Best Furnace (2026): How to Choose + ENERGY STAR Data

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The best furnace for a Front Range home is a condensing gas furnace that ENERGY STAR certifies for the northern U.S., sized from a real heat-loss calculation, set up for altitude, and vented by an installer who treats carbon monoxide safety as the main job. Those factors decide comfort, bills and safety far more than the brand on the cabinet. Many brands share one factory, and the same contractor can make any of them run well or badly. This guide explains AFUE, the difference between 80% and 95%-plus furnaces, why “U.S. South” certified models don’t count in Colorado, single-stage versus two-stage and modulating burners, and what home inspectors check after installation. On method: we don’t test equipment or rank brands on reliability. This guide explains how to choose and reports what ENERGY STAR’s certified furnace data shows.

Part of our Heating, Cooling & Ventilation guide — start there for the full picture →

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Quick Decision Table: Which Furnace Type Fits Your Home?

Price bands are typical installed costs for a replacement in a single-family home in 2026. They move with capacity, venting changes, whether the AC coil is replaced at the same time, ductwork fixes, permit fees and access (a tight attic install costs more than an open basement).

For a line-item view of quotes, see our furnace replacement cost guide. If you are open to going electric, our comparison of a heat pump vs furnace covers operating costs and comfort.

What Actually Matters When Choosing a Gas Furnace

AFUE: how much of the gas becomes heat

Annual Fuel Utilization Efficiency (AFUE) is the share of the fuel’s heat that ends up warming the house instead of going up the vent. An 80% furnace sends roughly a fifth of its fuel energy out the flue. A 96% furnace captures most of that by condensing water vapor out of the exhaust in a second heat exchanger. ENERGY STAR says certified gas furnaces in the northern half of the country are up to 18 percent more energy efficient than baseline models, and that certified furnaces combine higher AFUE ratings with higher-efficiency blower motors.

On the Front Range, where heating runs from October into May, the jump from 80% to the mid-90s usually pays back over the life of the furnace. The gap between 96% and 98% is much smaller. A 98% furnace isn’t twice as good as a 96%; it is two points better, and the cost of getting there often comes with features (modulation, communicating controls) that matter more than the number.

“All U.S.” vs “U.S. South Only”: the Colorado catch

ENERGY STAR splits gas furnaces by region. Furnaces certified for the U.S. South need an AFUE of 95% or higher. Furnaces certified for the U.S. North need 97% or higher. Both must keep cabinet air leakage at or below 2.0%. Colorado is on ENERGY STAR’s U.S. North list, along with neighbors such as Wyoming, Utah, Kansas and Nebraska.

That matters because a furnace advertised as “ENERGY STAR” may carry only the U.S. South label. ENERGY STAR marks those models with a separate “U.S. South” logo. A 95% or 96% furnace can be an excellent appliance, but if it is only South-certified it is not ENERGY STAR certified for a Colorado home, and it may not qualify for utility programs that require northern certification. Check the label, or look up the model number in the ENERGY STAR furnace product finder and confirm the region field reads “All.”

Staging and modulation

A single-stage furnace burns at full fire every time it runs. A two-stage furnace runs on a low fire (often around two-thirds) most of the time and goes to high fire on the coldest days. A modulating furnace adjusts its burner across a wider range in small steps. The more the furnace can turn itself down, the longer and gentler its cycles, which means fewer temperature swings, less noise and less dry, blasting air. Two-stage is the sweet spot for many homes. Modulating units make the most sense in larger houses, zoned systems and homes with rooms that never seem to settle.

The blower: ECM and variable speed

The blower moves air through the house in both heating and cooling seasons, so its motor uses a meaningful amount of electricity. Electronically commutated (ECM) motors use far less power than old permanent split capacitor motors, especially when the fan runs continuously for air filtration. Variable-speed ECM blowers also ramp up quietly and can improve air conditioner dehumidification. If you plan to run the fan often, this feature is worth more than another point of AFUE.

