Your furnace is one of the most predictable appliances in your home. Gas furnaces last 15-25 years; heat pumps last 10-15 years; boilers last 20-35 years. Beyond those windows, efficiency drops, failure risk rises, and — most importantly — the heat exchanger inside a gas furnace can crack, releasing carbon monoxide into your home's living air. A cracked heat exchanger is the single most dangerous HVAC failure, and it is often invisible until a CO detector catches it.
This guide walks through how to find your furnace's age, what signs warrant replacement, and what a new system costs in 2026.
This guide is organized the way the decision actually plays out in practice: what matters, what does not, and the reasoning behind each recommendation. Numbers and ranges reflect 2026 Connecticut, Massachusetts, and New York conditions and pricing.
Quick answer
Gas furnaces typically last 15-25 years. A cracked heat exchanger is a condemnable failure and a carbon monoxide risk — the furnace must be shut off immediately. Replacement costs in 2026: standard 80% AFUE gas furnace $3,500-$6,500; 95%+ high-efficiency condensing furnace $4,500-$9,500; heat pump (air source) $5,500-$12,000; dual-fuel (heat pump + gas backup) $8,500-$15,000; geothermal $20,000-$40,000+. Add $300-$700 for a new thermostat, $500-$1,500 for venting upgrades on high-efficiency models, and $150-$400 for permit fees. CO detectors on every level of the home are non-negotiable regardless of furnace age.
Field context
Cost ranges published in a guide like this are benchmarks, not guarantees. Each range reflects a band within which most fair-market invoices actually land — low end for a clean, uncomplicated job in normal business hours, high end for predictable complications and peak-season pricing. The middle is where most real invoices sit. The ranges are built from trade-association wage data, aggregated regional cost-guide benchmarks, manufacturer and retailer equipment pricing, and current utility rebate schedules. Three important caveats follow from how the ranges are built.
First, the ranges are not negotiating targets. Contractors price to their local market, their own overhead and schedule, and the specific scope of the job in front of them. A contractor whose bid comes in near the middle of the published range is not overcharging; a contractor whose bid falls 15% below the low end is usually missing scope rather than offering a better deal. The useful pattern is three bids on identical written scope, not a single bid compared to the published range.
Second, the ranges shift materially with seasonality, location, and labor market conditions. Peak heating and cooling seasons push HVAC and plumbing invoices 10-20% higher than shoulder seasons. Coastal Connecticut, Boston metro, and New York City metro labor rates run 15-25% above national averages. The ranges here are calibrated to 2026 CT/MA/NY conditions; readers in markedly different markets should adjust expectations.
Third, cost is not the same as value. The lowest number that completes the job correctly, with licensed work by a contractor who stands behind it, is usually the cheapest outcome over a 10-year horizon even when it is not the cheapest invoice in the quote stack. Most homeowners who look back at a major project with regret report choosing on price alone.
How to find your furnace's age
Look for the manufacturer's label inside the blower compartment or on the front/side of the unit. The serial number typically encodes the manufacturing date. Common patterns:
- Lennox, Trane, Carrier: first four digits of serial often encode week+year, e.g., "4521..." = week 45 of 2021
- Goodman, Amana: first four digits are typically YYMM, e.g., "1809..." = September 2018
- Rheem, Ruud: middle four digits encode month+year
Every manufacturer publishes a decoding chart. Search "[brand] furnace serial number age" for specifics. If the label is illegible, assume the unit is at least 15-20 years old and treat it as near-end-of-life.
Why heat exchanger cracks are so dangerous
A gas furnace burns natural gas or propane to heat a metal chamber — the heat exchanger. Air from your home is blown across the outside of this chamber, heats up, and circulates through your ducts. The combustion gases (including carbon monoxide) inside the chamber exit through the flue to the outdoors.
