Furnace Replacement: What Actually Drives Your Quote

Updated September 2026 · Home Project Resources

A furnace replacement is priced off six things: the heat load your house actually has, the efficiency tier you pick, whether the new unit can reuse the old venting, whether your ducts can move the required airflow, whether the gas line and electrical circuit are adequate, and how hard the old equipment is to get out. Everything else on the estimate hangs off those six.

That is why two houses on the same block get very different numbers for what looks like the same swap. One furnace sits in a wide open basement next to a properly sized chimney liner. The other is wedged into an attic knee wall, needs a new flue path cut to the roof, and shares a gas line with a range and a water heater that was already borderline.

Updated September 2026

A note on numbers. This page gives you drivers, line items and specifications, not dollar figures. Equipment and labor pricing swings hard by region, by season, and by how backed up your local trade is, so any number printed here would be wrong for most readers and stale for the rest. Get three written quotes and compare them with the checklist further down.

Sizing starts with conditioned square footage. Measure your heated space room by room with our free Square Footage Calculator, then check that figure against the load tables below. A furnace that is two sizes too big will short cycle, blow cool air at the registers between burns, and wear its heat exchanger faster than a right sized unit.

What drives a furnace replacement quote

Read a bid in this order. The first three explain most of the spread between contractors.

1. Venting category. This is the biggest hidden fork in the road. An 80% AFUE furnace vents hot flue gas up a metal or masonry chimney. A 90% or higher condensing furnace pulls so much heat out of the exhaust that the water vapor condenses, so it vents in PVC or polypropylene pipe out a sidewall and needs a condensate drain. Going from 80% to condensing means new pipe runs, a wall or roof penetration, a drain path, and often a condensate pump. If your water heater shared that chimney, the chimney is now orphaned and usually needs a smaller liner so the water heater still drafts properly.

2. Duct condition. A furnace is only as good as the air it can move. If the return side is undersized, static pressure climbs, airflow drops, and the limit switch starts cutting the burners off. A contractor who never put a manometer on your system does not know what your ducts do. Adding a return drop, resizing a trunk, or sealing an attic run all add labor and material.

3. Efficiency and staging tier. A single stage 80% unit and a modulating 98% unit with a variable speed blower are different products with different install requirements. The high tier usually needs a communicating thermostat, which means new wire if you only have four conductors at the wall.

4. Gas supply. Furnaces are sized in BTU input. If you are going up in input, or adding another appliance, the existing pipe may no longer carry it over the length involved. Upsizing a run from half inch to three quarter inch through finished space is real work.

5. Electrical. Most residential gas furnaces want a dedicated 15 amp circuit with a service switch within sight of the unit. Older installs are often fed off a shared circuit or a lamp cord splice, and an inspector will not pass that. If your panel is full or obsolete, that becomes its own project. Our guide to circuit breaker replacement covers what adding a dedicated circuit involves.

6. Access and removal. A basement next to a walkout door is easy. An attic platform reached by a pull down ladder, a crawl space unit lying on its side, or a furnace that has to be cut apart to get up the stairs all add hours.

How furnace sizing actually works

The honest answer is a Manual J load calculation. That is a room by room model built from your insulation levels, window area and type, air leakage, orientation, and your local winter design temperature. A contractor who sizes your furnace by reading the plate on the old one is copying somebody else's guess from decades ago.

The rough screening numbers below are for sanity checking a proposal, not for ordering equipment. They tell you when a bid is wildly off.

Two things to remember about that table. First, those are output BTU numbers, and furnaces are usually sold by input. A 100,000 BTU input furnace at 80% AFUE delivers 80,000 BTU of heat. The same input at 96% delivers 96,000. Second, a tight, well insulated house lands at the bottom of its band and a leaky 1950s house with single pane windows lands above the top of it. Air sealing and attic insulation can move you down a whole size, which is why it pays to read our attic insulation guide and the R-value chart before you buy equipment you will live with for twenty years.

Worked example: a 2,200 sq ft house in a mixed climate

Say you measured 2,200 sq ft of conditioned space, you have blown attic insulation at roughly R-30, double pane windows, and a mixed climate at about 35 BTU per sq ft.

1. Heat load: 2,200 x 35 = 77,000 BTU per hour of output.
2. Pick the efficiency: at 96% AFUE, the input needed is 77,000 / 0.96 = 80,200 BTU input.
3. Furnaces come in fixed sizes, usually 40, 60, 80, 100 and 120 thousand BTU input. The honest pick is the 80,000 BTU input unit.
4. Airflow check: gas furnaces are set to deliver a temperature rise inside the range printed on the data plate, often something like 40 to 70 degrees F. At 77,000 BTU of output and a 55 degree rise, airflow works out to 77,000 / (1.08 x 55) = 1,297 CFM. Your ducts have to move that. A single 14 inch return will not.
5. If the old furnace was a 120,000 BTU input unit, that is not proof you need one. It is proof somebody oversized it in 1998.

