"Sized for Heating" vs "Sized for Cooling": The Central Decision
A heat pump is one unit doing two opposite jobs. In summer the indoor coil pulls heat out of the house. In winter the same unit reverses and pulls heat from outside air into the house. The size you pick is almost always a compromise because cooling load and heating load rarely match for the same house in the same year.
In a 2,000 sq ft Atlanta home the cooling load might be 36,000 BTU and the heating load 40,000 BTU. Close enough that a 3 ton heat pump (36,000 BTU rated) covers both. In the same house in Boston the cooling load drops to 30,000 BTU but the heating load jumps to 60,000 BTU at 7°F. A 3 ton heat pump cools fine but only delivers about 20,000 BTU at 7°F because cold air holds less heat to extract. You need a bigger unit, a cold climate inverter, or backup heat. There is no fourth option.
The decision tree is simple. Zones 1, 2, 3 size to cooling. Zones 4 and 5 size to cooling and check that heating at 17°F covers the load. Zones 6, 7, and 8 size to the heating load and use a variable speed unit to keep summer cycle lengths reasonable. The numbers in the table below assume this same logic.
Heat Pump Tonnage by Climate Zone
The table below sizes a heat pump for combined heating and cooling. Cold zone numbers assume a cold climate ENERGY STAR unit holding rated capacity down to 5°F. Standard heat pumps in zones 5 to 8 would need to bump up half a ton, or accept that backup heat covers more hours.
| Home Size | Zone 1 to 2 | Zone 3 | Zone 4 | Zone 5 (CCHP) | Zone 6 to 7 (CCHP) |
|---|---|---|---|---|---|
| 1,000 sq ft | 2.0 ton | 1.5 ton | 2.0 ton | 2.0 ton | 2.5 ton |
| 1,500 sq ft | 3.0 ton | 2.5 ton | 2.5 ton | 3.0 ton | 3.0 ton |
| 2,000 sq ft | 4.0 ton | 3.0 ton | 3.0 ton | 3.5 ton | 4.0 ton |
| 2,500 sq ft | 4.5 ton | 3.5 ton | 3.5 ton | 4.0 ton | 5.0 ton |
| 3,000 sq ft | 5.0 ton | 4.0 ton | 4.0 ton | 5.0 ton | 5.0 + 2.0 split |
CCHP = Cold Climate Heat Pump (ENERGY STAR certified for cold weather performance). For zone 8 (interior Alaska, far northern Maine) most homeowners pair a heat pump with a gas or oil furnace as primary heat and use the heat pump only above 10°F. A standalone heat pump install in zone 8 is technically possible but the electric strip backup costs more than gas heat for most of the season.
Cold Climate Heat Pumps: Capacity at 5°F
Standard heat pumps are rated at 47°F outdoor. Capacity drops sharply as outdoor temperature falls. A 3 ton standard heat pump that produces 36,000 BTU at 47°F might only produce 18,000 BTU at 17°F and 12,000 BTU at 5°F. That is the reason heat pumps got a reputation for not working in cold weather.
Cold climate heat pumps (CCHP) use enhanced vapor injection, a second compression stage, or inverter compressors to hold capacity at low temperatures. The ENERGY STAR Cold Climate spec requires 70 percent of rated capacity at 5°F and a COP of at least 1.75 at 5°F. The leading units exceed that. Mitsubishi Hyper-Heat holds 100 percent capacity to 5°F on most models. Carrier Greenspeed holds 75 percent. Daikin Aurora holds 80 percent. Bosch IDS holds 100 percent through inverter modulation.
| Heat Pump Type | Capacity at 47°F | Capacity at 17°F | Capacity at 5°F | Useful Down To |
|---|---|---|---|---|
| Standard single stage | 100% | 55 to 65% | 35 to 45% | 15°F |
| Standard two stage | 100% | 65 to 75% | 45 to 55% | 5 to 10°F |
| Inverter (mid tier) | 100% | 80 to 90% | 60 to 75% | -5°F |
| CCHP (Hyper-Heat, IDS) | 100% | 100% | 85 to 100% | -15 to -20°F |
What Is a Balance Point and Why It Matters
The balance point is the outdoor temperature where heat pump output exactly matches the heating load of the house. Above the balance point the unit can keep up. Below it, backup heat has to fill the gap.
