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Calculator · Heat pump
Heat pump balance point calculator
The balance point is the outdoor temperature where a heat pump's heating capacity equals the heat the house loses. Below it the heat pump runs flat out and still falls short, and backup heat makes up the rest. Energy Vanguard's example is a house that loses 15,000 BTU/h at a 17°F design temperature, heated by a unit rated 17,800 BTU/h at 47°F and 10,700 BTU/h at 17°F. The two lines cross at 24.8°F, and on the design day the house needs 4,300 BTU/h of backup, 1.26 kW. Bring the load from the Manual J calculator and the nominal size from the heat pump sizing calculator.
- Run a Manual J for the house or zone and note the heating load and the outdoor design temperature it was run at.
- Take the heating capacity at 47°F and 17°F, and at 5°F if listed, from the AHRI certificate, the extended performance table or the NEEP cold-climate list. Use one column of one table, maximum capacity for a variable-speed unit.
- Enter the numbers. The result gives the outdoor temperature where the capacity line meets the load line and the backup heat the house needs at the design temperature, in BTU/h and kW.
Example result for the default inputs
15,000 BTU/h at 17°F, zero load at 65°F. Heat pump 17,800 BTU/h at 47°F, 10,700 BTU/h at 17°F.
Balance point
24.8 °F
Backup heat at design
1.3 kW
Heat pump share of the design load
71 %
| Design load | 15,000 BTU/h at 17°F |
|---|---|
| Zero-load temperature | 65°F |
| Load line, BTU/h per °F | 312.5 |
| Capacity at 47°F | 17,800 BTU/h |
| Capacity at 17°F | 10,700 BTU/h |
| Capacity line from 17°F to 47°F, BTU/h per °F | 236.7 |
| Heat pump capacity at 17°F | 10,700 BTU/h |
| Balance point, where the lines cross | 24.8°F |
| Backup heat at design, load minus capacity | 4,300 BTU/h |
| Backup heat at design, BTU/h / 3,412 | 1.26 kW |
- Enter capacities from one column of one table. For a variable-speed unit that is the maximum capacity at each temperature, as the NEEP cold-climate list gives it. An AHRI certificate may list rated capacity, and a line through rated figures understates what the unit can make in the cold.
- The backup above covers the gap on the design day with the compressor running. Emergency heat for a failed compressor is a separate decision that Manual S leaves to local codes, so check what the local code and the customer expect before you size the strips from this figure alone.
- This is the straight-line method. It leaves out defrost, which pulls capacity down below roughly 40°F, and a capacity curve that bends between the rating points. Manual S selection by equipment class, with the acceptable size range and a balance point on a typical capacity curve for the class, runs in the BuildSolver equipment selection tool on the Pro and Team plans.
For estimation purposes only. Consult a licensed engineer for final designs.
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How the balance point is calculated
The balance point comes from two straight lines drawn against outdoor temperature. One is the house: its heat loss is the design load at the design temperature and falls to zero at a mild outdoor temperature. The other is the heat pump: its capacity runs through the ratings at 47°F and 17°F, and through 5°F as well when the unit has that figure. Above the crossing the heat pump makes more heat than the house loses. Below it the heat pump falls short, and the gap at the design temperature is the backup heat the job needs.
- House load line
load(T) = design load x (zero-load T - T) / (zero-load T - design T)The load is taken as proportional to how far the outdoor temperature sits below the zero-load point. For 15,000 BTU/h at 17°F and zero at 65°F the line falls by 312.5 BTU/h for every degree warmer.- Heat pump capacity line
capacity(T) = cap 17 + (cap 47 - cap 17) / 30 x (T - 17)A straight line through the two rating points. 10,700 BTU/h at 17°F and 17,800 at 47°F give 236.7 BTU/h per degree. Below 17°F the page uses the line through 17°F and 5°F when you enter a 5°F capacity, and carries the upper line on when you do not.- Balance point
capacity(T) = load(T), solved for TEnergy Vanguard plots the two lines and reads the crossing as the balance point. For its house the lines cross at 24.8°F, the "about 25 °F" of the article.- Backup heat at design
backup = design load - capacity(design T); kW = backup / 3,412The heat the heat pump cannot make on the design day. In the Energy Vanguard house it is 4,300 BTU/h, or 1.26 kW of resistance heat at 3,412 BTU/h per kW. If the capacity line is above the load line at the design temperature, the backup is zero.
