A heat pump is the right size when the heat it delivers on your design day fits what your house loses that day. Neither number is on the box. In one example on this site, a heat pump sold as 3 tons makes about 19,160 Btu/h at 9°F, extending the maker’s figures, against a load of 36,000 Btu/h: 53% of it. In another, a house whose furnace puts out 64,000 Btu/h loses only about 19,400 to 22,600 Btu/h on a 9°F night, going by its gas bills. The label can fall short, and the old furnace can run far past the job.
This guide runs in the order you’d do the work, and each step leads to a calculator for your own numbers. They give estimates for checking a quote, not a replacement for a load calculation, and a qualified installer decides the final size. The Last Furnace sells and installs nothing.
Load comes before capacity
Load belongs to the house: the heat it loses or gains at a set outdoor temperature. Capacity belongs to the machine: the heat it can deliver at that temperature. Both are in Btu per hour, and DOE’s load guideline puts 1 nominal ton at 12,000 Btu/h. You need the load before any model number means much.
Step 1: Get a load number
ENERGY STAR tells homeowners that a contractor should verify system size with Manual J, the Air Conditioning Contractors of America’s residential procedure. Its result depends on what someone types in. DOE modeled a 2,223 sq ft Chicago house at 41,700 Btu/h, applied common “safety factors” to the inputs, and reached 64,700 Btu/h, 55% higher, without changing the method.
Your bills give a free check on the heating half. A September 2026 preprint from Purdue University and Trane Technologies researchers found Manual J reports, as practitioners ran them, averaging 2.35 times a bill-based estimate across a sample of 74 houses. It had no measured load to compare against, and its authors say Manual J can be highly accurate given accurate inputs. So a gap like that is worth a look at the inputs, and neither number is automatically right.
Our guide to HVAC load calculations explains how to read a Manual J report and estimates your heating load from winter fuel bills. It’s a check, not a Manual J. Bills can’t show cooling.
Treat floor area as a rough range at best. For the same 2,000 sq ft, EnergySage’s published estimate of 1 ton per 600 sq ft (updated 19 March 2026) gives 3.3 tons, while two rules Rewiring America describes give 4 and 5. Matching your old furnace isn’t a method either. Natural Resources Canada calls furnace output a last resort.
Step 2: Find your design temperature
A design temperature is the cold-weather figure a load calculation is built on, not the record low. It moves the answer: the furnace house above rises to about 20,800 to 24,500 Btu/h at 5°F.
ENERGY STAR publishes county figures, and its editions can differ by many degrees. Boston’s Suffolk County is 12°F in the 2015 edition and 1°F in the 2019 edition, whose figure comes from the Lowell station. These are limits for ENERGY STAR new homes, not ideal values. EPA recommends the Manual J value for the weather location closest to the home, so ask your installer which station they used. Our calculators carry a table to start from. Programs can use another percentile, as Efficiency Maine’s claim form does.
Step 3: Check capacity at that temperature
Rated tons aren’t winter tons. The Rheem sheet behind the opening example, an ordinary 3-ton pair and not a recommendation, lists 33,600 Btu/h at 47°F and 22,200 at 17°F. Extended to 9°F, that’s the 19,160 Btu/h above, leaving 16,840 Btu/h for backup heat. On the same example house, the same line’s 5-ton reaches about 73% of the load, also extended. A cold-climate Carrier size 54 reaches about 134%. Treat any figure below a sheet’s last row as a guess: Carrier’s sheet says extrapolation “is not an acceptable practice” for its data.
The figure to find is the balance point, where the house’s load line crosses the unit’s capacity line, near 25.5°F in the example. Above it the heat pump can carry the house alone. Below it, something else has to help. Our heat pump size calculator finds yours from your load, your design temperature and the maker’s figures for your exact outdoor and indoor pair, and prints the backup heat left over. For how output holds up in deep cold, read what really happens to a heat pump in cold weather.
Step 4: Decide what the heat pump is for
The target depends on the job. These are guidance from NEEP’s cold-climate guide, Natural Resources Canada and one state program, not US rules. We haven’t read Manual S, ACCA’s paid selection standard.
| The job | Capacity target | From |
|---|---|---|
| Replaces the system, no auxiliary heat | 100% to 115% of the load at design temperature | NEEP |
| Replaces the system, small supplemental source | 75% to 85% of the load at design temperature, plus just enough supplemental heat | NEEP |
| Does most of the heating, backup in deep cold | Close to or above the load at 17°F | Natural Resources Canada, option 4C |
| Existing heat stays as backup | Design-temperature capacity is a secondary concern; undersizing somewhat should improve efficiency | NEEP |
| Primary heat under a Maine rebate | At least 80% of peak load, 100% with supplemental heat, not counting a furnace or boiler | Efficiency Maine’s form |
The program decides what qualifies, so check its form before you sign. Our Maine rebate page has that state’s detail.
Keeping the furnace is a normal answer, and it can shrink the heat pump you need. A furnace can also be the cheaper winter fuel: our heat pump vs furnace guide sets out when it’s the better call, and the dual fuel calculator finds where to switch between them. NEEP adds a warning: a heat pump that satisfies a whole-house thermostat in very cold weather can leave basements and remote areas colder than before, and pipes may freeze there.
Mini splits: size by the room
A mini split is sized per zone, so run the numbers once for each indoor unit. ENERGY STAR’s chart, written for room air conditioners, puts 600 sq ft at 14,000 Btu/h of cooling. NEEA’s rough table shows heating overtaking cooling at about 550 sq ft in the -10°F to 5°F band, for insulated 2×4 walls and with its 125% margin included.
The number in a model’s name is a label. Two Mitsubishi pairs with the same 12,000 Btu/h rated cooling, shown as examples and not picks, list 14,690 Btu/h (MUZ-FS12NA) and 9,540 Btu/h (MUZ-GS12NA) at 5°F on distributor and retailer copies of the maker’s sheets. Our mini split size calculator estimates both loads for one zone, then checks a specific unit’s sheet against them.
Why one size up isn’t the safe choice
NEEP warns that gross oversizing can cause excessive cycling, low efficiency and poor summer dehumidification, and that load calculations already have safety factors built in, so they shouldn’t be padded. Compare the unit’s minimum capacity with your load at 47°F too: if the minimum is higher, expect it to cycle on mild days.
Undersizing has a cost too. Without backup, the house runs short on the coldest nights. With backup, more of the winter goes to the other system. In NREL’s 2023 field study of ducted heat pumps, two sites with spare capacity met more than 95% of the season’s heating with the compressor, while one sized at 63% of its design load met less than 80% in a colder-than-normal winter.
Natural Resources Canada warns that full-load sizing may not be possible on ducts designed for a traditional furnace, unless the shell has been improved. NEEP says to fix insulation and air leaks before buying where you can, because that reduces the size of equipment required.
What to get in writing
Ask for these on paper. Our heat pump installation guide covers the rest.
- The heating and cooling load, with the method, design temperature and air leakage figure behind it.
- Capacity at your design temperature for the exact outdoor and indoor pair, from the maker’s table, not the nominal size.
- The balance point, and what heats the house below it.
- Minimum capacity compared with your mild-day load.
- The largest system your ducts can take, tested, as ENERGY STAR’s bid checklist says to ask.
- Whether a permit and inspection are needed. Ask your local building department if you’re not sure.
If an installer sizes from square footage alone, or matches the old unit’s tonnage without a load number, ask them to run the calculation. If a heat pump will be the only heat in the house, or an estimate would decide how much backup you keep, get a proper load calculation from a qualified installer.
This page is general information, not professional, tax or legal advice.
Sources checked on 4 November 2026.
