Understanding your result
The headline is the size in tons for the load you chose to size for, cooling by default, with the balance point and the backup heat in the sub-line. The three bars put the heating load at the design temperature, the heat pump’s capacity at that temperature and the cooling load side by side; the gap between the first two bars is what the backup must supply on the coldest night of the year.
The table carries both sizes. Size for cooling is the half-ton that covers the summer load, as the central AC size calculator would give. Size for heating alone is the unit that would need no backup at the design temperature, and in a cold climate it is often absurdly large, which is the clearest illustration of why backup heat exists. The balance point is where the house’s heat loss line crosses the unit’s capacity line; the backup runs below it. The oversize for cooling row warns when a unit sized for heating would short-cycle in summer.
If the balance point is high and the backup is large, compare the running costs with the heat pump versus gas furnace calculator, which prices the electricity below the balance point against gas, and size the alternative with the furnace size calculator, which uses the same heating load. The heat pump installation cost calculator prices the unit, including cold-climate models.
How we calculate this
The loads per square foot come from the BTU per square foot table, and the heating figure is first converted to a heat loss per degree of indoor-outdoor difference so that it can be re-stated at whatever design temperature you enter. The capacity retention is a straight line through the two points manufacturers publish, the rating at 47 °F and the capacity at 17 °F, extended to your design temperature with a floor of 30%; real capacity tables curve a little, and the manufacturer’s extended data at your design temperature is the figure to use for a final selection, which is what ACCA Manual S requires.
The balance point uses 65 °F rather than 70 °F as the indoor base, because internal gains from people, appliances and sun supply the first five degrees, the same convention that defines heating degree days. The efficiency ratings table covers HSPF2 and COP, which describe how much electricity the unit uses to deliver its capacity.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Cooling and heating loads per sq ft | By climate zone, average insulation | Rule-of-thumb climate-zone charts; see the BTU per square foot table |
| Zone design temperatures | 50, 35, 25, 15, 0, −10, −20 °F | Typical 99% heating design temperatures (ASHRAE climatic data), rounded |
| Capacity retention, standard unit | 100% at 47 °F, about 61% at 17 °F, 45% at 5 °F | Straight line through typical AHRI-rated 47 °F and 17 °F capacities of single-stage air-source heat pumps |
| Capacity retention, cold-climate unit | about 80% at 17 °F, 73% at 5 °F | Typical of models on the NEEP cold-climate air-source heat pump list (capacity at 5 °F ≥ 70% of rated is a common threshold) |
| Backup heat steps | 5 kW | Electric heat strip kits sold in 5 kW increments |
| Balance-point base | 65 °F indoors | Degree-day convention: internal gains cover about 5 °F |
Assumptions last reviewed October 7, 2026.
The calculator does not model the defrost cycles that reduce delivered heat in damp cold weather, the fall in COP at low temperatures (which decides running cost rather than size), ground-source units whose capacity does not depend on air temperature, or the dual-fuel switchover temperature at which a furnace takes over entirely. The guide to heat pumps in cold climates covers the balance point, the backup options and the economics in detail.
Two worked examples
A standard heat pump in Chicago
2,000 sq ft in zone 5, 5 °F design temperature, average insulation, 8 ft ceilings, three people and a kitchen, standard unit, sized for cooling.
- Cooling load: 2,000 × 15 + 600 + 4,000 = 34,600 BTU/hr → 3 tons
- Heating load: 2,000 × 45 = 90,000 BTU/hr at 0 °F; heat loss per degree 90,000 ÷ 70 = 1,286 BTU/hr per °F; at 5 °F, 1,286 × 65 = 83,571 BTU/hr
- Capacity at 5 °F: 36,000 × 45% = 16,344 BTU/hr
- Backup: 83,571 − 16,344 = 67,227 BTU/hr = 19.7 kW, so 20 kW of strips
- Balance point: about 40 °F
The 3-ton unit alone covers the house down to about 40 °F, which in Chicago is most of October, November, March and April and a good share of the other winter months; below that the strips carry a growing share, and at 5 °F they do 80% of the work. The note says a cold-climate model of the same size would cut the backup to about 57,000 BTU/hr and run more of the winter on the compressor; a dual-fuel furnace is the other common answer in this climate.
A cold-climate unit in Atlanta
1,800 sq ft in zone 3, 20 °F design temperature, average insulation, three people and a kitchen, cold-climate unit, sized for cooling.
- Cooling load: 1,800 × 20 + 4,600 = 40,600 BTU/hr → 3.5 tons
- Heating load: 1,800 × 30 = 54,000 BTU/hr at 25 °F; 1,200 BTU/hr per °F; at 20 °F, 60,000 BTU/hr
- Capacity at 20 °F: 42,000 × 82% = 34,629 BTU/hr
- Backup: 25,371 BTU/hr = 7.4 kW, so 10 kW of strips
- Balance point: about 33 °F
Atlanta sees 20 °F a few nights a year, so a 10 kW strip kit that runs on those nights is a small part of the bill, and the 3.5-ton unit is only 3% above the cooling load. This is the case where a heat pump works almost unaided; a standard unit here would keep about 65% at 20 °F and need 15 kW of strips instead.
