What Is a Gas Furnace? Types, Efficiency, Costs, and Maintenance

A gas furnace is a cornerstone of home heating across much of the United States. This guide explains what a gas furnace is, how it works, the types available, and how to choose, maintain, and operate one safely and efficiently. It also covers costs, incentives, and how gas furnaces compare with electric furnaces and heat pumps.

What Is A Gas Furnace?

A gas furnace is a forced-air heating appliance that burns natural gas or propane to warm indoor air. The furnace heats a metal heat exchanger, and a blower pushes air across it into supply ducts, distributing heat throughout the home.

Most U.S. homes with central air conditioning share the same ductwork, so the furnace and AC often operate as one HVAC system. Gas furnaces are popular for fast heat, wide availability of fuel, and strong performance in cold climates.

Modern units use electronic ignition instead of standing pilot lights, advanced safety controls, and variable-speed blowers for quieter, more even heating.

How A Gas Furnace Works

The Heating Cycle

Call For Heat: The thermostat signals the furnace control board.
Pre-Purge: The inducer fan clears combustion gases; a pressure switch confirms proper draft.
Ignition: A hot-surface igniter or spark ignites the burners; a flame sensor proves flame.
Heat Transfer: Burners heat the primary (and in condensing units, secondary) heat exchanger.
Air Delivery: After a short delay, the blower pushes room air across the heat exchanger into ducts.
Shutoff: When the setpoint is met, burners stop; the blower runs briefly to cool the exchanger.

Key Components

Burners: Mix gas and air for combustion.
Heat Exchanger(s): Transfers heat to indoor air while keeping flue gases separate.
Inducer Motor: Pulls combustion gases through the exchanger and vent.
Blower (ECM or PSC): Circulates indoor air; ECM motors improve efficiency and comfort.
Gas Valve: Controls gas flow; can be single-stage, two-stage, or modulating.
Control Board & Sensors: Coordinates ignition, blower timing, and safety limits.

Condensing Vs. Non-Condensing Exhaust

Non-condensing furnaces (often 80% AFUE) vent hot flue gases through metal chimneys. Condensing furnaces (90%–98%+ AFUE) extract more heat, cooling exhaust enough to condense water vapor. They use PVC or CPVC venting and a condensate drain, often with a neutralizer.

Condensing models capture latent heat from water vapor, boosting efficiency but requiring proper condensate management and freeze protection for drain lines.

Types Of Gas Furnaces

By Firing Stages

Single-Stage: On or off at full fire. Lower upfront cost, more temperature swings, louder.
Two-Stage: Low fire for most heating, high fire in cold snaps. Better comfort and efficiency.
Modulating: Continuously varies output (often 30%–100%). Best comfort, quiet, and airflow control, highest cost.

Efficiency Category

Standard Efficiency: Typically 80% AFUE, non-condensing, metal flue, atmospheric or induced draft.
High Efficiency (Condensing): 90%–98.5% AFUE, PVC venting, secondary heat exchanger, condensate drain.

Combustion Air Source

Sealed Combustion (Direct Vent): Draws outside air for combustion and vents outdoors. Safer and more efficient, reduces backdraft risks.
Non-Sealed (Atmospheric): Uses indoor air; more sensitive to depressurization and backdrafting.

Type	Typical AFUE	Comfort	Noise	Upfront Cost
Single-Stage, 80%	80%	Basic	Higher	Lower
Two-Stage, 80%–95%	80%–96%	Better	Moderate	Moderate
Modulating, Condensing	95%–98.5%	Best	Lower	Higher

Efficiency, AFUE, And Operating Costs

AFUE (Annual Fuel Utilization Efficiency) measures how much of the fuel becomes usable heat over a season. An AFUE of 95% means 95% of gas energy becomes heat, 5% is lost up the flue.

The U.S. Department of Energy has long required minimum AFUE levels. In 2023, DOE finalized a rule raising the minimum efficiency for most new residential gas furnaces to 95% AFUE for units manufactured after 2028. See DOE guidance for specifics and exemptions.

DOE and ENERGY STAR offer resources on furnace efficiency. High-efficiency furnaces reduce gas use, improve comfort, and can qualify for incentives.

What Impacts Real-World Efficiency

Ducts: Leaky or poorly insulated ducts can waste 10%–30% of heat.
Blower Type: ECM motors use less electricity and maintain airflow better.
Sizing: Oversized furnaces short-cycle, lowering efficiency and comfort.
Thermostats: Smart, adaptive controls can trim runtime and avoid overshooting.
Installation Quality: Proper gas pressure, airflow, and combustion tuning are crucial.

Estimated Operating Cost Example

Assume a home needs 60 million BTU of heat over a season. One therm equals 100,000 BTU. At $1.40 per therm (typical U.S. residential average), seasonal costs vary by AFUE:

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AFUE	Therms Required	Estimated Cost
80%	750	$1,050
95%	632	$885
98%	612	$857

Actual costs depend on climate, home insulation, and gas prices. Improving duct sealing and insulation can rival AFUE upgrades in savings.

