Heat pump efficiency changes with outdoor temperature, and the difference can be dramatic. This guide explains heat pump efficiency by temperature, what COP and HSPF2 mean, how defrost and humidity affect performance, and when auxiliary heat should kick in. It also provides tables with typical COP values across temperatures, cost break-even math against natural gas, and practical controls to maximize savings and comfort in U.S. climates.
How Temperature Affects Heat Pump Efficiency
Air-source heat pumps move heat from outdoors to indoors. As outdoor air gets colder, there is less heat to extract and the compressor works harder. Efficiency (COP) and capacity drop as temperatures fall, and the drop is steeper on single-stage units than on variable-speed cold-climate models.
What COP Means
COP (coefficient of performance) is heat output divided by electrical input. A COP of 3 means the system delivers three units of heat for each unit of electricity. Higher COP is better. COP varies continuously with outdoor temperature, indoor setpoint, airflow, defrost behavior, and part-load modulation.
Performance Curves, Not Points
Manufacturers publish performance data at standard test points like 47°F and 17°F. Real homes experience a curve. Variable-speed inverter heat pumps flatten the curve by matching compressor speed to load, retaining capacity at lower temperatures and keeping COP higher than single-stage models.
Defrost And Humidity
Below roughly 40°F with high humidity, frost forms on the outdoor coil. The system periodically reverses into cooling to melt ice, using indoor heat and sometimes electric strips. Defrost temporarily lowers COP. Demand-defrost controls minimize cycles compared to timed defrost, improving cold-weather efficiency.
Indoor Temperature Effects
Raising indoor setpoint increases the temperature lift the compressor must achieve, reducing COP. Airflow also matters: too little airflow lowers capacity and COP, while too much can raise power draw. Clean filters and correct fan settings help maintain specs.
Typical COP By Outdoor Temperature
The table below shows approximate COP and capacity retention for three common system types. Real values vary by model, refrigerant, coil sizing, and controls. Use this as a directional guide when thinking about heat pump efficiency by temperature.
| Outdoor Temp (°F) | Standard Air-Source (SEER2 ~14–16, HSPF2 ~7.5–8.5) | Cold-Climate Air-Source (Inverter, NEEP Listed) | Ground-Source (Closed-Loop) |
|---|---|---|---|
| 60 | COP ~4.0 | Capacity ~110% | COP ~4.5 | Capacity ~115% | COP ~5.0 | Capacity ~100% |
| 47 | COP ~3.3 | Capacity ~100% | COP ~3.8–4.2 | Capacity ~105–120% | COP ~4.7 | Capacity ~100% |
| 35 | COP ~2.8 | Capacity ~90–95% | COP ~3.4–3.8 | Capacity ~110–120% | COP ~4.5 | Capacity ~100% |
| 25 | COP ~2.2–2.5 | Capacity ~80–90% | COP ~3.0–3.4 | Capacity ~100–115% | COP ~4.3 | Capacity ~100% |
| 17 | COP ~2.0–2.2 | Capacity ~70–85% | COP ~2.5–3.0 | Capacity ~90–110% | COP ~4.1 | Capacity ~100% |
| 5 | COP ~1.6–1.9 | Capacity ~60–75% | COP ~2.0–2.5 | Capacity ~80–100% | COP ~3.9 | Capacity ~100% |
| -5 | COP ~1.2–1.5 | Capacity ~50–65% | COP ~1.6–2.1 | Capacity ~70–95% | COP ~3.7 | Capacity ~100% |
Notes: Capacity values are relative to the 47°F rating point. Cold-climate models often use enhanced vapor injection and large coils to maintain capacity. Ground-source systems see steadier efficiency because the loop remains near the earth’s stable temperature.
Ratings That Predict Real-World Efficiency
U.S. ratings changed in 2023 to SEER2 and HSPF2. Understanding these helps translate lab data into practical expectations for heat pump efficiency by temperature.
HSPF2: Heating Season Efficiency
HSPF2 is seasonal heating output (Btu) divided by electrical input (Wh). Seasonal average COP ≈ HSPF2 ÷ 3.412. For example, HSPF2 9.0 implies an average seasonal COP of about 2.64. Cold-climate models often exceed HSPF2 9, while standard models land around 7.5–8.5.
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SEER2: Cooling Efficiency
SEER2 reflects cooling-season efficiency; it correlates weakly with heating performance. Many high-SEER2 models also feature variable-speed compressors that help in heating, but choose based on HSPF2 and low-ambient performance data if heating is a priority.
Rated Low-Ambient Capacity
Check manufacturer data for “100% capacity down to X°F” or capacity at 5°F. Capacity retention at low temperature prevents auxiliary heat use, which keeps costs and emissions down.
