Calculator inputs
Example data table
| Scenario | Current AFUE | New AFUE | Annual cost | Net cost | Year‑1 savings | Payback |
|---|---|---|---|---|---|---|
| Typical upgrade | 80% | 95% | $1,800 | $4,350 | $420 | 10.4 yrs |
| High rebates | 78% | 96% | $2,200 | $3,600 | $560 | 6.4 yrs |
| Lower fuel prices | 82% | 92% | $1,200 | $4,000 | $160 | 25.0 yrs |
Formula used
Efficiency scaling: For the same heat delivered, fuel input is inversely proportional to efficiency.
- New annual cost = Current annual cost × (Current AFUE ÷ New AFUE)
- Fuel savings = Current annual cost − New annual cost
- Net savings (Year 1) = Fuel savings + (Old maintenance − New maintenance)
- Net project cost = (Equipment + Install + Permits + Other) − Rebates − Credits
- Simple payback = Net project cost ÷ Net savings (Year 1)
- NPV = Σ[ Savingsy ÷ (1+discount)y ] − Net project cost
Savings can be escalated annually using the fuel price escalation rate.
How to use this calculator
- Pick an input method: annual cost, annual fuel use, or heating demand.
- Enter your current and proposed efficiency values (AFUE).
- Add project costs, then subtract rebates and tax credits.
- Adjust maintenance costs and optional controls savings if relevant.
- Set service life, fuel escalation, and discount rate for valuation.
- Click “Calculate savings” and review KPIs and yearly projection.
- Use the CSV or PDF buttons to export your results.
Efficiency and bill impact
A furnace upgrade changes fuel input required for the same delivered heat. The calculator scales annual heating cost by the ratio of current AFUE to new AFUE. For example, moving from 80% to 95% reduces modeled fuel spending by about 15.8%. If current heating cost is $1,800, the first-year fuel savings is roughly $284 before any controls or maintenance differences.
Project cost structure
Total project cost combines equipment, installation labor, permits, and other line items, then subtracts rebates and credits to produce a net project cost. Using the example inputs ($3,200 equipment, $1,400 labor, $150 permits, $100 other, $500 rebates), net project cost is $4,350. This net value is the starting point for payback, NPV, and ROI.
Savings beyond fuel
Maintenance and repair spending can shift materially after replacement. The calculator adds annual maintenance savings as (old maintenance minus new maintenance). If the old unit averages $200 per year and the new unit averages $120, that adds $80 to annual net savings. Optional controls savings can also be applied as a percentage reduction on the modeled new annual cost.
Payback and break-even timing
Simple payback equals net project cost divided by year-one net savings. With $4,350 net cost and $420 net savings, payback is about 10.4 years. The break-even year shown in the projection table uses cumulative, not discounted, savings; it indicates when total savings exceeds the upfront net cost.
Discounted value and sensitivity
NPV discounts future savings using the chosen discount rate and escalates savings using the fuel escalation rate. A higher discount rate reduces NPV, while higher escalation increases it. Adjust AFUE, fuel price, service life, and incentives to test scenarios. The Plotly chart visualizes net savings and cumulative savings to make sensitivity easier to interpret. For many homes, even small efficiency gains compound when fuel prices rise, turning modest year-one savings into meaningful long-run value steadily. This improves comparisons across realistic household scenarios. today today today today today today today today today today
FAQs
1) What should I use if I only know my total gas bill?
Use the “annual heating cost” option and enter the portion tied to space heating. If you are unsure, use winter bills and subtract typical summer usage.
2) Why does cost scale by AFUE ratio?
For the same delivered heat, required fuel input is inversely proportional to efficiency. That makes new cost approximately current cost multiplied by current AFUE divided by new AFUE.
3) Does this include duct leakage or insulation upgrades?
No. It assumes the home’s heat demand stays consistent, unless you use the heating use change input. Envelope upgrades should be modeled separately or reflected via a negative use change.
4) What discount rate should I choose?
Many homeowners use 3%–8% to represent opportunity cost and risk. Use a higher rate if you want more conservative present-value results.
5) How are rebates and credits applied?
They reduce net project cost directly. Enter only amounts you reasonably expect to receive or claim based on your eligibility and program rules.
6) Why can break-even differ from simple payback?
Break-even is the first year cumulative savings exceeds upfront cost, using escalated savings. Simple payback uses only year-one savings and ignores escalation.