SizingKitCapacity against temperature
Heat pump sizing calculator
A heat pump cannot be sized at one temperature, because its output falls as the outdoor air gets colder while the building’s demand rises — so the answer is a crossing point rather than a number. Put in the capacities your machine is rated at 47 °F and 17 °F, plus the design load and design temperature, and this free tool draws both lines, finds the balance point where they meet, and gives the supplemental heat below it in BTU/h and kW. It runs in the browser, takes no signup, and marks any row below 17 °F as extrapolated instead of pretending to know it.
- 100% free
- No signup
- 47 °F and 17 °F ratings
- Balance point in °F and °C
- Aux heat in kW
The machine, and the winter it has to get through
Two rated capacities describe the machine; a load and a design temperature describe the building. Either half works on its own — the curve appears with the machine, the crossing needs both.
AHRI 210/240 low-temperature heating rating point: capacity and input measured at 17 °F outdoor dry bulb.
From a room-by-room manual j calculator, in the same unit as the capacities above.
The ASHRAE 99% value for your own weather station. There is no city list on this site, deliberately.
Default 70 °F — ACCA Manual J, 8th edition, Table 1: the default indoor design dry-bulb temperature for heating. A design assumption. Manual J permits another value where the owner asks for one; the load moves by roughly 2–3% per degree.
Capacity against the load, degree by degree
BALANCE POINT
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SUPPLEMENTAL HEAT AT DESIGN
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CAPACITY THE BUILDING NEEDS AT DESIGN
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CAPACITY THIS MACHINE HAS AT DESIGN
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| Outdoor | °C | Machine | Building | Spare |
|---|---|---|---|---|
| 47.0 °F | 8.3 °C | — | — | — |
| 45.0 °F | 7.2 °C | — | — | — |
| 40.0 °F | 4.4 °C | — | — | — |
| 35.0 °F | 1.7 °C | — | — | — |
| 30.0 °F | -1.1 °C | — | — | — |
| 25.0 °F | -3.9 °C | — | — | — |
| 20.0 °F | -6.7 °C | — | — | — |
| 17.0 °F | -8.3 °C | — | — | — |
Capacities in BTU/h. A row marked * sits outside the 17–47 °F band the two ratings define, where the line between them is being extended past both ends rather than read between them.
Between 17 and 47 °F this interpolates straight between the two AHRI rating points, which is the field method. Below 17 °F it is extrapolating, and extrapolation is where it stops being trustworthy in both directions at once: a single-speed machine falls off faster than the line predicts and a cold-climate variable-capacity one holds far better. The manufacturer’s expanded performance data runs down to the temperatures that decide this, and where it exists it wins.
A design temperature comes from the ASHRAE Handbook—Fundamentals climatic design tables, which publish, for each of several thousand weather stations, the 99.6% and 99% heating dry-bulb temperatures and the 0.4%, 1% and 2% cooling dry-bulb temperatures with their mean coincident wet bulbs. ACCA Manual J uses the 99% heating and 1% cooling values. They are percentiles of the hours in a year, not extremes: at the 99% heating design temperature the weather is colder for roughly 88 hours a year, and the heating system is expected to fall behind for some of them. Look yours up by station — a design temperature is local, and a city's airport figure can be several degrees off a site twenty miles away at a different elevation. At the 99% value that is about 87.6 hours a year colder than the bottom row of your table. The cooling side of the same machine is selected separately — see the ac tonnage calculator — and if the answer here is that the machine cannot carry the winter, the comparison to make is against a furnace size calculator. Capacities read in tons rather than BTU/h convert on the btu to tons calculator.
How to find a heat pump's balance point
Two numbers describe the machine, two describe the winter, and the crossing falls out of them.
Take both rated capacities off the AHRI listing
You need the heating output at 47 °F and at 17 °F, in the same unit as each other. They are on the AHRI certificate for the specific indoor and outdoor combination, and in the submittal sheet's expanded performance table. The 47 °F figure alone is the one printed in advertising, and on its own it will size the machine too small for the coldest fifth of the season.
