AFUE changes a gas bill

AFUE is a percentage, so the math is direct: a 95 percent furnace needs less gas for the same heat than an 80 percent furnace. Take 60 MMBtu of delivered heat at $1.50 per therm. An 80 percent unit burns 60,000,000 / (100,000 x 0.80) = 750 therms, which costs 750 x $1.50 = $1,125. A 95 percent unit burns 631.6 therms and costs 631.6 x $1.50 = $947. The gap is $178 in one winter, or 15.8 percent of the older unit's bill.

The same 60 MMBtu season priced at $1.50 per therm, straight out of the calculator at four AFUE levels:

Over 15 winters that gap adds up to about $2,665. If the high-efficiency unit carries a $1,800 premium, the simple payback is 1,800 / 177.63 = 10.1 years. That is inside the life of the equipment, but not by a wide margin, so the decision turns on how long you stay and whether a rebate closes part of the price gap.

Efficiency thresholds for 2026 are worth knowing before you shop. ENERGY STAR furnace criteria require 97 percent AFUE in the northern region, 95 percent in the southern region, and 87 percent for oil furnaces. Federal minimum standards from the Department of Energy sit lower, at 95 percent in the North, 90 percent in the South, and 85 percent for oil.

One limit matters here. AFUE is measured at the furnace, so it excludes duct losses and the electricity the blower uses. A 96 percent furnace attached to leaky ducts in a vented attic does not deliver 96 percent of its fuel energy to the rooms.

Gas, propane, oil and electric side by side

Fuel prices move, so the ranking can flip between regions and between years. Run the same 60 MMBtu through four fuels with the defaults in the calculator: gas at $1.50 per therm, propane at $2.60 per gallon, heating oil at $4.00 per gallon, and electricity at 17.3 cents per kWh, with a 90 percent furnace for the three combustion fuels and 100 percent for resistance heat.

For scale, the EIA Electric Power Monthly puts the 2025 US average residential electricity price at 17.30 cents per kWh, which is where the default comes from. EIA data also put the 2025 average residential natural gas price at $15.34 per thousand cubic feet, or about $1.48 per therm once you allow for the energy in a thousand cubic feet. Heating oil is the volatile one: the EIA weekly residential survey for the week of October 12, 2026 showed a US average of $6.14 per gallon, well above the $4.00 used in the example, so oil heat in that market costs far more than the table shows.

A heat pump changes the comparison, because it moves heat instead of burning fuel. At a coefficient of performance of 3 and 17.3 cents per kWh, one MMBtu of delivered heat takes 97.7 kWh and costs $16.90. Gas at 90 percent AFUE costs $16.67 per MMBtu at $1.50 per therm. The two land close together, which is why the heat pump operating cost calculator is worth a look before you replace a working furnace.

Sizing and the cost of the wrong unit

An oversized furnace heats the house quickly, shuts off, and repeats. Short cycles leave rooms uneven, use more blower electricity per unit of heat, and put more wear on the heat exchanger. An undersized unit runs all winter without reaching the setpoint on the coldest days. Neither problem shows up in an AFUE rating.

Proper sizing comes from a room-by-room load calculation, usually Manual J, that accounts for insulation, window area, air leakage, and design temperature. Square footage rules of thumb skip all of that. If a bid arrives with a size and no calculation, ask for the load numbers, then check them against the furnace size calculator.

Two-stage and modulating furnaces cost more up front and run at lower output most of the winter. They do not raise AFUE much on their own, but they even out temperatures and reduce cycling. The savings are real in comfort and in blower power, and small in fuel.

What the estimate leaves out

The tool is a planning estimate, not a bid. Five things sit outside it.

• Duct losses. Heat that leaks from ducts in an attic, garage, or crawlspace never reaches the rooms, so the real fuel bill runs above the model.
• Blower electricity. The fan runs whenever the heat runs. A constant-torque motor draws more than an ECM, and the difference shows up on the electric bill, not the gas bill.
• Fuel price swings. A single winter price is a snapshot. Propane and heating oil can move by a third within a season.
• Rebates and tax rules. Utility rebates and federal incentives change with each program year. Confirm the current terms with the program before you count them.
• Repairs. The maintenance line covers routine service, not a failed inducer motor or heat exchanger in year nine.

Use the total as a way to compare two real quotes on equal terms, then keep a margin for the items above. If the gap between a cheap unit and an efficient one is small after 15 years, the quieter, better-commissioned installation is the better buy.