Furnace Efficiency Calculator

Understanding AFUE Ratings

Annual Fuel Utilization Efficiency (AFUE) measures how effectively your furnace converts fuel to heat over a typical year:

  • AFUE Rating Minimum standards: 80% non-weatherized gas, 81% weatherized gas, 83% oil furnaces. High-efficiency: 90-98.5% (condensing). Each 1% AFUE increase = 1% fuel savings.
  • Fuel Type Natural Gas: 80-98.5% AFUE possible. Propane: 82-96% AFUE typical. Oil: 83-89% AFUE common. Electric: 95-100% AFUE but higher operating cost.
  • System Age 0-5 years: -0-2% efficiency loss. 5-10 years: -3-7% loss. 10-15 years: -8-12% loss. 15+ years: -13-20%+ loss. Replace at 15-20 years.
  • Maintenance Annual tune-up: +2-4% efficiency gain. Clean burners: +1-2%. New filter: +1%. Sealed ducts: +10-20% system efficiency.

Efficiency Factors

Maintenance

Check combustion efficiency annually. Clean heat exchanger surfaces. Verify 325-400°F stack temperature. CO levels <100 ppm. Draft -0.02 to -0.04" W.C.

Filters

MERV 8-11 standard (change 3-6 months). MERV 13+ for allergens (2-3 months). Max pressure drop 0.5" W.C. Washable: clean monthly.

Ductwork

Insulate R-6 minimum in unconditioned space. Air leakage <10%. Static pressure 0.5-0.9" W.C. Supply/return balanced within 10%.

Controls

2-stage: 15% savings vs. single. Variable: 25% savings. Smart stats: 10-12% heating savings. Outdoor reset: 5-10% savings.

Cost Analysis

  • Operating Cost 80% AFUE: $1000/yr typical. 95% AFUE: $842/yr. Savings = (New AFUE - Old AFUE) × Annual Cost ÷ Old AFUE.
  • Upgrade ROI 80% to 95% AFUE: $3000-4500 cost. Annual savings: $158-200. Payback: 15-20 years. Utility rebates may reduce by 30%.
  • Energy Prices Natural Gas: $1-1.50/therm. Propane: $2-3/gallon. Oil: $2.50-4/gallon. Electric: $0.10-0.20/kWh. Check local rates.
  • Environmental Impact 80% to 95% AFUE reduces CO2 by 1-2 tons/year. NOx emissions: <40 ng/J for low-NOx units. ENERGY STAR: 15% more efficient.

Advanced Features

  • Modulation 40-100% firing rate range. 25-35% gas savings vs. single-stage. Better temperature control (±1°F vs. ±4°F).
  • Sealed Combustion Direct-vent required for 90%+ AFUE. PVC venting allowed. Intake air 20-60°F warmer. Eliminates backdrafting risk.
  • Heat Exchanger Primary: Aluminized steel (10-15 years) or stainless (15-20 years). Secondary: Condensing type adds 10-15% efficiency.
  • Blower Motor ECM: 60% electrical savings vs. PSC. Variable-speed: Better humidity control. Soft start reduces noise/wear.