Sizing the systems
Furnace installation cost
An 80% AFUE gas furnace runs about $4,200 installed and a 95% condensing one about $6,500 on the same 1,800 ft² house — but nearer $8,300 if your water heater shares the chimney, because the efficient furnace leaves that chimney and the water heater is then orphaned in it. That line is missing from almost every price guide and most first quotes, and it arrives after the old furnace is out.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-17 · Nothing you type leaves your browser
What are you heating?
The BTU number on a furnace is what goes in, not what comes out. So this works out what the house needs delivered, then what has to go in to deliver it at the efficiency you pick — which is why two quotes naming the same BTU are not offering the same heat.
Heated floor area. Leave out the garage and an unheated basement.
More air to heat, and the load rises with it.
Washington DC, Nashville, Kansas City, Portland
Burns about a fifth less gas for the same heat, and vents in PVC out through a wall. It also stops using the chimney, which is where the hidden costs live.
The default across most of the country and the cheapest to run where there is a gas main.
Insulated walls and attic to the standard of its decade. The baseline these figures assume.
Around 15% of floor area, double glazed.
Some sun on the house for part of the day.
The most expensive question on this page, and the one nobody asks until the old furnace is out.
A condensing furnace makes several litres of acidic water a day and it has to go somewhere.
Optional. Type what the contractor wrote and we check it against the load.
Used only to pick a regional labour rate. It stays in your browser.
Installed
$8,310
80,000 BTU in · 76,000 BTU/h out · 95% AFUE
$6,390 to $10,950 depending on who you ask
The house screens at 63,000 BTU/h. At 95% that needs 66,316 BTU/h going in, and furnaces are made in 20,000 BTU steps — so the nearest is 80,000, delivering 21% more than the house needs.
Input is not output
The figure on the box, and the figure that reaches your rooms.
The same 80,000 BTU box at 80% would deliver 64,000. Two quotes naming the same BTU figure are not offering the same amount of heat — ask for AFUE alongside it, every time.
Is condensing worth it here?
80% against 95%, on this house and this climate.
16 years
$4,150 more to fit, saving about $259 a year of gas. Inside the life of the furnace, so the money argument holds up on its own.
$2,550 of that difference is installation rather than equipment — venting, condensate and the chimney. That part varies enormously from house to house and it is why quotes diverge.
Shopping list
- Natural gas furnace, 80,000 BTU input, 95% AFUE
Delivers 76,000 BTU/h against a screening load of 63,000 BTU/h · two stage1 - PVC venting through the wall
A condensing furnace cannot use a masonry chimney — the flue gas is too cool and too wet. Intake and exhaust go through an outside wall, and how far the unit sits from one decides the price.$750 - Chimney liner for an orphaned water heater
If the water heater shared the chimney with the old furnace, it is now alone in a flue sized for two appliances. That flue will not warm up, so it condenses, corrodes and can backdraft. It needs relining to the right size — and this is the line that turns a cheap quote into an expensive one, halfway through the job.$1,800
What it costs
National average. Furnace, fitting, gas and electrical connection, start-up and removal of the old unit, plus the venting and chimney work a condensing furnace forces. Ductwork repairs, asbestos work and a gas meter upgrade are separate.
Cooling is a separate machine sharing the same ducts — see central air, and if both are due at once, a heat pump replaces the pair with one box. For the sizing on its own, the BTU calculator.
Worth knowing
- The house screens at about 63,000 BTU/h of heat. At 95% AFUE that needs 66,316 BTU/h going in, and the nearest size made is 80,000 — which delivers 76,000.
- The number on the box is INPUT, not output. A 80,000 BTU furnace at 95% delivers 76,000; the same box at 80% would deliver 64,000. Two quotes naming the same BTU figure are not offering the same amount of heat, and nobody points this out.