Sizing And Ductwork Considerations

Right-sizing is essential. Contractors should perform an ACCA Manual J load calculation to determine heating needs, then select equipment per Manual S and design ducts per Manual D.

Rules of thumb (like BTU per square foot) are unreliable. Proper sizing improves comfort, controls humidity in shoulder seasons, and protects the heat exchanger from rapid cycling.

Ductwork And Airflow

Static Pressure: High static from undersized ducts or restrictive filters reduces airflow and efficiency.
Returns: Ensure adequate return air; add returns to closed-off rooms if needed.
Filters: Use MERV 8–13 filters sized for low pressure drop; change regularly.
Leakage: Seal with mastic or UL-181 tape; insulate ducts in unconditioned spaces.

Commissioning should verify airflow (cfm), temperature rise, and static pressure against the furnace nameplate.

Installation Best Practices And Codes

Furnace installation should follow local codes and national standards such as NFPA 54/ANSI Z223.1 and manufacturer instructions. Permits and inspections are typically required.

Key Installation Steps

Gas Line Sizing: Confirm adequate capacity and proper manifold pressure.
Venting: Match vent category; slope and terminate per code; avoid sharing flues improperly.
Combustion Air: Provide sufficient makeup air or use sealed combustion.
Condensate: Trap, drain, and neutralize for condensing units; protect from freezing.
Electrical: Dedicated circuit, correct polarity, and proper grounding.
Clearances: Maintain service and combustible clearances.

Commissioning And Verification

Combustion Analysis: Check O2/CO2, CO, and draft; adjust gas pressure per spec.
Airflow & Temperature Rise: Verify within nameplate range for heat exchanger longevity.
Safety Controls: Test limit switches, pressure switches, and flame sensor operation.
Thermostat Setup: Program staging, blower profiles, and fan delays.

Quality installation often matters more than model selection for comfort, efficiency, and reliability.

Maintenance And Troubleshooting

Routine maintenance extends life and reduces breakdowns. Many manufacturers require proof of maintenance to honor warranties.

Routine Tasks

Filters: Inspect monthly; replace every 1–3 months or per pressure drop.
Annual Tune-Up: Clean burners, flame sensor, and blower; inspect heat exchanger.
Condensate: Flush trap and drain; check for leaks and slime; clean or replace tubing.
Vents: Inspect for corrosion, blockage, or loose joints.
Thermostat: Verify calibration and schedules; update firmware for smart models.

DIY Checks Before Calling A Pro

Confirm thermostat is on Heat and set above room temperature.
Check breakers, furnace switch, and service panel fuse.
Ensure gas valve is open; verify propane tank level if applicable.
Replace a clogged filter; clear supply and return grilles.
Look for error codes on the control board through the sight glass.

Common Symptoms And Actions

Symptom	Likely Cause	What To Do
No Heat, No Fan	Power off, tripped breaker, blown fuse	Restore power; call if it trips again
Ignites, Then Shuts Off	Dirty flame sensor, bad pressure switch	Clean sensor; pro testing for switches
Short Cycling	Oversized unit, high static, dirty filter	Check filter; have airflow and sizing assessed
Water Around Furnace	Condensate blockage or leak	Clear trap/drain; add condensate safety switch
Burning Smell At Start	Dust on heat exchanger	Normal briefly; persistent smells need service

If you suspect a cracked heat exchanger or smell gas, shut off the system and call a professional immediately.

Safety Essentials

Combustion appliances require attention to air quality and safe operation. The main risks are carbon monoxide (CO), gas leaks, and fire hazards.

CO Detectors: Install on each level and near sleeping areas; test monthly and replace per manufacturer guidance.
Backdrafting: Ensure proper venting and combustion air; avoid competing exhaust (e.g., powerful range hoods) without makeup air.
Gas Leaks: If you smell rotten eggs, evacuate and call the gas utility. Do not operate switches.
Clearances: Keep combustibles away from the furnace and flue; do not store solvents or gasoline nearby.
Heat Exchanger: Annual inspection helps catch cracks that can leak combustion gases.

Follow manufacturer instructions and local codes, and consider periodic testing with a calibrated combustion analyzer. See EPA guidance on CO for more safety tips.

Costs, Warranties, And Incentives

Total cost depends on furnace type, size, venting, gas line changes, and ductwork. Regional labor rates vary widely.

Typical Installed Cost Ranges

Standard 80% Single-Stage: About $3,800–$7,500 installed.
High-Efficiency Two-Stage: About $5,500–$9,500 installed.
Modulating, Condensing: About $6,500–$12,000+ installed.
Ductwork Additions/Corrections: $1,500–$6,000+ depending on scope.

Operating costs depend on gas prices and climate. Maintenance plans typically run $150–$300 per year and can include priority service and discounts.

Warranties

Heat Exchanger: Often 20 years to lifetime (parts); labor is usually shorter.
Parts: Commonly 5–10 years with registration.
Labor: Usually 1–2 years from the installer; extended plans available.