When the heat exchanger cracks, combustion gases mix with the air being blown into your home. Carbon monoxide is odorless, colorless, and tasteless. At moderate concentrations, it causes headaches, nausea, and confusion. At high concentrations, it kills.
Heat exchangers crack from thermal cycling fatigue (metal expanding and contracting over thousands of cycles), from corrosion (condensate acids in high-efficiency units), and from restricted airflow causing overheating.
Warning signs of furnace trouble
Six signs, in order of urgency:
- Your CO detector alarmed. Evacuate, ventilate, call the gas utility, and have an HVAC technician inspect before restarting.
- Yellow or flickering burner flame (should be crisp blue). Indicates incomplete combustion.
- Soot around the furnace or on vents. Indicates combustion problems.
- Rust streaks on the heat exchanger or vent pipe. Indicates corrosion or condensate issues.
- Higher than usual heating bills with no usage change. Efficiency is dropping.
- The unit is past its rated lifespan. Age alone is a replacement signal.
- Any CO detector alarm
- Any yellow or flickering burner flame
- Any unusual smell from the furnace
- Any visible rust, soot, or water around the furnace
- Any unit 15+ years old for an annual safety inspection
- Annual professional tune-up ($125-$300). Catches heat exchanger, burner, and venting issues before they become emergencies.
- Replace the air filter every 1-3 months during heating season. Restricted airflow overheats heat exchangers and shortens life.
- CO detectors on every level and outside every sleeping area. Test monthly, replace every 5-7 years.
- Keep the area around the furnace clear of stored items (especially flammable liquids and aerosols).
- Photograph the furnace annually with the labels and serial visible. Documents the equipment for insurance and resale.
- Gas Furnace Repair vs. Replace
- Boilers and Radiant Heat: Maintenance and Lifespan
- Oil Boiler Repair vs. Replace
- Oil Furnace Repair vs. Replace
- Stela Report — pre-purchase property intelligence with disclosure, condition, and risk flags.
- Repair Calculator — modeled cost ranges by category and ZIP, calibrated with regional and complexity multipliers.
- Stela Guides — step-by-step repair walkthroughs reviewed by licensed professionals, with safety callouts and disclosure.
- US Department of Energy — home heating systems and efficiency
- ENERGY STAR — furnace and heat pump ratings and rebate locator
- International Residential Code (IRC) Chapter 13 — general mechanical system requirements
- US Consumer Product Safety Commission — carbon monoxide safety
- Air Conditioning Contractors of America (ACCA) — Manual J and S sizing standards
System types and what they cost
Standard 80% AFUE gas furnace
Basic atmospheric-vented gas furnace. 80% of fuel energy delivered as heat; 20% lost up the flue. Vents through a metal chimney or B-vent.
Cost: $3,500-$6,500 installed.
Lifespan: 15-25 years.
Best for: homes with existing metal chimney venting, modest heating loads, cost-constrained replacement.
High-efficiency 95%+ AFUE condensing gas furnace
Two heat exchangers extract more heat from combustion; vents via PVC pipe to the side of the house. 95-98% efficient.
Cost: $4,500-$9,500 installed.
Lifespan: 15-20 years.
Best for: cold climates, high-use homes, homes ready to abandon chimney venting.
Air-source heat pump
Uses refrigeration cycle to move heat from outdoors into the home (and reverse for cooling). Modern cold-climate heat pumps work to -15°F or below.
Cost: $5,500-$12,000 installed (heat pump only, no gas backup).
Lifespan: 10-15 years.
Best for: mild to moderate climates, homes replacing both furnace and AC, homeowners pursuing electrification.
Dual-fuel (heat pump + gas backup)
Heat pump handles most heating load; gas furnace kicks in during extreme cold. Most efficient approach for cold climates.
Cost: $8,500-$15,000 installed.
Lifespan: 15-20 years (gas) + 10-15 years (heat pump).
Best for: homes where heat pump alone doesn't cover worst-case cold; electrification-friendly with gas backup for reliability.