- This is the expensive surprise on this job, and it is why a condensing furnace is not simply "the efficient one". A 95% furnace vents in PVC through a wall, so it stops using the masonry chimney. Your water heater was sharing that chimney and is now alone in a flue sized for two appliances: it will not warm the flue enough to draw, so the flue gas condenses, corrodes the liner and can spill back into the house. The fix is relining to the right size, about $1,800. Get it in the quote, in writing, before the old furnace comes out.
- Going from 80% to 95% costs $4,150 more here and saves roughly $259 a year of gas — about 16 years, inside the life of the furnace. Note that $2,550 of that extra is installation rather than equipment, which is the part that varies most from house to house.
- A screening figure, not a Manual J. Heating load is more forgiving than cooling — an oversized furnace is a minor fault where an oversized air conditioner is a real one — but the number to buy on still comes from a room-by-room calculation.
On this page — 6 sections
What a furnace costs installed
The short version
- About $4,200 for an 80% AFUE gas furnace, $6,500 for a 95% condensing one on the same house.
- The BTU on the box is input. An 80,000 BTU furnace delivers 64,000 at 80% AFUE. Ask for AFUE alongside every BTU figure.
- The efficient one orphans your water heater if it shared the chimney — about $1,800 to reline, and it is rarely in the first quote.
- Payback ranges from 2 years to never, entirely on climate. It is a question about your weather.
$4,160
80% AFUE, single stage
Keeps using the existing chimney
$6,510
95% AFUE, two stage
PVC venting, drain nearby
$8,660
95%, shared chimney, no drain
Liner and condensate pump added
$12,030
97% modulating, 2,600 ft², zone 5
A bigger house and the top tier
The gap between the first two is what everyone quotes as “the high-efficiency option”. The gap between the second and the third is the one nobody warns you about, and it arrives after the old furnace is already out.
MaterialsEstimator, HVAC equipment and installation price survey · checked 2026-09-16
The number on the box is not the heat you get
A furnace is sold by its input: how much fuel energy it burns per hour. What reaches your rooms is input times AFUE, and that difference is large enough to make two quotes incomparable.
output = input × AFUE and input needed = heating load ÷ AFUE
- heating load
- what the house needs delivered — about 63,000 BTU/h for 1,800 ft² in zone 4
- AFUE
- the share of fuel that becomes heat indoors: 80% or 95%
- input
- what the box says, and it only comes in 40, 60, 80, 100 and up thousand BTU steps
Worked through — 63,000 ÷ 0.80 = 78,750, so an 80,000 BTU furnace, delivering 64,000. The same 80,000 box at 95% AFUE delivers 76,000 — nineteen per cent more heat from an identically labelled machine.
The expensive surprise: an orphaned water heater
This is the part of the job with the worst ratio of cost to coverage anywhere in residential HVAC: it is expensive, it is required, it is entirely predictable — and it is missing from nearly every price guide and most first quotes.
An old 80% furnace is atmospheric. Its flue gas is hot, it rises up a masonry chimney on its own, and the water heater is very often vented into the same chimney, sharing it.
A condensing furnace is not atmospheric. Its flue gas has given up so much of its heat that it comes out barely warm and wet, and it has to be pushed out through a PVC pipe in a wall. So the furnace leaves the chimney — and the water heater is suddenly alone in a flue sized for two appliances.
A small water heater cannot warm that much brick. The gas cools before it reaches the top, condenses inside the chimney, and starts corroding it. Worse, the draught can fail altogether and combustion products spill back into the basement. It is a recognised code issue with a standard name — an orphaned water heater — and the fix is a correctly sized liner dropped down the chimney.
$900–3,500
Chimney liner
If the water heater shared the flue
$400–1,500
PVC venting
Distance to an outside wall sets it
$150–750
Condensate drain or pump
Several litres of acidic water a day
~33%
Of a condensing job
That these three can add up to
Whether condensing repays, by where you live
A 95% furnace burns about a fifth less gas than an 80% one for the same heat. Whether that is worth the extra cost is not a property of the furnace at all — it is a property of how much heating you do.