Balance Point, Sizing, And When Aux Heat Takes Over
The heating balance point is the outdoor temperature where the heat pump’s output equals the home’s heat loss. Below this, either the house cools or auxiliary heat is needed. Good design aims to push the balance point lower.
Sizing With Manual J/S/D
Accurate load calculation (ACCA Manual J), equipment selection (Manual S), and duct design (Manual D) are critical. Right-sized, variable-speed units run longer at low speed, improving comfort and average COP. Oversizing raises defrost penalties and cycling losses, while undersizing overuses strips.
Auxiliary Heat And Bivalent Point
Electric strip heat has COP ≈ 1. Dual-fuel setups pair a heat pump with a gas furnace. The bivalent point is the temperature where control switches to the backup source. Proper lockout settings avoid premature strip or furnace operation while maintaining comfort.
Control Strategies To Maximize Efficiency As Temperatures Drop
Controls make a major difference in real-world heat pump efficiency by temperature. Smart, outdoor-sensor-based logic can save significant energy in shoulder seasons and cold snaps.
- Outdoor Temperature Lockouts: Set an auxiliary heat lockout just below the heat pump’s economic break-even temperature. Keep the compressor enabled as low as it can still provide adequate capacity.
- Staging And Ramping: Use thermostats that prefer longer compressor run time before energizing strips. Limit strip engagement to large setbacks or fast recovery needs.
- Demand Defrost: Ensure the outdoor unit uses demand defrost, not fixed-interval defrost, to minimize unnecessary reversals.
- Moderate Setbacks: In cold weather, small setbacks (1–3°F) are safer. Large setbacks often trigger strips and erase savings.
- Fan Settings: Auto or low continuous fan reduces drafty air and keeps supply air warmer. Verify airflow per ton per the manufacturer to avoid coil icing.
- Humidity Management: In humid cold, accept slightly longer defrost intervals and check that the outdoor unit drains properly to prevent refreezing.
Home And Duct Factors That Shift The Efficiency Curve
The building envelope determines how low a temperature the heat pump can handle without backup. Improving the shell effectively raises the balance point and reduces power spikes.
- Air Sealing: Seal top plates, attic hatches, rim joists, and duct penetrations. Lower infiltration reduces heating load and drafts.
- Insulation: Upgrade attic to at least current code targets, insulate crawlspaces and basements, and consider wall insulation in older homes.
- Duct Sealing And Design: Duct leakage can exceed 20% in older homes. Seal and insulate ducts, size trunks and branches correctly, and balance flows for even room temperatures.
- Filter Resistance: High-MERV filters improve air quality but can reduce airflow if undersized. Use deeper media filters or larger return grilles to maintain static pressure.
- Windows And Shading: Tight windows and storm panels help, but often air sealing and attic insulation deliver higher value per dollar.
Cost And Emissions: Break-Even COP By Fuel And Rates
Whether a heat pump is cheaper than gas depends on rates, furnace efficiency, and COP at the current temperature. Use the formula below to set lockouts or choose dual-fuel switchover points.
Break-Even COP Formula
Cost per MMBtu via heat pump ≈ (Electricity Price $/kWh × 293.1) ÷ COP. Cost per MMBtu via gas ≈ (Gas Price $/therm ÷ AFUE) × 10. Break-even COP occurs when these are equal: COP ≈ (Electricity Price × 293.1 × AFUE) ÷ (10 × Gas Price).
Example With Typical U.S. Prices
Assume electricity $0.15/kWh, gas $1.50/therm, 95% AFUE. Break-even COP ≈ (0.15 × 293.1 × 0.95) ÷ (10 × 1.50) ≈ 2.7. If the operating COP at a given temperature is above 2.7, the heat pump is cheaper than gas; below that, gas is cheaper in a dual-fuel setup.
Time-Of-Use And Demand Charges
On time-of-use plans, preheat the house before peak hours, then let the heat pump cruise at low speed through the peak. Avoid resistive strips during peak windows if demand charges apply.
Climate-Specific Guidance For The U.S.
Different climates change the heat pump efficiency by temperature curve and the likelihood of auxiliary heat. The table outlines typical strategies.