Where to find your inputs
Design temperature. The 99% heating design temperature for your city is in the ASHRAE climatic data tables, in Manual J’s Table 1A, and in most utility and extension-service energy guides; the field’s hint gives the zone defaults. It is colder than the average January night and warmer than the record low.
Unit type. The manufacturer’s specifications give capacity at 47 °F and 17 °F; if the 17 °F figure is 75% or more of the 47 °F figure, treat it as cold-climate. The NEEP list of cold-climate models shows capacity at 5 °F.
Area, zone, insulation. As for the other sizing calculators: conditioned area only, zone by county, insulation by the age and condition of the house.
People and kitchen. Occupants and the kitchen add to the cooling load only; in winter they help.
Common mistakes
- Sizing a heat pump for the heating load in a cold climate. The unit will be two or three times the cooling load and short-cycle all summer; size for cooling and add backup, or go dual-fuel.
- Reading the 47 °F rating as the winter capacity. At 17 °F a standard unit has lost about 40%; the design-night capacity is what matters.
- Skipping the backup. Every air-source heat pump in zones 4 to 7 needs a plan for the coldest nights, whether strips, a furnace or a cold-climate model with margin.
- Oversizing the strips. Strips sized for the whole load, rather than the shortfall, run more than they should when the thermostat calls for them, and they cost three times what the compressor costs per BTU.
- Confusing capacity with efficiency. HSPF2 and COP describe electricity use; they do not change how many BTU the unit can deliver at 5 °F.
- Not checking the manufacturer’s table. The straight-line curve here is an estimate; the unit’s extended capacity data at your design temperature is what Manual S uses.
Questions people ask
- What size heat pump do I need for a 2,000 square foot house?
- Sized for cooling, as most are, a 2,000 sq ft house with average insulation needs about 3 tons in zones 4 and 5, 3.5 tons in zones 1 to 3 and 2.5 tons in zones 6 and 7. The heating side then decides how much backup you need. In zone 5 at a 5 °F design temperature, a standard 3-ton unit keeps about 45% of its capacity, roughly 16,000 BTU per hour against a load of about 84,000, so about 20 kW of resistance backup or a gas furnace covers the rest below a balance point near 40 °F.
- What is the balance point of a heat pump?
- The outdoor temperature at which the heat pump's heating capacity exactly equals the house's heat loss. Above it the unit cycles and has spare capacity; below it the unit runs continuously and the backup heat makes up the difference. For a standard heat pump sized for cooling, the balance point is typically 30 to 40 °F in the northern United States and 20 to 30 °F in the South. A cold-climate unit or a larger unit lowers it. The calculator solves for it from the load line and the capacity line.
- Should a heat pump be sized for heating or cooling?
- In most of the United States, for cooling, with backup heat for the coldest days, because a unit big enough for the heating load in a cold climate would be two or three times the cooling load and would short-cycle all summer. In mild climates (zones 1 to 3) the two loads are close and a unit sized for cooling often covers heating too. In cold climates a cold-climate heat pump sized somewhat above the cooling load, or a dual-fuel system with a furnace, is the usual compromise; the calculator shows both sizes and the summer oversize.
- What is a cold-climate heat pump?
- An air-source heat pump designed to keep most of its heating capacity at low outdoor temperatures, usually with a variable-speed inverter compressor and enhanced vapour injection. Where a standard unit falls to around 60% of its rating at 17 °F and 45% at 5 °F, a cold-climate unit holds 75 to 85% at 17 °F and 65 to 75% at 5 °F, and the better models still heat at −15 °F. The Northeast Energy Efficiency Partnerships maintains a public list of qualifying models with their capacity at 5 °F.
- How much backup heat does a heat pump need?
- Enough to cover the gap between the house's heat loss at the design temperature and the heat pump's capacity at that temperature. Electric strips come in 5 kW steps (5 kW is 17,060 BTU per hour), so a 67,000 BTU per hour shortfall needs 20 kW of strips. In a dual-fuel system the backup is a gas or propane furnace sized for the whole heating load, which takes over entirely below a switchover temperature. The backup runs for only a small share of the winter's hours if the balance point is low.
- Why does my heat pump run constantly in cold weather?
- Because below its balance point it is supposed to. A heat pump delivers warm air at 90 to 105 °F rather than the 130 °F a furnace produces, so it has to run longer to move the same heat, and below the balance point it runs all the time while the backup fills in. Constant running at mild temperatures, say 45 °F, suggests a unit that is undersized, low on refrigerant or iced up, and is worth a technician's visit.