Finding your balance point. Plot two lines on a graph. The first is heating load, which goes up as outdoor temperature drops (a straight line from 0 BTU at 65°F to maximum BTU at your design temperature). The second is heat pump output, which goes down as outdoor temperature drops (a curve from 100 percent at 47°F to maybe 40 percent at 5°F for a standard unit). The temperature where the two lines cross is your balance point.
A right sized heat pump in zone 4 usually balances around 25 to 30°F. In zone 5 with a cold climate unit, balance point lands at 15 to 20°F. In zone 6 with a CCHP, balance point lands at 5 to 10°F. Anything below your balance point requires backup heat to make up the difference, which is why backup strategy is the next decision after sizing.
Backup Heat Strategy: Electric Strips vs Gas Furnace (Dual Fuel)
Every heat pump in zones 4 through 8 needs some form of backup heat for the hours when outdoor temperature drops below the balance point. The two real options are electric resistance strips inside the air handler or a gas (or propane or oil) furnace running as the second heat source.
Electric resistance strips are simple, cheap to install ($300 to $800 added to the air handler price), and produce 1 BTU per watt-hour. At $0.15 per kWh, that is roughly $4.40 per therm equivalent. If your heat pump has a COP of 3 at 30°F, every kWh produces 3 times the heat, costing $1.47 per therm equivalent. Once the COP drops below about 1.5 (usually around 15 to 20°F for standard units), strip heat becomes cheaper per BTU than running the heat pump.
Gas furnace backup (called dual fuel or hybrid heat) costs $3,000 to $6,000 more to install but produces heat at $1.20 per therm in most US markets. A control board locks out the heat pump when outdoor temperature drops below a switchover setpoint, usually 30 to 35°F. Gas backup wins in zones 5, 6, and 7 if you already have a gas service. It loses in mild zones because the gas furnace barely runs. It also loses if you do not have gas service at the street, because adding gas service typically costs $1,500 to $4,000 on top of the furnace install.
SEER2 and HSPF2 Minimums by Climate Zone
The DOE updated minimum efficiency standards in January 2023. The standards split the US into a southern region (zone 1, 2, 3) and a northern region (zone 4, 5, 6, 7, 8). The minimums apply to new installations, not replacements of existing units.
| Region | Federal Minimum SEER2 | Federal Minimum HSPF2 | ENERGY STAR SEER2 | ENERGY STAR HSPF2 |
|---|---|---|---|---|
| South (Zone 1, 2, 3) | 14.3 | 7.5 | 15.2 | 8.1 |
| North (Zone 4 to 8) | 13.4 | 7.5 | 14.3 | 8.1 |
| ENERGY STAR Cold Climate | 15.2 | 9.0 | n/a | n/a |
The federal IRA tax credit (25C) requires ENERGY STAR Most Efficient or cold climate certification to qualify. The credit is 30 percent of the install cost up to $2,000 per year. Most state and utility rebates ($500 to $4,000 depending on state) layer on top of the federal credit. Check the IRS site and your state energy office for current rules.
How to Verify a Heat Pump Sizing Recommendation
A heat pump quote is more complex than a furnace or AC quote because you are buying one unit for two seasons. Ask these four questions before signing.
- What heating capacity at my design temperature? The Manual J should output a heating load number at your winter design temperature (5°F to 15°F for most cold climates). The heat pump performance sheet shows capacity at multiple outdoor temperatures. The capacity at your design temperature should equal or exceed the heating load. If the contractor cannot show both numbers from the same paperwork, the unit is being sold from a catalog instead of sized.
- What is the balance point? Ask for the exact temperature where the heat pump output equals the heating load. A real install plan names this number. If the answer is "uh, around 30 maybe," the contractor has not run the math.
- Is this an ENERGY STAR Cold Climate unit? Required for zone 5 and colder if you want full IRA tax credit eligibility. The CEE directory and ENERGY STAR product finder list certified models. AHRI certificate number on the quote should match.
- How many hours per year will backup heat run? Should be a number from the load calc, not a guess. In zone 5 with a CCHP, expect 100 to 300 hours per year. In zone 6 expect 300 to 700 hours. If the contractor says "almost never" without a calculation, the backup sizing is also guessed.
The residential load calculator walks through both the heating and cooling loads needed to verify these numbers. For the cooling side specifically, the AC tonnage calculator covers the sensible vs latent breakdown.
Frequently Asked Questions
How do you size a heat pump differently than an AC?