The jaggedness in the middle of that chart is real and worth explaining, because it looks like an error. Furnaces come in 20,000 BTU steps. In some climates the move to 95% AFUE drops you a whole size and the equipment barely costs more; in others both efficiencies land on the same box and you pay the full premium. Whether the size drop happens can matter more than the efficiency gain, and no price guide accounts for it because it depends on your particular load.
| 80% AFUE, single stage | 95% AFUE, two stage | |
|---|---|---|
| This house, installed | $4,160 | $6,510 |
| Uses the masonry chimney | Yes | No — PVC through a wall |
| Orphans a shared water heater | No | Yes, and that needs a liner |
| Makes condensate | No | Several litres a day, acidic |
| Gas burnt for the same heat | 100% | About 84% |
| Payback in Minneapolis | — | About 3 years |
| Payback in Houston | — | About 37 years |
MaterialsEstimator, Screening load survey, BTU per square foot by climate zone · checked 2026-09-16
What to ask before you sign
Get these in writing, from every contractor
- Input BTU and AFUE together. Either one alone tells you nothing about how much heat you are buying.
- What load calculation produced that size. 'The same as the old one' is not a load calculation, and the old one was probably oversized.
- Whether the venting is included, and where the PVC exits the building. Distance to a wall is what moves this price.
- Where the condensate goes, and whether a pump or a neutraliser is needed.
- What else vents into the chimney today — and if it is the water heater, whose job the liner is.
- Whether the existing ductwork has been checked for size and leakage. A modern blower wants more airflow than an old one.
- Whether the thermostat is included, and whether a two-stage or modulating unit needs a communicating one.
- Gas line size. A bigger input than the old furnace can need a larger line or even a meter upgrade.
- The labour warranty separately from the parts warranty. The manufacturer's parts cover is close to worthless without labour.
What goes wrong
01Comparing quotes by BTU alone
The number is input. An 80,000 BTU furnace delivers 64,000 at 80% AFUE and 76,000 at 95% — nineteen per cent more heat from an identically labelled box.
Instead — Ask for input BTU and AFUE together, then multiply. Compare output against output.
02Going condensing without checking the chimney
A 95% furnace stops using the masonry flue, and if the water heater shared it, that water heater is now orphaned in an oversized chimney that will condense and can backdraft. It is about $1,800 to put right, and it surfaces after the old furnace is out.
Instead — Look at what vents into the chimney before you choose the efficiency, and get the liner priced in the quote or explicitly excluded in writing.
03Buying high efficiency in a warm climate for the savings
The payback runs from two years in Fairbanks to never in Miami, because it depends on how much gas you burn. In zone 2 the upgrade takes around thirty-seven years on a furnace that lasts twenty.
Instead — Run the payback on your own zone. If it exceeds the life of the furnace, buy 80% and put the difference into insulation, which reduces the gas rather than burning it better.
04Matching the size of the old furnace
It assumes somebody once did a load calculation, and usually nobody did. The house may also have new windows, new insulation and a new roof since.
Instead — Screen the load from the house as it is now and ask for a Manual J. A size down is cheaper to buy and more comfortable to live with.
05Worrying about oversizing as much as with air conditioning
The advice gets copied across from cooling, where it is serious. An oversized furnace cycles a bit and can feel drafty; it does not leave the house humid, and sizes only exist in 20,000 BTU steps anyway.
Instead — Take the nearest size up without anxiety. Spend the attention on the chimney and the ducts instead — that is where the money and the safety are.
06Ignoring the ductwork
Ducts leaking into an unheated attic or crawl space lose a fifth of the heat before it arrives, and ducts sized for an old furnace may choke a modern variable-speed blower.
Instead — Have them pressure-tested and sealed at the same time. It can drop you a furnace size, which pays for a good part of the sealing.