Notice step 4. That airflow number is where most furnace projects quietly fail. The equipment is fine and the ductwork was never checked.

80% AFUE vs 90% and up: the fork that changes the whole job

AFUE is the share of fuel energy that ends up as heat in your house over a season. The classes are simple. The install differences are not.

The upgrade case is strongest where the heating season is long. In a very cold climate the fuel saved every winter is meaningful. In a place that runs the furnace six weeks a year, the extra install work rarely earns itself back, and an 80% unit into an existing lined chimney is a clean, fast job.

One thing catches people out: some jurisdictions have effectively raised the minimum efficiency allowed for new residential furnaces, so ask your contractor what is legal to install in your area before you assume the 80% option is even on the table.

Single stage, two stage or modulating

This is the comfort question, not the code question.

• Single stage. The burner is on at full fire or off. Simple, cheap to repair, fewest parts. It heats the house in short blasts and can feel drafty in the shoulder seasons.
• Two stage. Runs at roughly 65 to 70% of capacity most of the time and steps up on the coldest days. Longer, gentler cycles, better evenness between floors, quieter.
• Modulating. The gas valve ramps continuously, often down to 35 or 40% of full fire, paired with a variable speed ECM blower. The most even heat and the lowest fan energy, plus the most electronics to fail after the warranty ends.

If your upstairs has always been cold while the thermostat floor is comfortable, staging plus a variable speed blower helps more than raising the AFUE. If your house is small and your ducts are good, a single stage unit is not a downgrade.

The line items on a real quote

Ask for the estimate broken out this way. A single lump sum tells you nothing, and it makes bid comparison impossible.

1. Equipment. Model number, input BTU, AFUE, stage count, blower type, cabinet width, and the matching coil if you have central air conditioning.
2. Removal and disposal. Old furnace out, old vent connector out, haul away, plus refrigerant recovery if a coil is involved.
3. Venting. Material, diameter, total length, number of elbows, termination type and location, and any chimney liner work for an orphaned water heater.
4. Gas. Any pipe replaced or upsized, sediment trap, new shutoff, leak test.
5. Electrical. Dedicated circuit, service switch, low voltage wire to the thermostat, condensate pump circuit.
6. Condensate. Drain routing, trap, pump if there is no gravity path, and where it discharges.
7. Sheet metal. Plenum transitions, new filter rack, return drop, any duct modifications.
8. Controls. Thermostat model, and whether it is communicating or conventional.
9. Permit and inspection. Who pulls it, and who meets the inspector.
10. Startup and commissioning. Gas pressure set, temperature rise measured, static pressure measured, combustion analysis run, and the readings written on the paperwork.
11. Warranty. Parts term, heat exchanger term, labor term, and whether registration is required to get the full parts coverage.

Item 10 is the one almost nobody asks for and the one that separates a real install from a swap. A commissioned furnace has a documented manifold pressure, a measured temperature rise inside the plate range, and a static pressure reading. Ask for those three numbers in writing.

Repair or replace: how to decide

A cracked heat exchanger is not a judgment call. That furnace comes out. If a contractor reports one, ask to see the evidence: a camera photo, or the failed combustion readings. This is also one of the most common upsells in the trade, so a contractor who will not show you anything is telling you something. Our contractor red flags page lists the other patterns worth recognizing.

Permits, code items and things people forget

• Permit and inspection. Almost everywhere requires one for gas appliance replacement. A contractor who suggests skipping it is transferring risk to you, and unpermitted work tends to surface at resale.
• Carbon monoxide alarms. Many jurisdictions require an alarm on every level and near sleeping areas as a condition of the permit.
• Combustion air. A sealed mechanical closet needs a defined path for combustion air, sized from the code table. Finishing a basement around an old furnace can starve equipment that used to work fine.
• Drip leg and shutoff. A sediment trap and an accessible manual gas shutoff at the appliance are standard.
• Vent termination clearances. Sidewall vents have minimum distances from windows, doors, gas meters, decks and finished grade. Snow depth matters in cold climates.
• Return air location. Pulling return air from a garage, or from the furnace closet itself, is not allowed.
• Duct sealing. Some energy codes require testing of new or replaced duct sections.

How to compare three bids apples to apples

Contractors bid different scopes. Comparing bottom lines without normalizing the scope is how people end up picking the worst install.

1. Force the same size. Ask each one what heat load they calculated and how. If one says 80,000 input and another says 120,000 for the same house, one of them is wrong.
2. Force the same efficiency tier. Compare 80% to 80% and condensing to condensing. If one bid quietly includes a chimney liner because it converts to condensing, that is scope, not padding.
3. Force the same staging. Two stage against two stage.
4. Line up the ductwork. If one bid includes a new return drop and the others do not, add that scope to the others before you compare anything.
5. Check the coil. If you have central air, ask whether the existing coil is being reused and whether it physically matches the new cabinet.
6. Read the warranty rows. Parts, heat exchanger and labor are three separate terms.
7. Ask who does the work. In house crew or subcontracted, and whether the person who sold the job will be on site.
8. Get the commissioning promise in writing. Measured temperature rise, static pressure, gas pressure, combustion analysis.