Worked examples from the published sources
Six cases below run through the same functions the form uses. The first comes from Energy Vanguard and the second from the PE Mechanical study guide. Four more are examples chosen for the page: the same house with the zero-load point moved, the same unit on a smaller load, and a cold-climate unit with a 5°F rating.
| Case | Setup | Result |
|---|---|---|
| Energy Vanguard house | 15,000 BTU/h at 17°F, zero load at 65°F; 17,800 at 47°F, 10,700 at 17°F | Balance point 24.8°F; backup at design 4,300 BTU/h (1.3 kW) |
| PE Mechanical study guide example | 60,000 BTU/h at 5°F, zero load at 70°F; 36,000 at 47°F, 22,000 at 17°F | Balance point 36.4°F; backup at design 43,600 BTU/h (12.8 kW) |
| Energy Vanguard house, zero load at the 70°F setpoint (chosen for the page) | 15,000 BTU/h at 17°F, zero load at 70°F; 17,800 at 47°F, 10,700 at 17°F | Balance point 25.3°F; backup at design 4,300 BTU/h (1.3 kW) |
| Energy Vanguard house, zero load at 60°F (chosen for the page) | 15,000 BTU/h at 17°F, zero load at 60°F; 17,800 at 47°F, 10,700 at 17°F | Balance point 24.3°F; backup at design 4,300 BTU/h (1.3 kW) |
| Same unit on a 9,000 BTU/h load (chosen for the page) | 9,000 BTU/h at 17°F, zero load at 65°F; 17,800 at 47°F, 10,700 at 17°F | Balance point below 17.0°F; backup at design 0 BTU/h (0.0 kW) |
| Cold-climate unit with a 5°F rating, -5°F design (chosen for the page) | 30,000 BTU/h at -5°F, zero load at 65°F; 36,000 at 47°F, 30,000 at 17°F, 24,000 at 5°F | Balance point 6.8°F; backup at design 11,000 BTU/h (3.2 kW) |
Here are the two lines of the Energy Vanguard house, read every 10°F. Where the capacity column drops below the load column, the house needs backup heat.
| Outdoor, °F | House load, BTU/h | Heat pump, BTU/h | Backup, BTU/h |
|---|---|---|---|
| 57 | 2,500 | 20,167 | 0 |
| 47 | 5,625 | 17,800 | 0 |
| 37 | 8,750 | 15,433 | 0 |
| 27 | 11,875 | 13,067 | 0 |
| 17 | 15,000 | 10,700 | 4,300 |
Standards and data sources
- Energy Vanguard, A Simple Way to Calculate Heat Pump Balance Point (Allison Bailes) - the method, built on the heating load at the 99% design temperature, the capacity at 17°F and 47°F, zero load at 65°F, straight lines for both, and "Where the two lines cross is the balance point." The worked example, 15,000 BTU/h at 17°F with 10,700 and 17,800 BTU/h from the AHRI Directory, gives about 25°F. Comments under the article add defrost and capacity that is not linear between the rating points.
- Open Exam Prep, PE Mechanical HVAC study guide, 9.6 Heat Pumps and VRF - the same method with the zero-load point at a 70°F setpoint and the backup step. 60,000 Btu/hr at 5°F with 36,000 at 47°F and 22,000 at 17°F give a balance point of 36.4°F, 16,400 Btu/hr from the heat pump at 5°F, 43,600 Btu/hr of supplemental heat and 12.8 kW.
- NEEP, Guide to Sizing and Selecting Air-Source Heat Pumps in Cold Climates - the two sizing strategies for full replacement and new construction, capacity at the design temperature of 100-115% of the heating load without auxiliary heat, or 75-85% of the load with "just enough supplemental heat" to make up the difference, and the caution that published tables "may not show maximum capacity at colder temperatures."
- AHRI Standard 210/240 heating rating points - the 47°F and 17°F heating tests behind the two capacities most certificates list, and the 5°F test required for units certified as cold climate.
- U.S. Energy Information Administration, British thermal units - the conversion of backup heat to kilowatts, "1 kilowatthour=3,412 Btu".
Limitations of this calculator
- The load is an input. The balance point is only as good as the heat loss you type in, so bring a Manual J figure run at the same design temperature.
- Both lines are straight. Real heat loss rises with wind, and real capacity bends between the rating points and dips when the outdoor coil defrosts, which commenters on the Energy Vanguard article put below about 40°F. The true crossing can sit a few degrees off the one shown here.
- Below the coldest rating you enter, the capacity is the line carried on. A unit can fall off faster or hold on longer than that, so check the lowest temperature in the manufacturer's table and its cut-out temperature.
- The zero-load temperature is a judgment call. Internal gains and sun carry a house some way below the thermostat setting, so 65°F is the usual choice, and a tight house with large gains can sit lower still.
- Capacities must come from one column of one table. A variable-speed unit has a minimum, a rated and a maximum capacity at each temperature, and only the maximum shows what it can carry on a cold night.
- The backup figure is supplemental heat with the compressor running. Emergency heat for a failed compressor is left to local codes in Manual S, and it can call for more than this page shows.
- This is the thermal balance point. A dual-fuel switchover temperature set by fuel and electricity prices is a separate calculation this page does not run.