07Replacing on a diagnosis you have not seen
A cracked heat exchanger is a genuine reason to replace. A failed igniter is a $200 repair, and the two get described in the same sentence on a cold morning when you are not in a position to argue.
Instead — Ask for a photograph of the crack. If it cannot be produced, get a second opinion before committing several thousand dollars.
08Replacing the furnace alone when both are old
The evaporator coil for the air conditioning sits on the furnace and has to come off anyway. Doing them separately pays the same labour twice and risks a mismatched pair that never delivers its rated efficiency.
Instead — If both are near the end, price them together — and price a heat pump against the pair, because it replaces both with one machine.
- 01Measure the heated areaHeated floor area only. The garage and an unheated basement are not being paid for.
- 02Screen the heating loadClimate zone, insulation, glazing and shading. This is output — what has to reach the rooms.
- 03Divide by AFUE to get inputThat is the number on the box. A 63,000 BTU load needs 78,750 of input at 80% and 66,300 at 95%.
- 04Round up to a size that existsFurnaces come in 20,000 BTU steps, so some rounding is unavoidable. Oversized heating is a minor fault, not a serious one.
- 05Check what else uses the chimneyBefore anything else. If the water heater shares it and you go condensing, the chimney needs relining and that is not a small line.
- 06Find where the condensate will goA condensing furnace makes several litres of acidic water a day. No drain nearby means a pump.
- 07Work out the payback on your own climateThe condensing upgrade repays in two years in Fairbanks and never in Miami. It is a question about your weather, not about the furnace.
The indoor coil sits on the furnace, so if both are near the end they are one job and one lot of labour — and a matched pair is what the efficiency ratings assume in the first place.
Cooling too?- AFUE
- Annual Fuel Utilisation Efficiency. The share of the fuel you buy that becomes heat in the house rather than heat up the flue. An 80% furnace sends a fifth of your gas outside.
- Input BTU
- How much fuel energy goes into the furnace per hour. It is the number printed on the box and quoted by contractors, and it is not what reaches your rooms.
- Output BTU
- Input times AFUE — what actually heats the house. An 80,000 BTU furnace delivers 64,000 at 80% AFUE and 76,000 at 95%.
- Condensing furnace
- One efficient enough that the flue gas cools below its dew point and gives up its latent heat. That is where the extra efficiency comes from, and it is why it produces water and cannot use a masonry chimney.
- Orphaned water heater
- A water heater left alone in a chimney it used to share with the furnace. The flue is now far too big for it, so it never warms up, condenses, corrodes, and can spill combustion gases back into the house.
- Chimney liner
- A correctly sized metal flue dropped inside the masonry chimney so a single appliance can draw properly. It is the fix for an orphaned water heater and it is rarely in the first quote.
- Condensate
- The acidic water a condensing furnace produces — several litres a day. It needs a drain, often a pump, and in some places a neutraliser before it enters the waste.
- Single stage, two stage, modulating
- How finely the burner can turn down. Single stage is on or off; modulating adjusts continuously, which is quieter and keeps the temperature steadier.
- Heat exchanger
- The metal chamber separating flue gas from the air you breathe. It is what cracks at the end of a furnace's life, and a cracked one is the legitimate reason to replace rather than repair.
- Design temperature
- The outdoor temperature a heating system is sized for — roughly the coldest it gets in a normal year, not the coldest ever recorded. Sizing for the record makes the furnace oversized for every other day.
- Heating degree days
- A measure of how cold a year is, and the thing that decides whether a high-efficiency furnace ever repays. It is why the same upgrade is obvious in Minneapolis and pointless in Miami.
- Manual J
- The room-by-room load calculation that says how much heat a house actually needs. Skipped on most replacements in favour of matching whatever was there.
Common questions
- How much does it cost to install a furnace?
- About $4,200 for an 80% AFUE gas furnace and around $6,500 for a 95% condensing one on the same 1,800 ft² house — and closer to $8,700 if the water heater shared the chimney and there is no drain nearby, because the condensing unit forces both a liner and a condensate pump. The equipment gap is the smaller part of that difference.
