Whether a heat pump costs less to run than a furnace can turn on a single input. So every running cost on The Last Furnace shows its formula, uses a winter gas price, labels each assumption, and prints the result the same way whichever side wins. Rules and rebates are quoted from their source with the date we read them. Installed prices are not part of this site: our sister site CostPlain handles those. This page sets out how each part works and where its limits are.
How running costs are computed
Every comparison follows the same steps, and the calculators let you change each input:
- Find the heat the house used. Fuel burned × the efficiency of the system that burned it. Your bills count fuel, not heat, so this step removes what went up the flue.
- Put it in common units. We use EIA’s factors: 3,412 Btu per kWh, 100,000 Btu per therm, 1,036 Btu per cubic foot of gas, 138,500 Btu per gallon of heating oil and 91,452 Btu per gallon of propane. EIA calls these sample factors.
- Cost the fuel option. Heat ÷ the new system’s efficiency × your fuel price.
- Cost the heat pump. Electricity in kWh = heat ÷ (seasonal COP × 3,412). Multiply by your rate.
- Find the break-even. Break-even COP = the cost of a million Btu of electric resistance heat ÷ the cost of a million Btu delivered by the fuel. Above it, the heat pump costs less to run. Below it, the furnace or boiler does.
Here is the opening example of our heat pump vs gas furnace calculator, at US average prices. It is all our arithmetic, and the 80% furnace, the 95% furnace and the COP of 3.0 are our assumptions.
| Step | Working | Result |
|---|---|---|
| Gas for heating | EIA RECS 2020 US average, 44.9 million Btu | 449 therms |
| Gas price | $14.77 per Mcf ÷ 10.36 therms per Mcf | $1.43 a therm |
| Furnace you have, 80% | 449 therms × gas price | $640 a year |
| Heat delivered | 44.9 million Btu × 80% | 35.9 million Btu |
| New furnace, 95% | Heat ÷ 95% × gas price | $539 a year |
| Heat pump electricity | Heat ÷ (3.0 × 3,412) | 3,509 kWh |
| Heat pump | 3,509 kWh × 18.16¢ | $637 a year |
| Saving against a new furnace | $539 − $637 | −$98 a year |
| Break-even COP against a new furnace | $53.22 ÷ $15.01 per million Btu | 3.55 |
A negative saving means the furnace costs less to run. We print that in the same plain style as the opposite result, and either one is a normal answer. For oil, propane and electric resistance heat, the heat pump savings calculator runs the same steps with that fuel’s factor.
Why we use winter gas prices
The latest monthly gas price is the wrong number for a heating decision. EIA’s US residential price was $27.05 per Mcf in August 2026. For the first quarter of 2026 it was $14.77. Homes burn very little gas in summer, so fixed charges spread over tiny volumes and push the average per unit up. You buy your heating gas in winter, at the winter figure.
The difference is enough to change the winner. Run the example above at each price:
| Gas price used | New 95% furnace | Heat pump, COP 3.0 | Break-even COP | What it appears to show |
|---|---|---|---|---|
| $14.77 per Mcf, Q1 2026 | $539 a year | $637 a year | 3.55 | The furnace costs less to run |
| $27.05 per Mcf, August 2026 (wrong) | $987 a year | $637 a year | 1.94 | The heat pump appears to save $350 a year |
So no calculator here uses the latest month. The defaults are EIA’s Short-Term Energy Outlook residential prices for the first quarter of 2026, by Census division, from the October 2026 edition. That is a regional figure, so your state’s price can differ. Your own winter bill is better than any default, and the calculators let you enter it. On a winter bill, subtract the fixed monthly charge and divide by the units used.
Where the other defaults come from
Every default carries its date in the calculator’s help text. Each has a limit worth knowing:
| Input | Source and date | The limit |
|---|---|---|
| Electricity by state | EIA Electric Power Monthly, table 5.6.a, August 2026, preliminary, released 23 October 2026 | A summer month, and an average that includes fixed charges. The US figure was 18.16¢, against 17.83¢ in EIA’s first-quarter 2026 outlook figure, but we have not measured that gap by state. Winter and heat pump rates can differ. |
| Heating oil and propane | EIA weekly prices for oil and propane, 26 October 2026 | Exclude taxes. October propane runs below winter peaks. EIA publishes no figure for many states. Use the price you actually paid. |
| Typical heating use | EIA RECS 2020, table CE5.4.ST | Fuel burned in one survey year, as an example only. Your bills are better. |
| Design temperature | ENERGY STAR Design Temperature Limit Reference Guide, 2015 edition, revised 2019 | A limit table at the 99% heating level for certified homes, not a table of ideal values. Where a county has several stations it takes the lowest. Check the value for your location with your installer. |
| Degree days | NOAA NCEI U.S. Climate Normals, 1991-2020 | 30-year normals, not last winter’s weather. |
| Capacity and COP | Makers’ own data sheets | Named as examples, never as picks. Check the model suffix and the indoor match. We report the maker’s figures as the maker’s. |
What is an assumption
Some inputs have no public source, or depend on your house. We mark those “our assumption” and let you change them:
- Seasonal COP. It drives the running-cost result and it is the hardest number to know. The default of 3.0 is an example, not a rating and not a promise.