Incentives And Credits

Federal Tax Credits: Section 25C may offer up to 30% of project cost, capped amounts for qualifying high-efficiency equipment; specifications change over time. Verify current criteria with the IRS and ENERGY STAR.
Utility Rebates: Many gas utilities offer rebates for ≥95% AFUE furnaces or ECM blowers.
Low-Income Programs: Weatherization Assistance and state programs can help with upgrades.

Check state/local incentives at the DSIRE database and confirm eligibility before purchase.

Gas Furnace Vs. Electric Furnace Vs. Heat Pump

Choosing the right heating system depends on climate, energy prices, and home specifics. Each option has strengths.

Gas Furnace

Pros: Strong heat in cold weather, fast warm-up, relatively low fuel cost in many regions.
Cons: Combustion safety and venting needs, CO emissions, gas service required.

Electric Furnace (Air Handler With Electric Heat)

Pros: Simpler installation, no flue, pairs easily with AC.
Cons: High operating cost where electricity is expensive; less efficient than heat pumps.

Heat Pump

Pros: Very efficient, provides heating and cooling; cold-climate models work well in many northern areas.
Cons: Performance drops in extreme cold; may need backup heat; duct or ductless options affect fit.

Dual-fuel systems pair a heat pump with a gas furnace, using the heat pump in mild weather and gas in colder conditions. This can reduce costs and emissions while maintaining comfort.

Comfort Features And Smart Controls

Modern furnaces offer enhancements that improve comfort, sound, and energy management.

Variable-Speed ECM Blowers: Quieter operation, better filtration, and steadier temperatures.
Advanced Thermostats: Learning algorithms, geofencing, and humidity control can optimize staging.
Zoning: Motorized dampers and multiple thermostats allow tailored temperatures by area.
Enhanced Filtration: Media filters or electronic air cleaners improve air quality with proper sizing.

Ensure controls are compatible with the furnace’s staging or modulation to realize full benefits.

Environmental Impact And Future Trends

Burning natural gas produces carbon dioxide—about 11.7 pounds per therm. A typical heating season can emit a few thousand pounds of CO2, depending on usage and weather.

Efficiency improvements, weatherization, and smart controls help reduce emissions. High-efficiency condensing furnaces and dual-fuel heat pump strategies are common transitional paths.

The DOE’s forthcoming 95% AFUE minimum for many furnaces manufactured after 2028 will shift the market toward condensing models. Meanwhile, advances in cold-climate heat pumps expand non-combustion options. Local policies, utility incentives, and fuel prices will influence adoption.

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Buying Checklist

Load Calculation: Request ACCA Manual J results in writing.
Duct Assessment: Measure static pressure; plan for sealing or resizing if needed.
Efficiency: Target ≥95% AFUE where feasible; consider ECM blowers.
Comfort: Choose two-stage or modulating for quieter, steadier heat.
Venting & Condensate: Verify routing and freeze protection for condensing units.
Incentives: Confirm rebates and tax credits before purchase.
Commissioning: Get documentation of combustion analysis, airflow, and temperature rise.
Warranty & Service: Register equipment; consider a maintenance plan.

Glossary Of Common Terms

AFUE: Seasonal measure of furnace fuel efficiency.
ECM: Electronically commutated motor; efficient variable-speed blower.
Condensing Furnace: High-efficiency unit with secondary heat exchanger.
Sealed Combustion: Uses outdoor air for combustion via dedicated piping.
Manual J/S/D: Industry standards for load, equipment selection, and duct design.
Therm: 100,000 BTU of energy; common billing unit for natural gas.

Frequently Asked Questions

How Long Does A Gas Furnace Last?

Typical lifespan is 15–20 years with proper maintenance. Heat exchanger condition and installation quality are key determinants of longevity.

Is A 98% AFUE Furnace Always Worth It?

It depends on gas prices, climate, and installation costs. In colder climates, high AFUE often pays off. In milder areas, duct upgrades or a dual-fuel heat pump may yield better value.

Can A Gas Furnace Work Without Electricity?

No. Modern furnaces require electricity for controls, fans, and ignition. A backup generator or battery system is needed for outages.

What Filter MERV Rating Should Be Used?

MERV 8–13 balances filtration and airflow for most homes. Ensure the filter is properly sized to avoid excessive static pressure.

How Often Should A Furnace Be Serviced?

Annually is recommended. Service includes cleaning, safety checks, combustion analysis, and confirmation of temperature rise and airflow.

Resources

How to Get the Best HVAC Prices

Firstly, keep in mind that installation quality is always the most important thing for residential HVAC project. So never sacrifice contractor quality for a lower price.
Secondly, remember to look up the latest rebates as we talked above.
Thirdly, ask for at least 3 bids before you make the decision. You can click here to get 3 free estimates from your local contractors, and this estimate already takes rebates and tax credit into consideration and filter unqualified contractors automatically.

Lastly, once you chose the right contractor, remember to use the tactics from this guide: Homeowners Tactics When Negotiating with HVAC Dealer to get the final best price.