- What size furnace do I need?
- An 1,800 ft² house in the mixed middle of the country screens at about 63,000 BTU/h of heat delivered. At 80% AFUE that means 78,750 BTU of input, so an 80,000 BTU furnace; at 95% it means 66,300, which is also an 80,000. Furnaces are made in 20,000 BTU steps, so some rounding up is unavoidable.
- Is the BTU rating on a furnace input or output?
- Input, almost always — and it is the single commonest misunderstanding in this purchase. An 80,000 BTU furnace at 80% AFUE delivers 64,000 to the house; the same 80,000 at 95% delivers 76,000. Two quotes naming the same BTU are not offering the same amount of heat, so ask for AFUE alongside the BTU every time.
- Is a 95% AFUE furnace worth the extra money?
- It depends entirely on your climate, and the range is enormous: on the same house it pays back in about two years in Fairbanks, three in Minneapolis, nine in Washington DC, thirty-seven in Houston and effectively never in Miami. A condensing furnace saves roughly a fifth of the gas you burn, so it repays where you burn a lot. In the warm zones it is a comfort and emissions decision, and anyone selling it there as a money decision is doing arithmetic worth asking to see.
- Why does a high-efficiency furnace need a chimney liner?
- Because it stops using the chimney. A condensing furnace vents cool, wet flue gas through a PVC pipe in an outside wall. If your water heater shared the masonry chimney, it is now alone in a flue sized for two appliances — it cannot warm that much brickwork, so the gas condenses instead of rising, corroding the chimney and potentially spilling back into the house. The fix is a correctly sized liner, about $1,800, and it is the line most often missing from the first quote.
- What is an orphaned water heater?
- A water heater left as the only appliance in a chimney it used to share. It is the standard term for it, it is a recognised code issue, and it is created deliberately every time an atmospheric furnace is replaced with a condensing one. If a contractor quoting a 95% furnace has not mentioned it and your water heater vents into the chimney, ask.
- Is an oversized furnace a problem?
- Far less than an oversized air conditioner, and it is worth saying plainly because the advice gets copied across from cooling. An oversized furnace cycles more, loses a little efficiency and can feel drafty as it blasts and stops. It does not leave the house humid, and the sizes only come in 20,000 BTU steps anyway. Worth knowing, not worth paying to avoid.
- Gas, propane or electric?
- Natural gas where there is a main, every time. Propane is the same furnace with a different orifice kit and roughly double the running cost, which is exactly the situation where a heat pump becomes worth pricing. Electric resistance is cheap to install and expensive to run — it is 100% efficient, which sounds unbeatable until you notice a heat pump moves about three units of heat for one of electricity.
- Should I replace the furnace and air conditioner together?
- If both are near the end, yes — the labour overlaps, the indoor coil sits on the furnace and has to come off anyway, and a matched pair is what the efficiency ratings assume. It is also the moment to price a heat pump, which replaces the pair with one machine.
- How long does a furnace last?
- Fifteen to twenty-five years for a gas furnace, and the thing that ends it is usually a cracked heat exchanger. That is a genuine reason to replace rather than repair, because a crack lets flue gas into the air you breathe. A failed igniter or blower motor is not.
- Why are two furnace quotes thousands apart?
- Different AFUE, different input BTU, and — the big one — whether the venting, condensate and chimney work are in the price. On a condensing installation those three can be a third of the job, and they are the easiest things to leave out of a quote that wants to look cheap.
- Can I keep my existing ductwork?
- Usually, and it is what makes a replacement affordable. But ducts sized for an old, oversized furnace may be undersized for the airflow a modern variable-speed unit wants, and leaky ducts in an unheated attic or crawl space waste a fifth of the heat before it arrives. Have them checked at the same time — it can drop you a size.
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About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
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