Altitude derating

Air at Denver’s elevation holds less oxygen per cubic foot than air at sea level. Gas appliances therefore need their input adjusted at higher elevations. Fuel gas codes and manufacturer instructions commonly require derating above 2,000 feet, often expressed as roughly 4% of input per 1,000 feet, unless the manufacturer specifies otherwise. Local gas utilities may already deliver gas with a lower heating value, which is part of the calculation. In practice, the installer should follow the furnace’s high-altitude instructions, which can include different burner orifices or a high-altitude pressure switch, and then verify gas pressure and temperature rise with instruments. Ask the contractor how they handle altitude; a confident, specific answer is a good sign.

Sizing

Many older furnaces are oversized, sometimes dramatically. Oversized furnaces short-cycle, overshoot the thermostat and wear components faster. ENERGY STAR’s HVAC buying guidance names the standard tool contractors should use: a Manual J load calculation that accounts for insulation, windows and air leakage. If you have added insulation or replaced windows since the old furnace went in, the right size may be smaller than what is there now. Our guide on furnace short cycling explains the symptoms of a mismatch.

Natural gas, propane and capacity ratings

Nearly every certified gas furnace in the ENERGY STAR list is rated for natural gas and can be converted to propane with a manufacturer kit. Rural homes on propane in Elbert County or the foothills should make sure the installer actually performs and labels that conversion, since running a natural-gas setup on propane is a safety hazard. When comparing quotes, look at both input and output capacity in Btu per hour. Input is what the burner consumes; output is the heat delivered to the house. Two furnaces sold as “80,000 Btu” can deliver quite different output depending on AFUE, and at altitude the delivered output drops further after derating. The heat-loss calculation should be compared with the derated output, not the nameplate input.

If your current furnace still runs, the decision to replace it usually comes down to age, safety and repair history. A unit well past 15 years with a failed heat exchanger, repeated control-board or inducer failures, or rising gas bills is a stronger candidate than a 10-year-old unit needing a single igniter.

What the ENERGY STAR Furnace Data Shows

We pulled ENERGY STAR’s certified furnace list for the U.S. market, which holds 3,697 model listings: 3,668 gas furnaces (natural gas or natural gas convertible to propane) and 29 oil furnaces. Only 1,307 of the gas listings are certified in all U.S. regions. The other 2,361 are certified for the U.S. South only.

Two things stand out. First, most South-only models sit at 95% or 96% AFUE, right in the range many sales sheets call “high efficiency.” Second, the “All” group is dominated by 97% models, which matches the current northern threshold. A few hundred “All” listings show 95% to 96.9% AFUE; that may reflect when they were certified, so for any specific model, confirm the region on the live ENERGY STAR product finder before relying on it.

Manufacturer families behind the brand names

Furnace brands cluster into a handful of manufacturing partners. Carrier Corporation’s furnace certifications, for example, span Carrier, Bryant, Payne, Heil, Tempstar, Day & Night, Comfortmaker and Arcoaire. Daikin Comfort Technologies covers Daikin, Goodman and Amana. Rheem and Ruud share a partner, as do Trane and American Standard. Buying a different badge from the same family often means buying very similar hardware from a different dealer network.

How to read this honestly:

• Listing counts measure catalog breadth, not quality. One furnace family can generate dozens of listings across capacities, cabinet widths and sister brands.
• The “All” share reflects product strategy. A family with many South-only listings may simply sell heavily into southern markets. It is not a rating of the company’s northern models.
• Top AFUE differences are small. Every major family offers certified models at or above 97%, the northern threshold. The best gas furnace for your house is the right capacity and staging from a dealer who services what they sell.
• ENERGY STAR does not measure reliability or noise. Use the data to confirm eligibility and efficiency, then judge the installer.

Installation and the Home-Inspection Angle

A furnace is a combustion appliance inside the living space, so installation errors can be dangerous, not just inefficient. These are the items inspectors look at most closely.