- Furnace and boiler efficiency. The 80% and 95% are examples. Use the AFUE on your own label and on your quote.
- Balance-point base. Our heat pump size calculator draws the load from zero at 65°F. That base is an assumption.
- Load per square foot. Rule-of-thumb load factors have no primary source, and we label them that way. A published figure, such as a ton per so many square feet, is quoted as a published estimate.
- Load share and hours. How hard a unit works and for how long depends on your house and your habits, so these are your inputs.
A rated figure is a test result, not what a home gets. HSPF2 ÷ 3.412 gives a seasonal COP for that rating test, and we say so when we use it. Field results run differently: a British Columbia study of 26 systems at 24 occupied homes estimated average seasonal heating COPs of 2.4 for ductless, 2.6 for single-stage central and 3.3 for variable central systems. Those are older systems in one region, not a US average and not a figure for your home. We show such results beside the rating and never instead of your own numbers.
How sizing estimates work
Our sizing tools ask two questions: how much heat does the house need on a design day, and what can this unit deliver at that temperature? The first can come from your bills, using degree days, in our load calculator. The second comes from the maker’s capacity figures at listed outdoor temperatures. Neither replaces a load calculation done for your house. Our sizing pages say that an estimate from bills is not a Manual J, and a qualified installer sizes the real system.
How rules and rebates are checked and dated
Every tax rule, rebate, standard and program a page uses goes first into a rules register, with its primary source, the date we read it and one of three labels:
- Confirmed: we read the statute, notice, rule or program document and checked the wording.
- Partly confirmed: we saw the primary source only in part, or it conflicts with another. The page either says what is missing or leaves the claim out.
- Unconfirmed: only a news report, a seller or nothing. We do not state it as fact.
Where the text is clear, we quote it. The federal credit is the plain case: section 25C “shall not apply with respect to any property placed in service after December 31, 2025” (26 USC 25C(i)), so no running-cost or payback figure here includes it. Where the text is not clear, we say that too. DOE Program Notice 26-2 removes fuel-switching upgrades and also lets homes keep existing fossil-fuel systems when installing a heat pump. We have not found a settled reading of how those lines fit together, so the rebates page leaves it open and sends you to your state energy office.
Rebates are rechecked monthly and everything else in the register at least every three months. Rebates that change on 31 December 2026 are rechecked in mid-December and again in January, when 2027 sheets appear. Program pages can change without notice, so confirm the current terms with the program or the utility before you sign anything. Only the program decides whether you qualify.
Installed prices live on CostPlain
The Last Furnace has no install-price tables, and no page here gives its own estimate of what a system should cost. CostPlain, our sister site for US home repair costs, does that work, with a build-up from dated prices and government wage data. Here, a price appears only as your own input in a calculator or as a named, dated published estimate. We never average published figures, because a marketplace quote, a survey and a maker’s estimate measure different things.
What we do report is the extra cost over what you would buy anyway, which often matters more than the sticker price. The source is TECH Clean California’s cost study, a December 2023 survey of 64 registered California contractors about a defined simple job. It covers California only, and those are contractors’ reported averages, not invoices or bids, so we say that wherever we cite it.
The cost register
Every shared price, factor and data default goes into one cost register before a page uses it, with the value, the source and its link, the date we read it and what it measures. Pages draw from the register, so the same figure shows the same way everywhere and a change is made in one place.
Volatile figures are refreshed through the heating season: EIA electricity and winter gas prices monthly as new data come out, and oil and propane at least monthly to March. Makers’ sheets are rechecked every three months, and fixed files such as RECS once a year. A page shows an updated date only when its content really changed.
What we don’t use, and why
- The latest monthly gas price. It can flip the answer, as shown above.
- Ratings as home results. A rated COP is never written as what a home will get.
- Sellers’ savings claims as fact. A maker’s or installer’s claim is reported as theirs, and state programs’ examples are shown with their stated assumptions.
- Standards we can’t read. ACCA’s Manual J and Manual S, the National Electrical Code and UL standards are sold or sit behind a sign-in. We quote no limit from them, and we reproduce no tables from them.
- Figures we can’t trace. If we can’t find where a number came from or what it measures, we leave it out or say plainly that it is unconfirmed.
What an estimate can’t know
- Your house. Insulation, air leaks, ducts and layout change both the load and what a heat pump delivers.
- Your real COP. It depends on the equipment, the install, your climate and how the system is set up with any backup heat.
- Your rate plan. Tiered, time-of-use, winter and heat pump rates can all move the result. Fixed gas charges end only if you close the gas account.
- Next winter. Fuel and power prices change from year to year, and our defaults are a snapshot.
- Your program. Eligibility, funds and deadlines are set by the program, not by us.
So our numbers are estimates, not quotes, and never a verdict on what you should buy. Use them to ask better questions. A qualified installer, the program or a tax professional decides what applies to you. Our editorial policy covers how pages are checked, and our disclaimer sets out the limits: this is general information, not professional, tax or legal advice. If a figure looks wrong, please tell us.
Sources checked on 4 November 2026.