Add the design heating load and the temperature it was calculated at
The pair has to match: a load of 42,000 BTU/h means nothing without the outdoor temperature it was worked out for. The two together give the building's conductance in BTU/h per °F, which is what lets the page draw the demand line across the whole table rather than at the one point you entered.
Read the crossing, then read the row at your design temperature
Above the balance point the machine carries the house on its own; below it the Spare column goes negative and that shortfall is what the auxiliary heat has to make up. Size the strips on the deficit at design, not on the whole load — sizing them for the whole load is how a heat pump ends up with a resistance furnace bolted to it that runs whenever the thermostat jumps.
Technical specifications
| Rating points read | 47 °F and 17 °F outdoor dry bulb — the two heating conditions AHRI 210/240 publishes a capacity at. Every figure on this page is built from those two and nothing else. |
|---|---|
| Between them | Straight-line interpolation across the 30 degrees between the rating points, which is the field method. The line cannot show the dip near 35 °F where frost forms fastest and part of each hour goes to defrost; AHRI tests that condition separately. |
| Below the low rating point | Extrapolated and marked as such with an asterisk, never silently. A single-speed machine falls off faster than the line below 17 °F and a cold-climate inverter holds far better, so the line is wrong in opposite directions on different equipment. |
| Indoor design temperature | 70 °F by default, editable between 50 and 85. It is a Manual J assumption rather than a setting: the load moves by roughly 2 to 3% per degree you change it. |
| Outdoor design temperature | Yours to supply — the ASHRAE 99% heating dry bulb for your own weather station, read between −60 °F and 65 °F. There is no city list here; a station twenty miles away at a different elevation is several degrees out, and the tables are revised per Handbook edition. |
| Hours below design | About 87.6 a year at the 99% value — a design temperature is a percentile of 8,760 hours, not a record low. The cooling side is chosen at the 1% dry bulb. |
| Supplemental heat, in kW | Reported beside the BTU/h figure, since one kilowatt of resistance heat is 3.41 BTU/h and strips are ordered in kilowatts. Resistance heat is COP 1.0 by definition, which is the bar the heat pump above the balance point is beating. |
| What this page does not know | The machine's low-ambient cutout, its defrost strategy and its compressor-lockout setpoint — all three can stop a heat pump well above the temperature where its capacity line runs out, and all three come from the equipment's own literature. |
Frequently asked questions
What is the balance point, and why is it not 32 °F?
It is the outdoor temperature at which the heat pump's falling capacity exactly equals the building's rising heat loss, and it has nothing to do with freezing. Freezing is a property of water; the balance point is a property of a particular machine in a particular house, and the same equipment lands at 15 °F in a tight new build and at 40 °F in a leaky old one. It is the single most useful number about an installed heat pump, because every hour of the year colder than it is an hour the auxiliary heat contributes and the running cost changes character.
Should a heat pump be sized on the heating load or the cooling load?
On whichever season dominates, and the two rarely agree. In most of the US the heating load is larger, so a machine sized to carry winter without auxiliary heat will be oversized for summer and will leave the house damp — the same failure an oversized air conditioner causes. The usual resolution is to size nearer the cooling load, accept a balance point somewhere in the 25 to 35 °F region and let strip heat cover the tail; in a cooling-dominated climate the question does not arise. What makes the trade-off decidable is seeing both numbers, which is why this page reports the shortfall rather than just a verdict.
Why is the 47 °F rating the wrong number to size on?
Because almost no heating happens at 47 °F. It is a laboratory rating point chosen to be reproducible, and at that temperature a house needs a fraction of its design load, so a machine matched to the design load at 47 °F will be far short of it at 5 °F — exactly when it is needed. The number that governs is capacity at your own design temperature, which is why this page asks for the second rating point: with only one, the slope of the curve is unknown and the extrapolation is a guess.
How much auxiliary heat do I actually need, and in what size?
The deficit at your design temperature, converted to kilowatts and rounded up to the strip sizes the air handler accepts. That is a much smaller number than the whole design load, because the heat pump is still producing most of it even at the bottom of the table — a machine 12,000 BTU/h short at design needs about 3.5 kW of resistance heat, not the 12 kW that covering the entire load would take. Two things push it up: an outdoor thermostat or a compressor lockout that turns the pump off below some temperature, and a specification that requires the strips to carry the house alone during a defrost cycle or an equipment failure.