A heat pump has to do two jobs that pull in opposite directions. In summer it cools, and the sizing is based on cooling load like any AC. In winter it heats, and the unit loses capacity as outdoor temperature drops. Sizing strictly to the cooling load leaves you short of heat on a 10°F morning. Sizing strictly to the heating load gives you a unit that short cycles all summer. The right answer depends on your climate zone and where the loads cross.
What is a heat pump balance point?
The balance point is the outdoor temperature where the heat pump output exactly equals the heating load of the house. Above the balance point the heat pump can keep up. Below it you need backup heat. For a 3 ton heat pump in a 60,000 BTU heating load home, the balance point usually lands between 25°F and 35°F depending on the unit. A cold climate heat pump pushes the balance point down to 5°F or even below 0°F.
Do heat pumps actually work in cold weather?
Yes, with the right unit. Standard heat pumps lose about half their rated capacity by 17°F and most stop being useful below 10°F. Cold climate heat pumps (ENERGY STAR Cold Climate certified) hold 70 to 80 percent of rated capacity at 5°F and still deliver useful heat down to -15°F. Brands like Mitsubishi Hyper-Heat, Bosch IDS, Carrier Greenspeed, and Daikin Aurora are the most common cold climate options.
What is the difference between SEER2 and HSPF2?
SEER2 measures cooling efficiency in summer. HSPF2 measures heating efficiency over a full heating season. Both are seasonal averages that account for partial load operation. The federal minimums set in 2023 are 14.3 SEER2 / 7.5 HSPF2 for southern states and 13.4 SEER2 / 7.5 HSPF2 for northern states. ENERGY STAR Cold Climate units typically hit 16+ SEER2 and 9.0+ HSPF2.
Should my backup heat be electric strips or a gas furnace?
Electric resistance strips cost less to install ($300 to $800 added to the air handler) but use 3 to 4 times more electricity per BTU than the heat pump itself. They are fine if your balance point is around 20°F and you only need backup for a few dozen hours per year. A gas furnace as backup (dual fuel setup) costs $3,000 to $6,000 extra but lets you switch to cheaper gas heat whenever the heat pump COP drops below the gas equivalent, usually around 30°F. Dual fuel makes sense in zones 5, 6, and 7 if you already have gas service.
What size heat pump do I need for 2,000 square feet?
In a mild zone like Charleston (3A) a 3 ton heat pump handles a 2,000 sq ft home for both heating and cooling. In a cold zone like Boston (5A) the same house needs 3 tons for cooling but 4 to 4.5 tons of heat at 0°F, which is why you either oversize the heat pump or add backup heat. A cold climate inverter heat pump can stay 3 tons because it maintains rated capacity down to 5°F.
Should I size to the heating load or the cooling load?
In zones 1 through 3, size to the cooling load. The heat pump is mostly cooling the house and the heating load is small enough that any sized cooling unit covers it. In zones 4 and 5, size to the cooling load but verify the heating capacity at 17°F meets your design heating load. In zones 6, 7, and 8, size to the heating load at your winter design temperature, and accept that the unit will short cycle in summer unless you go variable speed.
Are cold climate heat pumps worth the extra money?
In zones 5 through 8 they are usually the only heat pumps that make sense. A standard heat pump in Minneapolis would call electric strip backup for 30 to 40 percent of the heating season. A cold climate unit handles 95 percent of the season without backup. The install cost is $1,500 to $4,000 higher, but you avoid 1,000+ hours per year of $0.15/kWh strip heat. Payback is typically 4 to 7 years in cold climates and effectively never in warm ones.
How do I find my heating design temperature?
The ASHRAE 99 percent winter design temperature is the temperature your zip code stays above for 99 percent of the heating hours. It is colder than the average winter temperature but warmer than the record low. Atlanta is 26°F. Boston is 7°F. Chicago is -2°F. Minneapolis is -11°F. The DOE Building America program publishes a county lookup table. You size the heat pump to deliver your heating load at this temperature, not at the coldest temperature on record.
Can I run a heat pump and a furnace at the same time?
Not at the same time. A dual fuel setup runs the heat pump above the switchover temperature (usually 30 to 35°F) and runs the gas furnace below it. The control board locks out the opposite system so they never run together. The switchover point is set based on local gas and electric rates, which is why a good install adjusts it once you have a few months of utility bills.