Our full walkthrough on how to get contractor bids includes an email template for requesting a matched scope, which saves an enormous amount of back and forth.

Questions to ask before you sign

• What heat load did you calculate, and can I see the output?
• What is the input BTU and the AFUE of the exact model you are proposing?
• What temperature rise range is printed on that furnace's data plate?
• Will you measure static pressure before and after, and give me the numbers?
• Where does the vent terminate, and how far is that from the nearest window and the gas meter?
• If we go condensing, what happens to the water heater that shares the chimney?
• Where does the condensate go, and does it need a pump?
• Is there a dedicated circuit and a service switch already, or is that in your scope?
• Who pulls the permit, and who meets the inspector?
• What is not included in this price?

That last question is the most useful one on the list. It moves the surprises from the middle of the job to the kitchen table.

What makes an estimate go up after work starts

Some change orders are legitimate discoveries. Knowing the usual ones lets you ask about them in advance instead of finding out on day two.

• Asbestos on old duct joints or an old boiler jacket. Abatement is its own trade with its own rules.
• A chimney that fails inspection once the furnace is disconnected and the liner is finally visible.
• Gas pipe that turns out to be undersized for the new input once the total connected load is added up.
• A panel with no space for the dedicated circuit, or an obsolete panel nobody will add to.
• A missing return path where a joist bay was used as a return and it leaks into the wall cavity.
• A coil that will not mate to the new cabinet, forcing a coil change.
• A floor drain that does not drain, so the condensate needs a pump and a new discharge route.

Ask each bidder which of these they checked. The one who already looked at your gas meter and your electrical panel is the one least likely to hand you a surprise.

How this fits with the rest of your HVAC

If your air conditioner is also near the end of its life, doing both at once usually saves labor, because the coil, the plenum and the electrical get handled once. Our HVAC replacement guide covers matched systems, and the heat pump installation page covers the case where the smarter move is to replace the furnace with a heat pump, or to add one and keep the furnace as backup heat in a dual fuel setup. If you are only replacing the cooling side this year, start with air conditioner installation.

Frequently asked questions

How long does a furnace replacement take?

A straight like for like swap in an accessible basement is usually a one day job, roughly six to ten working hours including startup. Add a day if the venting changes, the chimney needs a liner, or sheet metal has to be fabricated on site. Attic and crawl space installs run longer because everything gets carried in pieces.

How long should a furnace last?

Fifteen to twenty years is the normal band for a gas furnace that got annual service. Units that were oversized, or that ran for years with a clogged filter and high static pressure, fail earlier because the heat exchanger cycles harder. Units in dry, mild climates with short heating seasons often go longer.

Can I install a furnace myself?

In most places, no. Gas appliance work usually requires a licensed installer and a permit, homeowners insurance may exclude damage traced to unpermitted gas work, and manufacturer warranties are commonly void without licensed installation. Combustion venting is the part that hurts people, and it is not a good place to learn.

Do I need a bigger furnace if I finished my basement?

Not automatically. A finished basement is mostly surrounded by soil, which is a much milder boundary than outside air, so it adds less heat load per square foot than above grade space. It does need its own supply and return, though, and that ductwork is what most basement projects skip. Add the space to your measurements in the Square Footage Calculator and have the load recalculated.

Is a high efficiency furnace worth it in a mild climate?

Often not. The extra work for condensing venting, condensate handling and a possible chimney liner is paid once, but the fuel savings only accumulate during the heating season. Short seasons pay it back slowly. Long, cold seasons pay it back fast. Ask each contractor for the estimated annual fuel use at each efficiency tier and weigh that against the scope difference.

Why does my new furnace short cycle?

The three usual causes are oversizing, high static pressure from restricted ducts or a clogged filter, and a limit switch tripping because airflow is too low for the fire rate. Have the installer measure temperature rise. If it reads above the range printed on the data plate, airflow is the problem, not the furnace.

Should I replace the thermostat at the same time?

If the new furnace is two stage or modulating, yes, because a single stage thermostat cannot control the extra stages. Communicating systems need the manufacturer's own thermostat and sometimes an extra conductor at the wall. If you are installing a single stage 80% unit and your existing thermostat works, keeping it is fine.

What should I do the week before installation?

Clear a path from the door to the equipment, move stored boxes at least three feet back from the furnace, make sure the crew can reach the panel and the gas meter, and photograph the old data plate and any duct labels before they disappear. Then put the list of commissioning numbers on the fridge so you remember to collect them before the truck leaves.