Does this work for a cold-climate or variable-capacity heat pump?
It will understate one badly, and that is the honest limitation of a two-point line. Cold-climate inverter equipment is engineered to hold capacity down to temperatures where a single-speed machine has given up, and its real curve is convex where this page draws a straight line — using the straight line will put the balance point too warm and buy far more strip heat than the machine needs. Those manufacturers publish full expanded performance tables at 5 °F, −5 °F and lower for exactly this reason, and where such a table exists it replaces everything on this page below 17 °F.
Why are the rows below 17 °F marked with an asterisk?
Because they are outside the two measurements the curve is built from, and this site marks an extrapolation rather than hiding it. Between 47 and 17 °F the line sits between two published capacities and is a fair approximation; below 17 °F it is being extended past both, and the error grows in a direction that depends on the compressor rather than on the arithmetic. The table still draws them because a design temperature of 0 °F is a real condition somebody has to plan for — it just does not pretend they carry the same weight as the rows above.
Does defrost show up in these numbers?
No, and its absence is visible as a smoothness the real curve does not have. Frost accumulates fastest around 35 °F, where the outdoor air is cold enough to freeze condensate on the coil and humid enough to deposit a lot of it, so a real machine loses both output and run time to defrost cycles in that band — and AHRI 210/240 measures that condition as a separate test rather than folding it into the 47 and 17 °F capacities. A straight line between two frost-free ratings will therefore read slightly high in the middle of the table. It does not move the balance point much, because the balance point in most climates sits below the worst of the frosting band.
About the two lines, and the temperature they cross at
Every other piece of heating equipment has a capacity that is a constant. A furnace makes the same output at 40 °F as at −10 °F, so sizing it is one comparison at one condition, and a furnace size calculator is a division. A heat pump does not have a capacity; it has a curve, and the curve slopes the wrong way. It works by moving heat out of outdoor air, and colder air holds less of it at a lower pressure, so the compressor moves less refrigerant and delivers less heat at exactly the hours the house wants the most. Two lines on the same axis — one falling, one rising — cross at one point, and that crossing is the whole of heat pump sizing. It is why AHRI publishes two heating capacities instead of one, and why a quote that names only the 47 °F figure has told you almost nothing about the winter.
What happens below the crossing is where the money is. The shortfall is made up by electric resistance heat, which has a coefficient of performance of exactly 1.0 by definition — every watt in becomes a watt of heat and no more — while the heat pump above it is delivering two or three watts of heat per watt drawn. So the strips are not merely a backup; they are the part of the season that costs two to three times as much per unit of heat, and the balance point is the thermostat setting on that expense. This is the reasoning behind sizing the auxiliary heat on the deficit at design rather than on the whole load, and behind checking, before anything else, whether a control is locking the compressor out at a temperature well above where its capacity actually ran out. A resistance element that comes on at 35 °F because somebody set an outdoor thermostat conservatively will cost more than every other decision on this page put together.
None of this settles the summer, and the summer is half the machine. The same box has to satisfy a cooling load inside a much tighter window — 90 to 125% of it — which frequently conflicts with what the winter wants, and that selection is the ac tonnage calculator. Both sides need a real load to work against rather than a square-foot estimate, which is a manual j calculator, and both sides read capacities that arrive in tons, kW or MBH depending on who wrote the sheet — the btu to tons calculator converts between them. Once the machine is in, whether it is making its rated capacity at all is a refrigerant-side question rather than a sizing one, answered at the gauges on the superheat calculator. And this page is arithmetic on two published numbers, not a design: a permit, a utility rebate or a load-calculation review wants a signed calculation from someone who has measured the building.
Where the curve is drawn
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The capacity table redraws as you type and prints from the browser as it stands, which is the form it is most useful in: stapled to a proposal beside the equipment submittal, with the balance point and the strip-heat figure the customer is being asked to pay for visible on the same sheet.