| Climate/Region | Winter Profile | Recommended Approach |
|---|---|---|
| Hot-Humid (e.g., Miami, New Orleans) | Mild winters; defrost on humid cool mornings | Standard inverter ASHP; prioritize SEER2 and HSPF2 balance; ensure demand defrost |
| Mixed-Humid (e.g., Atlanta, Washington, DC) | Frequent 20s–40s°F; occasional teens | Inverter ASHP with good low-ambient data; aux lockout near economic COP; moderate setbacks |
| Cold (e.g., Chicago, Boston) | Teens common; single digits | Cold-climate ASHP listed by NEEP; size for 99% design temp with high capacity retention |
| Very Cold (e.g., Minneapolis) | Subzero spells | Top-tier cold-climate ASHP or ground-source; consider dual-fuel; tight envelope and demand defrost |
| Marine (e.g., Seattle, Portland) | Cool and damp; many defrost cycles | Inverter ASHP; demand defrost and large coil; manage humidity and drainage |
| Dry Mountain (e.g., Denver, Salt Lake City) | Cold nights, sunny days; low humidity | Cold-climate ASHP; leverage solar gains; preheat before TOU peaks; high-altitude derating check |
Choosing Equipment: Tech Differences That Matter At Low Temperatures
Not all heat pumps behave the same in the cold. Selecting the right technology is crucial for efficiency as temperatures fall.
- Inverter Compressors: Variable-speed systems modulate from very low to high output, maintaining comfort and COP under light loads and pushing capacity at low ambient without heavy strip use.
- Enhanced Vapor Injection (EVI): Many cold-climate models inject vapor mid-compression to boost mass flow and low-ambient capacity, improving COP at 5°F and below.
- Coil Surface Area: Larger outdoor coils increase heat pickup at lower temperature, raising COP and reducing defrost frequency.
- Refrigerants: R-410A is common; newer R-32 and R-454B can improve efficiency and reduce GWP. CO₂ (R-744) air-to-water units excel at cold domestic hot water production.
- Crankcase Heaters: These prevent liquid slugging but consume standby power. Smart controls limit heater use based on ambient and runtime history.
- Demand Defrost Sensors: Coil temperature and pressure sensors trigger defrost only when needed, saving energy during long cold snaps.
Translating Ratings To Your Temperature: A Quick Method
To estimate COP at a given temperature for a specific model, combine published data with climate information.
- Find Manufacturer Data: Look for capacity and power at multiple temperatures (e.g., 47°F, 17°F, 5°F).
- Compute COP At Points: COP = Capacity (Btu/h) ÷ (Power (W) × 3.412). Graph these points.
- Interpolate: For intermediate temperatures, interpolate COP linearly as a rough estimate, adjusting downward 5–10% for frequent defrost in humid cold.
- Check Balance Point: Compare model capacity vs. Manual J load across temperatures to see when strips may be needed.
Defrost Behavior And Its Real Impact
Defrost energy use depends on climate and controls. In marine and mixed-humid climates, frost is common around 25–40°F. In very cold and dry climates, frosting is less frequent but can be severe near freezing following snow.
- Cycle Frequency: Demand defrost may run every 60–120 minutes or longer under light frosting, versus fixed 30-minute intervals in older systems.
- Duration: Each defrost may last 3–10 minutes. During defrost the heat pump cools the indoor coil; auxiliary heat may temper supply air.
- Annual Effect: Over a season, defrost may reduce heating seasonal efficiency by several percent; good controls limit the penalty.
Ductless Vs. Ducted In Cold Weather
Ductless mini-splits often achieve excellent low-ambient performance thanks to oversized coils and fine modulation. Ducted systems can match this if ducts are tight and properly sized.
- Ductless Advantages: High HSPF2, zoned control, minimal distribution losses.
- Ducted Advantages: Whole-home distribution, hidden equipment, compatibility with existing ducts.
- Hybrid Designs: Mix ducted air handlers for core spaces with ductless heads for difficult rooms to reduce strip heat reliance.
Practical Setup: Lockout Temperatures And Setbacks
Set lockouts based on economics and capacity, not rules of thumb. The following typical values can be a starting point, then fine-tune with data.
- Aux Heat Lockout: 25–35°F for standard ASHP; 10–25°F for cold-climate ASHP, assuming adequate capacity.
- Compressor Cutoff: Many cold-climate models can run to -5°F or below; only lock out if coil protection or comfort requires it.
- Setback Strategy: Keep setbacks small in cold weather to avoid strip engagement. Use “smart recovery” features to ramp up early.
Maintenance That Preserves COP In The Cold
Routine maintenance prevents efficiency declines that become most visible at low temperatures.
- Outdoor Coil Cleanliness: Keep fins clean and unobstructed; clear snow drifts and ensure proper drainage to avoid refreezing.
- Filter Changes: Replace or clean filters per manufacturer guidance; verify static pressure stays within limits.
- Refrigerant Charge: Undercharge or overcharge harms COP; have a technician verify charge with low-ambient procedures.
- Airflow Verification: Check supply and return airflow per ton; correct duct restrictions or blower settings as needed.
- Thermostat Updates: Keep firmware updated on smart thermostats; confirm outdoor sensor readings for accurate lockouts.
Frequently Asked Questions About Heat Pump Efficiency By Temperature
Do Heat Pumps Work Below Freezing?
