Why Your Fuel Bills Beat a Square-Foot Rule
Most heat pump quotes start from the size of the house. That number can't see your insulation, your windows, how leaky the attic is, or whether you keep the house at 64 or 72. Two houses with identical floor plans can need heat pumps a ton apart.
Your fuel use has already accounted for all of it. Every gallon of oil or therm of gas you burned last winter replaced heat that left the house through those walls, windows and gaps, across a full season of real weather. Dividing that by how cold the season was gives your home's heat loss per degree of temperature difference — a measured figure, not an estimate. It's the same billing-analysis method New York's housing agency sets out for space-heating load calculations in its clean-energy programs.
How the Calculation Works
- Remove hot water. If the same boiler or fuel heats your water, that share has nothing to do with the house's heat loss. A summer month's use, times 12, is the cleanest way to take it out. Otherwise we estimate it from household size (about 25 gallons of hot water per person per day).
- Convert to heat delivered. Heating fuel × energy content (138,500 BTU per gallon of oil, 91,500 per gallon of propane, 100,000 per therm, 3,412 per kWh) × system efficiency.
- Divide by the season. Heat delivered ÷ (heating degree days × 24) gives BTU per hour for every degree the outside is colder than 65°F.
- Apply your design day. Multiply by (65°F − design temperature). That's the heat load on a night as cold as your system needs to handle.
For the default example — a Boston house burning 800 gallons of oil a year at 80% efficiency, with three people on the same boiler for hot water — the result is about 30,000 BTU/hr at 8°F, or roughly 2.5 tons. A typical oil boiler in a house like that is rated somewhere above 100,000 BTU/hr.
Don't Size the Heat Pump to the Boiler
The most common sizing shortcut is to match the old equipment's rating. That carries forward whatever oversizing the boiler had, and boilers have historically been installed far larger than the houses they heat — partly to recover hot water quickly, partly as a safety margin nobody ever revisited. If you enter your current boiler's output above, the calculator shows how its rating compares with your measured load.
An oversized heat pump costs more to buy and runs worse: it spends more of the season cycling on and off instead of running long and slow, which is where inverter heat pumps are most efficient.
The Number to Check on a Spec Sheet
The capacity in a heat pump's model name or headline is measured at 47°F. Heat pumps lose capacity as it gets colder, so that figure tells you little about your design night. What matters is the capacity at your design temperature — or at 5°F, the lowest standard rating point, if your design temperature is near or below it.
Cold-climate models list their capacity and efficiency at 5°F, 17°F and 47°F. NEEP's cold-climate specification, which many utility rebate programs use as their qualifying list, requires ducted systems to reach a COP of at least 1.75 at 5°F. The calculator gives your load at 47°F, 17°F, 5°F and your design temperature so you can line it up against those columns directly.
If you plan to keep the boiler as backup, you don't need to cover the design load in full. A heat pump that meets the load down to the high teens covers every hour warmer than that on its own — in coastal Northeast cities, where January averages around 30°F, that's most of the winter — and the boiler picks up the coldest nights. Whether your existing baseboard or radiators could run on a heat pump instead is a separate question — our baseboard heat pump calculator answers it with the load from this page.
Where This Method Goes Wrong
- Heat the bills didn't see. A wood stove, space heaters, a gas fireplace, or rooms left closed all carry part of the load without showing up in your fuel figure. The result will come out low.
- An unusual year. A mild winter, or months when the house was empty, skews a single year. Average two or three years, and if you can, use the actual degree days for those years from degreedays.net rather than the long-term normals built into the city list.
- Delivered fuel timing. Oil and propane are bought in deliveries, not by the month. Twelve months of deliveries can easily contain thirteen or eleven months of burning.
- Efficiency. An old boiler's real seasonal efficiency is often below its rating. Using a lower efficiency gives a smaller load, so when in doubt err high.
- Changes since. New windows, insulation or air sealing after the billing year lower the load; an addition raises it.
The city list uses NOAA 1981–2010 normal heating degree days and ASHRAE 99.6% heating design temperatures. Recent winters have generally run a little milder than those normals; a professional Manual J load calculation is still the right step before a large purchase, and a good installer will welcome your fuel-use figure as a cross-check.