AFUE: How Much of Your Fuel Becomes Heat
Annual Fuel Utilization Efficiency (AFUE) is the primary efficiency rating for furnaces. Modern gas units typically run 80%–98% AFUE. A 90% furnace turns 90% of its fuel into usable heat and loses ~10% up the vent. Electric furnaces convert nearly 100% of input electricity into heat because there’s no flue loss—yet that doesn’t guarantee lower bills; electricity usually costs more per unit of heat than natural gas.
Quick check:
Delivered BTU = Input BTU × AFUE
Example: 100,000 BTU input at 95% AFUE → 95,000 BTU output.
“High efficiency” gas = condensing (secondary heat exchanger, PVC venting). “Standard efficiency” = non-condensing (metal vent/chimney).
For electric air handlers paired with heat pumps, see Air Handlers.
BTU Output & Right-Sizing (Why Capacity Isn’t a Guess)
BTU (British Thermal Unit) describes heating capacity. Many high-efficiency gas furnaces land around 75,000–100,000 BTU/h; older/inefficient equipment can be higher. The “right” size depends on heat loss construction, insulation, windows, infiltration, and design temperature. Oversizing creates short cycles, noisy starts, and poor comfort; undersizing struggles on design days.
Field workflow that works:
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Perform or request a Manual J.
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Confirm duct capacity (static pressure, cfm per ton) and gas/electrical service.
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Match staged or modulating heat to load profile.
Use our Sizing Guide and, if you want a second set of eyes.
Electric Heating, HSPF & When Heat Pumps Win
AFUE applies to resistance heat (near 100%), but for heat pumps you’ll see HSPF (Heating Seasonal Performance Factor). Typical modern ranges are 7.5–13.0; higher is better. HSPF measures seasonal performance, not one operating point, and it reflects real-world cycling/defrost. In many climates, a high-HSPF heat pump with an electric or gas backup (“dual fuel”) beats straight resistance heat on operating cost while keeping your install simpler than a full gas rip-and-replace.
Heat Exchangers & CO Safety (What Techs Inspect)
In a gas furnace, the heat exchanger separates combustion gases from indoor air. Burners heat the exchanger; the blower moves house air across it to pick up heat. A cracked exchanger can allow combustion products—including carbon monoxide (CO) into supply air. That’s why annual inspections matter: visual checks, combustion analysis, and verifying pressure differentials and flame behavior.
Risk indicators: abnormal flame movement when the blower starts, soot, rust flakes, nuisance rollout switch trips, or elevated CO in supply air.
Questions about what a “cracked HX diagnosis” entails? Visit our Help Center.
Combustion Air: Sealed vs. Atmospheric (Why It Matters)
Atmospheric furnaces pull combustion air from the room; sealed-combustion units bring air in from outdoors via dedicated piping. Sealed combustion improves safety (no depressurization/backdrafting), reduces infiltration, and keeps burner air cleaner—especially valuable in tight homes or rooms with negative pressure (exhaust fans, dryers, mechanical rooms). It also pairs with condensing designs that reclaim latent heat for higher AFUE.
If you’re evaluating a swap from atmospheric to sealed: verify vent routing and exterior termination clearances. Need accessories for terminations and line sets on split systems? See Accessories.
Venting Systems & Draft (Direct-Vent vs. Conventional)
Proper venting safely ejects combustion byproducts. Direct-vent (two-pipe) systems supply outside air and exhaust through dedicated pipes—the safest, most consistent approach for modern condensing furnaces. Conventional venting uses a single flue to a chimney and relies on buoyancy/draft—more prone to issues if the chimney is oversized, cold, or competing with house depressurization.
Simple idea sketch:
[Outside] ==> Intake Pipe ==> [Sealed Burner] ==> Exhaust Pipe ==> [Outside]
If your project points toward all-in-one equipment, compare R-32 Packaged Systems.
Operating Cost vs. Installation Cost (The Whole-Bill View)
Upfront, electric furnaces are typically simpler and cheaper to install—no gas piping, venting, or combustion air. Ongoing, many households see higher operating costs with resistance heat because electricity often costs more per delivered BTU. Typical figures: natural gas heating around $540/year, electric heating roughly $900–$2,500/year (utility rates and climate drive the swing).
Gas furnaces cost more up front (venting, condensate, intake/exhaust penetrations) but commonly win on lifetime operating cost where gas is available and winters are severe.
Check budget options, promos, and payments: Lowest Price Guarantee.
Maintenance & Diagnostics: Gas vs. Electric
Gas furnaces need annual combustion checks: burner cleanliness, flame signal, manifold pressure, heat-exchanger inspection, inducer/pressure switch operation, vent integrity, and flame color (steady blue is ideal). Techs also test for leaks and verify condensate management on condensing units.
Electric furnaces are mechanically simpler with no combustion so maintenance focuses on filters, blower, sequencers/relays, elements, and connections (torque and thermal discoloration checks). Both types benefit from proper airflow (clean filters, correct blower tap, duct static within spec).
Browse DIY-friendly systems when appropriate: Ductless Mini-Splits. Always follow manufacturer commissioning steps.
Lifespan & Replacement Planning (What to Expect)
Expect gas furnaces to last 15–20 years with routine care. Heat exchangers see thermal stress and moisture (in condensing units), and ignition components wear. Electric furnaces often run 20–30 years because there’s no combustion; elements and controls are straightforward to service.
Replace sooner if: repair costs exceed ~20% of replacement, you have a cracked HX, chronic nuisance trips, or an oversized single-stage unit short-cycles and kills comfort/indoor air quality. Align replacement with duct or IAQ upgrades so you only open ceilings once.
Stretch your budget with Scratch & Dent finds and buy with confidence under.
Climate & Environmental Impact (CO₂, Grid, and Fit)
In colder climates, gas furnaces shine with fast, powerful heat and consistent performance at low outdoor temps. In milder regions, electric options including heat pumps often pencil out thanks to lower install cost and efficient shoulder-season operation.
Emissions lens: burning natural gas emits about 11.7 lb CO₂ per therm at the appliance. Electric furnaces have zero on-site emissions; their footprint depends on your grid mix. The good news: the grid keeps decarbonizing clean electricity and reaches roughly 39% of global generation in 2024 so electric and heat pump options get cleaner over time.
See combined options: R-32 AC & Gas Furnaces.
Recommended Next Steps (CTA)
Not sure which path fits your home and utility rates? Here’s a field-tested checklist:
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Size it right: Start with our Sizing Guide and share photos/specs via Quote by Photo. We’ll sanity-check BTU needs, duct capacity, and venting routes.
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Run the numbers: Tell us your last 12 months of gas and electric usage. We’ll compare 10-year operating cost for (a) high-efficiency gas, (b) heat pump + electric backup, and (c) dual fuel.
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Verify site constraints: Available gas line? Chimney condition? Electrical panel space/ampacity? We’ll map equipment that fits the site, not just the brochure.
Have technical questions or need a second opinion on a bid? Start at our Help Center or message us through the Design Center. We’ll keep it practical, code-compliant, and aligned with your climate and utility rates.