- Manual S selection by equipment class, with the acceptable size range, a balance point on a typical capacity curve for the equipment class and the cooling side of the same unit, runs in the BuildSolver equipment selection tool on the Pro and Team plans, not on this page.
- Preliminary sales-phase estimate. Not ACCA-approved software output and not accepted for permit submission.
Common balance point mistakes
- Mixing rated and maximum capacity. AHRI certificates for variable-speed units often list rated capacity, while the NEEP list and the extended tables give maximum capacity too. A line from rated capacity at 47°F to maximum capacity at 17°F can even slope the wrong way. Take both figures from the same column.
- Using a rule-of-thumb load. The balance point moves with the load. In the Energy Vanguard house, 15,500 BTU/h in place of 15,000 moves it from 24.8°F to 25.6°F, and a square-foot guess can be off by far more than that. NEEP calls for a full Manual J or equivalent.
- Drawing the load line to the thermostat setting. With zero load at 65°F the Energy Vanguard house balances at 24.8°F. Draw the line to 70°F and the same house balances at 25.3°F, so the backup looks needed on milder days than it is. The backup at the design temperature does not change. Pick the zero-load point on purpose and say which one you used.
- Reading the backup at the balance point. At the balance point the backup heat is zero by definition. The backup to size is the gap at the design temperature, 4,300 BTU/h or 1.26 kW for the Energy Vanguard house, and the circuit for it is sized from those kilowatts.
- Trusting the line far below the ratings. The PE study guide example designs at 5°F with ratings only down to 17°F, so its 16,400 Btu/hr at 5°F is an extrapolation. Fine for a textbook, but on a real job pull the 5°F figure or the manufacturer's low-temperature table before you size the strips.
Common questions about the heat pump balance point
What is a heat pump balance point?
It is the outdoor temperature where the heat pump's heating capacity equals the heat the house loses. Above it the heat pump keeps up on its own. Below it the heat pump runs flat out and still falls short, so backup heat has to make up the rest.
How do you calculate the balance point of a heat pump?
Draw the house load as a straight line from the design load at the design temperature down to zero at about 65°F. Draw the heat pump capacity as a straight line through its ratings at 47°F and 17°F. The two lines cross at the balance point. For the Energy Vanguard example, 15,000 BTU/h at 17°F against 17,800 BTU/h at 47°F and 10,700 BTU/h at 17°F, the crossing is 24.8°F.
How much backup heat does a heat pump need?
The difference between the design load and the heat pump's capacity at the design temperature. Divide that by 3,412 for kilowatts of resistance heat. The Energy Vanguard house needs 4,300 BTU/h, 1.26 kW. The PE study guide example needs 43,600 Btu/hr, or 12.8 kW. That is supplemental heat. The compressor keeps running. Emergency heat for a failed compressor is a separate decision, left to local codes in Manual S.
What is a good balance point?
There is no single target temperature. NEEP's cold-climate sizing guide gives two strategies. Size the heat pump so its capacity at the design temperature is 100-115% of the heating load, which puts the balance point at or below the design temperature and needs no auxiliary heat. Or size it for 75-85% of the load and add just enough supplemental heat to make up the difference.
Why does the load line go to zero at 65°F?
Sun, people and appliances heat a house for free, so it stops needing heat from the system some degrees below the thermostat setting. Energy Vanguard takes that point as 65°F. The PE study guide example uses the 70°F setpoint, and a tight house with large internal gains can need no heat until it is colder than 65°F. Change it in the form.
Where do I find the heat pump capacity at 47°F and 17°F?
Start with the AHRI certificate for the matched system. It lists heating capacity at 47°F and 17°F. Manufacturer extended performance tables and the NEEP cold-climate list give minimum, rated and maximum capacity, and the NEEP list adds 5°F. For a variable-speed unit, use maximum capacity at every temperature. Never mix columns.
Does this page find the balance point for a specific model?
It finds the balance point of the straight line through the capacities you enter, which is the simplified method. Manual S selection by equipment class, with the acceptable size range and a balance point on a typical capacity curve for the equipment class, runs in the BuildSolver equipment selection tool on the Pro and Team plans.
Is the balance point the same as the dual-fuel switchover temperature?
No. The thermal balance point is the coldest temperature where the heat pump can heat the house alone. A dual-fuel switchover is often set by comparing the cost of heat from electricity and from gas, and it can sit above the thermal balance point. This page calculates the thermal one only.
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.
Used on the same job
From the balance point to the equipment
The straight lines show whether a unit is in the right range and how much backup to plan for. Selecting the unit itself means checking its size against the Manual S limits for its equipment class, with the cooling side checked at the job's design conditions. The Manual S calculator runs that selection on the Pro and Team plans. The backup strips then go through the AC breaker size calculator for the circuit.
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