Yes. Modern cold-climate heat pumps deliver useful capacity and COP >2 near 5°F and can operate below 0°F. Performance depends on model and installation quality.
Why Does My Heat Pump Blow Cooler Air In Winter?
Supply air from heat pumps is often 85–105°F, which can feel cooler than furnace air. During defrost, the air can temporarily cool. Longer, steadier runs still heat the space efficiently.
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Is Ground-Source Always Better?
Ground-source systems have higher and steadier COP, but higher upfront cost and site constraints. In many homes, a well-selected cold-climate air-source unit achieves excellent life-cycle savings.
What About R-32 Or R-454B?
Next-generation refrigerants can improve efficiency modestly and reduce global warming potential. System design and controls matter more than refrigerant alone for low-ambient performance.
Will A Bigger Heat Pump Solve Cold-Weather Issues?
Oversizing can increase cycling, reduce dehumidification in shoulder seasons, and raise defrost penalties. Right-size using Manual J, and select a unit with strong low-ambient capacity instead of simply going bigger.
Example: Estimating Operating Cost Across Temperatures
Assume a cold-climate ASHP with COP values from the table and electricity at $0.15/kWh.
- At 35°F (COP 3.4): Cost per MMBtu ≈ 0.15 × 293.1 ÷ 3.4 ≈ $12.93.
- At 17°F (COP 2.7): Cost per MMBtu ≈ 0.15 × 293.1 ÷ 2.7 ≈ $16.29.
- At 5°F (COP 2.2): Cost per MMBtu ≈ 0.15 × 293.1 ÷ 2.2 ≈ $19.98.
If gas is $1.50/therm with a 95% furnace, cost per MMBtu ≈ ($1.50 ÷ 0.95) × 10 ≈ $15.79. Result: The heat pump is cheaper than gas at 35°F, similar at 17°F, and may cost more at 5°F unless the local electric rate is lower or gas is higher.
How To Read Manufacturer Low-Temperature Data
Before buying, request expanded performance tables, not just nameplate ratings.
- Capacity At 17°F And 5°F: Look for ≥90–100% capacity retention at 17°F and strong output at 5°F for cold-climate performance.
- Input Power: Compare power draw at each temperature to compute COP. Lower watts per Btu means higher efficiency.
- Defrost Strategy: Confirm demand defrost and review any low-ambient kit requirements or crankcase heater power.
- Sound And Vibration: Inverters are quiet at low speed; ensure mounting minimizes resonance in cold, dense air.
Rebates, Standards, And Data Sources
Programs and databases can help identify models with strong heat pump efficiency by temperature and lower total cost of ownership.
- NEEP Cold-Climate ASHP List – Verified low-ambient capacity and COP data for cold-climate models.
- ENERGY STAR Heat Pumps – High-efficiency models and cold-climate specification.
- DOE Efficiency Standards – SEER2/HSPF2 details and minimums by region.
- ACCA Manuals J/S/D – Professional design methods for correct sizing and duct design.
- DSIRE Incentives Database – State and utility rebates and tax credits.
Key Takeaways To Optimize Efficiency As Temperatures Shift
- Pick The Right Platform: In cold regions, choose a NEEP-listed cold-climate inverter; consider ground-source where feasible.
- Design Matters: Do Manual J/S/D, ensure duct tightness, and verify airflow to maintain COP in cold weather.
- Control Intelligently: Use auxiliary lockouts, demand defrost, and moderate setbacks to avoid resistive heat spikes.
- Watch The Economics: Compute break-even COP using local rates; adjust dual-fuel switchover accordingly.
- Maintain The System: Clean coils, correct charge, and updated controls keep low-ambient performance on spec.
Glossary
- COP: Coefficient of performance, heat output divided by electrical input.
- HSPF2: Heating seasonal performance factor (Btu/Wh) per 2023 test method; HSPF2/3.412 ≈ seasonal COP.
- SEER2: Seasonal energy efficiency ratio for cooling under 2023 test method.
- Balance Point: Outdoor temperature where heat pump output equals home heat loss.
- Bivalent Point: Temperature where control shifts from heat pump to backup heat.
- Demand Defrost: Control strategy that initiates defrost only when sensors detect frost buildup.
How to Get the Best HVAC Prices
- Firstly, keep in mind that installation quality is always the most important thing for residential HVAC project. So never sacrifice contractor quality for a lower price.
- Secondly, remember to look up the latest rebates as we talked above.
- Thirdly, ask for at least 3 bids before you make the decision. You can click here to get 3 free estimates from your local contractors, and this estimate already takes rebates and tax credit into consideration and filter unqualified contractors automatically.
Lastly, once you chose the right contractor, remember to use the tactics from this guide: Homeowners Tactics When Negotiating with HVAC Dealer to get the final best price.