Geothermal (ground-source heat pump)
Uses ground loops to exchange heat with the earth. Most efficient HVAC system available; longest equipment life.
Cost: $20,000-$40,000+ installed.
Lifespan: 20-25 years (equipment); 50+ years (ground loops).
Best for: long-term owners, large lots, cold climates, homes with high heating/cooling bills.
Boiler (hydronic heat)
Heats water that circulates to radiators or radiant floor. Common in older Northeast and Midwest homes.
Cost: $5,500-$12,000 for standard; $9,000-$18,000 for high-efficiency condensing boiler.
Lifespan: 20-35 years.
Best for: homes with existing radiator or radiant infrastructure.
What replacement actually costs in 2026
National ranges.
| Scope | Low end | Typical | High end |
|---|---|---|---|
| HVAC technician diagnostic visit | $100 | $175 | $350 |
| Heat exchanger inspection with combustion analyzer | $150 | $275 | $450 |
| Single-stage 80% AFUE furnace | $3,500 | $4,800 | $6,500 |
| Two-stage 95%+ AFUE condensing furnace | $5,500 | $7,500 | $9,500 |
| Modulating 97% AFUE variable-speed furnace | $7,500 | $9,800 | $12,000 |
| Air-source heat pump (replacing furnace + AC) | $5,500 | $9,000 | $12,000 |
| Cold-climate heat pump (high-performance) | $8,500 | $12,500 | $16,500 |
| Dual-fuel system (furnace + heat pump) | $8,500 | $12,000 | $15,000 |
| Geothermal heat pump system | $20,000 | $28,000 | $40,000+ |
| Condensing boiler (hydronic) | $9,000 | $13,000 | $18,000 |
| PVC venting upgrade (for high-efficiency furnace) | $500 | $950 | $1,500 |
| New thermostat (smart/programmable) | $150 | $350 | $700 |
| Duct modifications during replacement | $500 | $2,500 | $8,000 |
| Permit and inspection fees | $150 | $350 | $700 |
| Federal/state/utility rebates (typical deduction) | -$500 | -$2,000 | -$8,000 |
The 2022 Inflation Reduction Act provides significant tax credits for heat pumps (up to $2,000) and geothermal systems (30% of cost, uncapped). Many utility companies add rebates on top. Ask your installer to pull all applicable rebates.
When to call a professional
All furnace, boiler, and heat pump work is professional-only. Gas, high-voltage electrical, refrigerant handling, and combustion safety are code-regulated and require licensed installers.
Call a licensed HVAC technician immediately for:
Stela Home earns no referral fees from contractor connections.
Preventing the next failure
Diligence and documentation
Diligence on cost management centers on three practices. First, written scope before any contractor conversations. A scope document listing every line item — equipment, labor, materials, permits, disposal, warranty terms — standardizes quotes and exposes where contractors are pricing differently. Second, three competitive bids on identical scope, not three contractor interviews followed by loose estimates. Third, license and insurance verification through the relevant state registry, plus two references on similar jobs completed in the preceding two years. These steps take a few hours and routinely save five to fifteen percent on the final invoice, independent of any negotiation.
Documentation at the back end matters as much as diligence at the front. A paid invoice with itemized scope, photographs of the completed work, and a record of any permits pulled belongs in the homeowner's records — not just for warranty claims but for the eventual resale, where a documented maintenance and improvement history routinely adds real value at closing. The homeowners who build this habit from day one of ownership tend to recover disproportionately more of their project costs when they sell; the homeowners who treat records casually tend to give money back at inspection.
Bottom line
The honest bottom line on cost: the right number is rarely the lowest quote. It is the lowest quote attached to complete scope, licensed work, and a contractor whose license, insurance, and references check out. Every one of those three items has quietly saved more money than bid negotiation in the long arc of home ownership.
Related Stela Home coverage
How Stela Home helps
Three Stela Home tools work together on this kind of decision: