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Calculator · Heat pump
Heat pump sizing calculator - the rating, and what is left of it on the design night
A heat pump is rated at 47°F and installed where it is colder than that. Give this page the heating load you already have and the outdoor design temperature it was figured at, and it works out what a unit has to carry on its 47°F nameplate to cover the house on the design night. The capacity factors and the sizing call are the same code the BuildSolver assistant runs in chat.
- Enter the whole-house heat loss at design conditions in BTU/h. The Manual J calculator produces one from a ZIP code and a short description of the house.
- Use the 99% winter design dry-bulb the load was figured at. The design temperatures page gives it by ZIP.
- A standard split system and a cold-climate heat pump hold very different capacity in the same weather, so the class changes the answer more than the design temperature does.
- The result shows the factor, the capacity the unit needs on its 47°F nameplate, the nominal size that clears it, and what that size delivers in your weather. Add a specific unit's 47°F rating to see its shortfall and the electric backup that covers it.
Example result for the default inputs
36,000 BTU/h heating load at 17°F outdoor, standard split system.
Capacity factor at 17°F
72% of the 47°F rating
Capacity needed at 47°F
50,000 BTU/h
Nominal size at 47°F
60,000 BTU/h
| Heating load at design | 36,000 BTU/h |
|---|---|
| Equipment class | standard split system |
| Where the factor came from | Table row at 17°F, factor 0.72 |
| Capacity needed at 47°F | 50,000 BTU/h |
| Nominal size at 47°F | 60,000 BTU/h |
| That nominal size at the design temperature | 43,200 BTU/h |
| Margin over the load at the design temperature | 7,200 BTU/h |
For estimation purposes only. Consult a licensed engineer for final designs.
The Manual J calculator returns a whole-house heating load from a ZIP code and a short description of the house.
How the size is calculated
Two lines of arithmetic carry this page. The calculator takes the heating load you already have, reads the capacity factor for your outdoor design temperature out of the same table the BuildSolver heating tool uses, and divides one by the other. Every constant behind those two lines is named in the rows below.
- Capacity factor
derate = linear interpolation of the class table at the outdoor design temperatureThe standard split table runs 1.00 at 47°F, 0.90 at 35°F, 0.72 at 17°F, 0.55 at 5°F, 0.45 at -5°F and 0.35 at -15°F. The cold-climate table runs 1.00 at 47°F, 0.95 at 35°F, 0.90 at 17°F, 0.82 at 5°F, 0.75 at -5°F, 0.65 at -15°F and 0.55 at -22°F. Both are BuildSolver averages, not a rating for any one model.- Outside the table
colder than the last row → factor held at the last rowThe factor is clamped, never extrapolated. A held factor says the table ran out, and it says nothing about whether the equipment still runs down there.- Capacity needed at 47°F
required = heating load ÷ derateThe number is carried through unrounded. This is what you compare against the heating capacity on a spec sheet, which is published at 47°F.- Nominal size
smallest of 18K, 24K, 30K, 36K, 48K, 60K ≥ requiredThe nominal series BuildSolver's engine uses for residential equipment, 1.5 through 5 tons at the AHRI 47°F heating rating. Rounding goes up, because a unit that misses the load leaves the house cold. Your supplier may carry sizes between these.- Capacity at design
delivered = size × derateWhat that nominal size still puts out at the design temperature. This is the number to hold against the load.- Above the ceiling
required > 60,000 → no nominal sizePast 60,000 BTU/h at 47°F the calculator returns no size, and the job moves to a cold-climate unit, a dual-fuel pairing, or two units.- Below the small-load line
load < 12,000 → no nominal sizeBuildSolver returns no size at all under 12,000 BTU/h, the same line the heating tool uses when it hands back an electric strip size instead of equipment. Between that line and the bottom of the series the smallest nominal size already carries more than the load, and the result shows how much more. A single-zone ductless unit sized to the space is the usual answer down there.- A unit you name
at design = rated at 47°F × derate; shortfall = load − at designOptional. Enter the 47°F heating capacity from a spec sheet and the page shows what that unit delivers at the design temperature and how far short it falls, if it does.- Electric backup
kW = shortfall ÷ 3,412, rounded up to 0.5 kWThis line is unit arithmetic. One kilowatt-hour is 3,412 Btu (EIA), so a kilowatt of resistance heat makes 3,412 BTU/h. Rounding up to the half kilowatt is a BuildSolver rule, and the kit list for the air handler you are quoting has the sizes it actually ships in.
One 36,000 BTU/h load in five different winters
Every row below carries the same 36,000 BTU/h heating load. Only the design temperature and the equipment class change. Capacity factors are shown rounded to two places; the calculator interpolates the exact value.
| Job | Design temp | Capacity factor | Needed at 47°F | Nominal size | Delivered at design |
|---|---|---|---|---|---|
| Marine winter, standard split system | 26°F | 0.81 | 44,444 | 48,000 | 38,880 |
| Mixed climate, standard split system | 17°F | 0.72 | 50,000 | 60,000 | 43,200 |
| Cold climate, standard split system | 5°F | 0.55 | 65,455 | past the ceiling | no size |
| Same job, cold-climate heat pump | 5°F | 0.82 | 43,902 | 48,000 | 39,360 |
| Far north, cold-climate heat pump | -15°F | 0.65 | 55,385 | 60,000 | 39,000 |
The load never moves, and the capacity the unit needs at 47°F climbs by a factor of 1.47 between the 26°F and 5°F standard split rows. At 26°F a standard unit keeps 0.81 of its rating, so a 48,000 BTU/h nameplate covers the house. At 5°F the same class holds only 0.55, the load asks for 65,455 BTU/h at 47°F, and that is above the 60,000 BTU/h top of the nominal series, so the calculator returns no size at all.
Compare the two 5°F rows. Same house and the same load, two equipment classes. The cold-climate unit holds 0.82 where the standard one holds 0.55, which drops the required rating from 65,455 BTU/h to 43,902 BTU/h and puts a 48,000 BTU/h unit back on the table. 0.55 against 0.82 at 5°F is why this job gets a cold-climate unit.
When you already have a unit in mind
Take the same 36,000 BTU/h load at 5°F and a real cold-climate heat pump rated 36,000 BTU/h at 47°F. Enter that rating in the optional field and the calculator reports what it delivers at the design temperature, 29,520 BTU/h at 5°F.
36,000 BTU/h at 47°F · 5°F outdoor · Table row at 5°F, factor 0.82
- Heating load 36,000 BTU/h
- Shortfall 6,480 BTU/h
- Electric backup 2.0 kW to cover it
Backup at one temperature only · 1 kW = 3,412 BTU/h (EIA) · computed by the calculator above
That 2.0 kW covers the gap at one temperature and stops there. How many hours a year the strips actually run, and the temperature below which they take over, come out of the unit's own capacity curve against the house load line. That crossing point is the balance point, and finding it is Manual S work.
How much of the rating survives, by outdoor temperature
These are the factors the calculator interpolates between, printed straight from the same constants the BuildSolver heating tool uses. Every value is a fraction of the unit's capacity at 47°F, so 0.55 means a 36,000 BTU/h nameplate is making about 19,800 BTU/h.
| Outdoor air | Standard split | Cold-climate | Note |
|---|---|---|---|
| 47°F | 1.00 | 1.00 | AHRI H1 test point, the nameplate rating |
| 35°F | 0.90 | 0.95 | |
| 17°F | 0.72 | 0.90 | AHRI H3 test point |
| 5°F | 0.55 | 0.82 | AHRI H4 test, required for cold-climate certification |
| -5°F | 0.45 | 0.75 | |
| -15°F | 0.35 | 0.65 | standard table stops here |
| -22°F | off the table | 0.55 | cold-climate table stops here |
Three rows are real test conditions. AHRI Standard 210/240 Table 8 puts the H1Full heating test at 47.0°F dry bulb and 43.0°F wet bulb entering the outdoor unit, the H3Full test at 17.0°F and 15.0°F, and the H4Full test at 5.0°F and 4.0°F, all with 70.0°F dry bulb and 60.0°F wet bulb entering the indoor unit. Table 7 of the same standard marks the H4 test required only for systems certified as cold climate heat pumps, which is why the standard column falls off a cliff right where the cold-climate column is still being tested. What happens between and below those points is a BuildSolver average across the NEEP cold climate database and common standard-efficiency split systems, so treat the shape of this curve as an assumption and your own unit's published curve as the answer.
Below -15°F for a standard unit and below -22°F for a cold-climate one, the interpolation runs out of rows and the factor is held rather than lowered. NEEP calls the bottom of a manufacturer's published table the lowest cataloged temperature, and below it the manufacturer publishes no data.
Standards and data sources
- ACCA Manual J 8th Edition - the procedure behind the heating load this page asks for. The load is an input here. Run it in the BuildSolver Manual J calculator, in ACCA-approved software, or by hand, then bring the number to this form.
- ASHRAE Handbook - Fundamentals, Chapter 14 - the definition of the 99% heating design dry-bulb, the second input. It is the temperature the outdoor air stays at or above for 99% of the hours in a year, so it is colder than that for roughly 88 hours out of 8,760.
- AHRI Standard 210/240 (I-P) - the heating rating points the capacity table is anchored to. Table 8, "Test Conditions", lists the H1Full test at 47.0 / 43.0°F air entering the outdoor unit, the H3Full test at 17.0 / 15.0°F and the H4Full test at 5.0 / 4.0°F, all with 70.0 / 60.0°F entering the indoor unit, where note 2 reads "Values listed are dry-bulb temperature / wet-bulb temperature, °F."
- AHRI 210/240 definition of a cold-climate heat pump - section 3.2.10, "A heat pump for which both low-temperature compressor cut-out and cut-in temperatures are specified to be less than 5°F and for which capacity for the H4 full test (at 5°F) is specified to be at least 70% of the capacity for the nominal full capacity test conducted at 47°F (H1Full or H1Nom)." The 5°F test is optional for everything else: Table 7, note 9, marks it "Required for all systems certified as cold climate heat pump."
- NEEP Cold Climate Air Source Heat Pump Specification, Version 4.0 - the specification the cold-climate product list qualifies against, effective January 1, 2023. The capacity averaging on this page used that product list as it stood in 2024. The specification also names the limit of any published table: "Lowest Cataloged Temperature (LCT) is defined as the lowest outdoor dry bulb temperature at which the manufacturer offers published performance data in technical manuals or product documentation."
- U.S. Energy Information Administration, British thermal units - the backup heat conversion, "1 kilowatthour=3,412 Btu". A kilowatt of resistance heat running for an hour delivers 3,412 BTU/h.
- ACCA Manual S - equipment selection against a specific model's expanded performance data, including the acceptable size range and the balance point. That step comes after this one and runs on the Pro and Team plans.
What this estimate leaves out
- The load is an input here. Everything on this page is only as good as the heating load you type in, so run a real Manual J rather than a rule of thumb.
- The capacity factors are a BuildSolver average across the NEEP cold-climate database and the AHRI 210/240 rating points. They describe a class of equipment, and the unit you plan to sell can sit off them. At 5°F the two columns of the table on this page are 27 points apart, and a real unit can land anywhere in that span. Take the curve from the manufacturer's expanded performance tables.
- Two classes are offered, standard split and cold-climate. Inverter units with a documented boost range, packaged equipment, water-source and ground-source systems all behave differently.
- The nominal size carries the whole load on the compressor alone. No credit is taken for backup heat, which is why the size reads conservative for anyone planning a dual-fuel or strip-heat system.
- Below the coldest row a -20°F design day and a -40°F design day return the same capacity, so pull the manufacturer's low-temperature table before you quote either one.
- Defrost cycles, wind exposure, setback recovery and the low end of an inverter's turndown are outside this model.
- No seasonal numbers come out of this page. HSPF, HSPF2, COP and annual energy use need a bin-hour or hourly model, and BuildSolver does not run one.
- The balance point, the acceptable size range and the check against a model's AHRI data are Manual S work. They live in the Pro and Team equipment selection tool.
- Sizing to the heating load alone can put the cooling capacity above what Manual S allows. Check the cooling side of the job before you order the equipment.
- Preliminary sales-phase estimate. Not ACCA-approved software output and not accepted for permit submission.
Common heat pump sizing mistakes
- The 47°F nameplate is not a sizing number. A 36,000 BTU/h heat pump is a 36,000 BTU/h heat pump at 47°F and nowhere else. Match the load against the capacity at your design temperature, which is what the calculator above returns.
- Reading tonnage as heating capacity. Tons describe the cooling side. On the heating side the same box slides down its capacity curve all winter, so a three-ton unit can be a two-ton heater in January and a one-ton heater during a cold snap.
- Assuming a premium brand means cold-climate. Cold-climate is a rating with a test behind it: cut-out and cut-in below 5°F, and at least 70% of the 47°F capacity at the 5°F test. Look the model up in the NEEP list before you promise winter performance.
- Taking a held factor as a rating. Past the coldest row the table stops moving. Get the manufacturer's own low-temperature table before you quote a job that cold, and read the compressor cut-out temperature while you are in there.
- Backup heat left out of the quote. Whatever the compressor cannot carry on the design night comes from strip heat or a furnace. Price that equipment, the electrical service it needs and the control strategy at the same time as the outdoor unit.
- Treating the design temperature as the coldest night. The weather runs below the 99% design dry-bulb about 88 hours a year. Size for the record low and the unit short-cycles through the other eight thousand.
Where this page stops
This calculator answers one question and stops. Everything above it is sizing at a single temperature, the design temperature. Here is what it does not do.
- The balance point. Where a real unit's capacity curve crosses the house load line, and therefore where backup heat starts carrying the house, needs that unit's expanded performance data. Manual S does it, and BuildSolver runs Manual S on the Pro and Team plans.
- The acceptable size range. Manual S sets the limits on how far a selection may sit above the load, cooling and heating separately, and checks a model against its own AHRI data. Same Pro tool.
- Seasonal performance. HSPF2, COP and annual energy cost are not calculated anywhere in BuildSolver. Anyone quoting them off a design-condition capacity is guessing.
Free in chat, with no account, you get the whole-house load for heating and cooling, design conditions by ZIP, dew point and psychrometrics, and a first-pass duct size for one run. Room-by-room loads and full Manual D are on the Pro and Team plans. The chat also takes what this form has no room for, such as measured R-values and a furnace AFUE, and the same free heating tool returns a dual-fuel flag with the load. That flag marks the climate. Choosing the temperature where a dual-fuel system hands over to the furnace is Manual S work on the Pro and Team plans. What the chat and the load pages allow per day is in the questions below. This page is pure arithmetic in your browser, so it costs you nothing either way.
Questions about heat pump sizing
What size heat pump do I need?
Start from the load, never from floor area. Once you have the heating load, this page converts it into a size at the temperature you design for. In the worked example a 36,000 BTU/h load at a 17°F design temperature on a standard split system needs 50,000 BTU/h at 47°F, so the nominal size is 60,000 BTU/h, and that size delivers 43,200 BTU/h at 17°F. That 60,000 BTU/h is a rating at 47°F rather than what the unit makes on a design night: at 17°F it holds 72% of its 47°F rating, and the 43,200 BTU/h that comes out is the figure the 36,000 BTU/h load is matched against. If you do not have a load yet, run one in the Manual J calculator first.
Why is the capacity needed at 47°F larger than the heating load?
Because the rating and the load are measured in different weather. The load is what the house loses at the design temperature. The rating is what the unit makes at 47°F. Dividing the load by the capacity factor at your design temperature converts one into the other, so the colder the design night, the further the nameplate climbs above the load.
Where does the capacity table come from?
It is a BuildSolver average, published openly on this page so you can see what drives the answer. The anchors are the AHRI 210/240 heating rating points at 47°F, 17°F and 5°F. Both columns were averaged from the NEEP cold climate product list as it stood in 2024. For the standard column BuildSolver took the non cold-climate tier of that list together with five leading standard-efficiency split systems, the Carrier 25HCB, Trane XR15, Lennox 14HPX, Goodman GSZ14 and Rheem RP15. The cold-climate column is the tier average from the same list. The specification those products qualify against is a separate document, NEEP ccASHP version 4.0, effective January 1, 2023. It is an assumption of this calculator, not a figure quoted from a standard, and a specific model can sit well off it.
What happens when my design temperature is below the table?
The capacity factor, the share of a unit's 47°F rating it still makes at your design temperature, is held at the coldest row instead of dropping further, which means 0.35 for a standard split system below -15°F and 0.55 for a cold-climate unit below -22°F. Read that as the table running out, not as a capacity rating. AHRI 210/240 requires heating tests at 47°F and 17°F, and the 5°F test only for units certified as cold climate, so anything colder comes from the manufacturer's own published table if one exists.
Why does the result say there is no nominal size?
Two cases come back empty. Above the line, the load divided by the capacity factor asks for more than 60,000 BTU/h at 47°F, which is past the top of the nominal series BuildSolver uses for residential single units, and the honest options are a cold-climate unit, a dual-fuel pairing with a furnace, two units, or planned backup heat below a crossover temperature. Below the line, a load under 12,000 BTU/h gets no size either, because the smallest unit in the series would cycle against a load that small. That is where a single-zone ductless unit or resistance heat goes, and it is the same rule the BuildSolver heating tool follows.
Does this page find the balance point?
No. The balance point is where a specific unit's capacity curve crosses the house load line, and finding it takes that unit's expanded performance data. It is Manual S work, and BuildSolver runs Manual S in the equipment selection tool on the Pro and Team plans. This page answers for one temperature, the design temperature.
Does it give HSPF2, COP or running cost?
No. Those are seasonal numbers and this calculator works at design conditions only. Capacity at a temperature and efficiency across a heating season are different calculations, and BuildSolver does not run the second one.
Does this calculator count against a daily limit?
No. The math runs in your browser, so you can use it as often as you like with no account and nothing sent to a server. Two other things do have ceilings. The pages that call a BuildSolver load tool, such as the Manual J calculator, share 10 calculations a day and 50 a month from one address, with or without an account. The chat counts separately, at 3 questions a day for an anonymous visitor and 10 a day and 50 a month once you have a free account.
Used on the same job
- Replacement AC sizing The cooling side of the same change-out, where the Manual S oversize cap bites.
- Duct sizing calculator Heat pumps move more air than the furnace they replace. Check the trunk before you quote.
- CFM calculator Airflow per ton and per room, for the blower settings on the new equipment.
Get the load in chat, then size it here
Describe the house and the assistant runs the whole-house Manual J for heating and cooling, gives you the design conditions for the ZIP, and asks for anything it needs. Bring the heating load back to this page to see what a unit does with it in January.
Open the chat