Venting and carbon monoxide

Condensing furnaces vent through plastic pipe (usually PVC, CPVC or polypropylene, as the manufacturer specifies), typically out a side wall or the roof. Inspectors check that the intake and exhaust terminations keep required clearances from windows, doors, gas meters and the ground, and that they sit above expected snow depth, a real concern in Front Range winters. Pipes should slope back toward the furnace so condensate drains, and joints should be glued and supported.

Non-condensing 80% furnaces vent through metal B-vent or a lined chimney. When a condensing furnace replaces one that shared a chimney with a gas water heater, the water heater is left alone in a flue sized for two appliances. That “orphaned” water heater can draft poorly and spill exhaust, so the chimney may need a liner or the water heater may need new venting. Ask about it in the quote.

EPA lists furnaces with leaking chimneys, back-drafting and improperly sized, blocked or disconnected flues among indoor carbon monoxide sources. CPSC recommends CO alarms on every level of the home and outside sleeping areas. Our guides to the furnace flue pipe and a cracked heat exchanger explain warning signs on existing systems.

Condensate, clearances and combustion air

• A condensate drain with a trap that is primed, routed to a proper drain, and protected from freezing if it runs through a garage or crawlspace. A condensate pump, if used, should have a safety switch.
• Clearances to combustibles and service clearance in front of the furnace, as listed on the data plate.
• Adequate combustion air for any non-direct-vent appliances in the same room, especially in tight utility closets.
• A gas shutoff valve and sediment trap at the furnace, and no stored solvents or paint nearby.
• Filter access that is easy to reach and a filter size that matches the cabinet.
• A temperature rise within the range on the data plate, which confirms airflow is correct.

Ducts and the AC coil

Replacing a furnace without looking at the ductwork can leave the new unit fighting the same restrictions as the old one. ENERGY STAR points homeowners to duct sealing as a way to get better heating and cooling performance. If you have central air, discuss whether the evaporator coil on top of the furnace should be replaced at the same time; it may need to match a new outdoor unit, and changing it later can mean paying for the same labor twice.

Permits, warranty registration and paperwork

Furnace replacements in Front Range jurisdictions generally require a mechanical permit, and the inspection checks gas piping, venting and combustion safety. Make sure your installer pulls it. Ask them to register the warranty, since extended parts coverage often depends on registration soon after installation. Keep the model and serial numbers and the ENERGY STAR or AHRI listing with your home records. When you sell, an inspector will look for permits on recent HVAC work. Our furnace inspection guide covers what an inspection includes.

How to vet a furnace installer

Ask each contractor for a written heat-loss calculation, the exact model number and its ENERGY STAR region, how they will handle altitude, the vent route and termination location, what happens to the water heater venting, who pulls the permit, and how long their own labor warranty lasts. A contractor who measures the house, asks about drafty rooms and checks the ducts is more likely to deliver a good result than one who quotes the same size from the doorway. Our guide to hiring heating and cooling pros lists more questions. If you’re comparing a furnace with an all-electric option, see our guide to the best heat pump for cold climates.

Where to Buy a Furnace

Furnaces are almost always bought through an installer. Big-box stores run installed-sales programs that subcontract to local HVAC companies, and independent dealers sell the brands they are trained on. Get at least three written quotes, compare model numbers and ENERGY STAR region rather than brand names, and check your utility (such as Xcel Energy Colorado) and the ENERGY STAR rebate finder for current furnace rebates before you sign.

References

• Furnaces Key Product Criteria (U.S. North and U.S. South) — ENERGY STAR (U.S. EPA)
• Furnaces: Features and Buying Guidance — ENERGY STAR (U.S. EPA)
• ENERGY STAR Certified Furnaces Product Finder — ENERGY STAR (U.S. EPA)
• Carbon Monoxide’s Impact on Indoor Air Quality — U.S. EPA
• Carbon Monoxide Information Center — U.S. Consumer Product Safety Commission
• Duct Sealing — ENERGY STAR (U.S. EPA)