Furnace Replacement Cost

A furnace quote is two numbers wearing a lot of sales language: how much heat it makes, and how efficiently it makes it. Get those two straight and the quotes become comparable. Leave them vague and you are comparing brands, which is what the brochure wants.

What a furnace costs, installed

Heating is quoted by BTU output - the heat that actually reaches the house, not the gas that goes into the burner.

Bands are indicative 2026 US contractor ranges from published cost guides (HomeGuide, Angi). They are not a quote.

Input is not output, and the label shows input. A 100,000 BTU furnace running at 80% AFUE delivers 80,000 BTU/h of heat. At 96% it delivers 96,000. When a contractor says “a hundred thousand BTU furnace,” ask which number he means, because the house only ever feels the second one.

Efficiency: the other half of the price

AFUE is the only piece of jargon you need here, and it is simpler than it looks: it is the share of the gas you buy that ends up as heat in your house. An 80% AFUE furnace turns 80 cents of every gas dollar into heat and sends the rest up the flue. A 96% one keeps nearly all of it.

That is roughly double, for the same heat. You are buying lower gas bills with cash up front, and whether it pays back is a real calculation with your climate, your gas price and your remaining years in the house in it - not a moral question.

Two practical notes the brochure will not lead with. A condensing furnace exhausts through PVC rather than the old metal flue and produces acidic condensate that needs a drain, so the venting is genuinely different work. And if you live in Canada the decision is largely made for you - 95% AFUE has been the legal minimum since 2019, so a condensing furnace is not an upgrade you choose, it is the only one you can buy.

Sizing: the rule of thumb, and why it is wrong on purpose

The estimating rule is about 40 BTU/h per sq ft of conditioned floor, so a 2,000 sq ft house lands near 80,000 BTU/h. That is enough to price the job and to catch a quote that is wildly off. It is not a design.

The size that should actually be installed comes from a room-by-room load calculation - Manual J in the US, CSA F280 in Canada - which the codes require by name. It accounts for your insulation, your windows, your air leakage and your climate, and it routinely lands on a smaller system than the floor-area rule does. The rule of thumb systematically oversizes, and an oversized furnace is not a safer furnace: it short-cycles - runs in short bursts, blasting the thermostat satisfied and shutting off again - which is hard on the equipment and leaves the rooms furthest from it cold.

Get the load calculation before anyone orders equipment. It is the cheapest part of the job and the only part that decides whether the expensive parts were the right size.

How the price is built

BTU/h needed = floor area × BTU per sq ft   → round UP to the next furnace built

furnace replacement cost = BTU/h × $ per 1,000 BTU
                         + ductwork (seal, repair or new)
                         + venting, if moving to a condensing unit
                         + permit and inspection
                         + removal of the old unit

The calculator sets that $ per 1,000 BTU from the size you land on, and lets you type over it - which is how you price a high-efficiency unit, since a 96% furnace costs more per BTU than an 80% one.

Heating rounds up. A furnace that cannot keep the house warm on the coldest night it was sized for is simply not big enough, and there is no 64,000 BTU furnace on the shelf - the next one is 80,000. (Cooling rounds the other way, and AC unit cost explains why.) Run yours through the furnace replacement cost calculator for a low-typical-high band.

What moves the total

• Output, which should come from a load calculation and not from the floor area.
• Efficiency, which is close to a doubling from 80% to 96%+.
• The ductwork. A new furnace hung off leaking ducts is money spent twice - sealing and repair is $400 to $1,900, new whole-house ductwork $2,000 to $12,000.
• The venting, if you are moving from a metal flue to a condensing unit’s PVC.
• Where it sits. An attic or a crawlspace furnace costs more to swap than one standing in an open basement.
• Whether the AC comes with it. One set of trips, one permit, one matched system.

Worked example

A 2,000 sq ft house, existing 80% AFUE gas furnace, ducts in reasonable shape.

1. Size for pricing: 2,000 sq ft × 40 BTU/h = 80,000 BTU/h, which sits in the 70,000-90,000 band - $5,100 - $9,500 installed.
2. Choose the efficiency tier. Like-for-like at 80% AFUE lands nearer $3,800 - $6,200. A 96%+ condensing unit is $7,500 - $12,000, plus the new PVC venting.
3. Price the ducts honestly. If they leak, $400 - $1,900 to seal and repair, and it is worth doing while the furnace is out.
4. Then get the load calculation - and expect it to come back smaller than 80,000. That is normal, and it saves you money on step 1.

Note which step is last and which one the salesman does first.

Where furnace quotes go wrong

• Sizing off the floor area and calling it done. The rule of thumb oversizes. Pay for the load calculation.
• Confusing input BTU with output BTU. The house feels the output.
• Buying efficiency without doing the arithmetic. It doubles the price. It might still be right - but check it, do not assume it.
• Ignoring the ducts. The best furnace in the world cannot push heat through a leaking return.
• Budgeting on a tax credit without checking it still exists. The US federal 25C credit was repealed for anything placed in service after 31 December 2025 (One Big Beautiful Bill Act, P.L. 119-21), and Canada’s Greener Homes Grant is closed. Check before you count on it.
• Replacing the furnace in isolation when the AC is the same age and shares its coil. See HVAC replacement cost.