At What Temperature Do Heat Pumps Lose Efficiency

Heat pumps transfer heat rather than create it, making them efficient for many climates. Their performance, however, varies with outdoor temperature. This article explains when and why heat pumps lose efficiency, how much efficiency declines at specific temperatures, differences between air-source and ground-source systems, and practical steps to maintain comfort and efficiency.

Outdoor Temperature Range	Typical Efficiency Impact	Notes
Above 40°F	High Efficiency (COP 2.5–4+)	Optimal operating range for most air-source heat pumps
20–40°F	Moderate Efficiency (COP 1.5–2.5)	Efficiency begins to fall as heat extraction becomes harder
0–20°F	Reduced Efficiency (COP ~1–1.8)	Auxiliary heat often used; energy cost rises
Below 0°F	Significant Loss (COP <1–1.2)	Many systems rely heavily on backup heat; cold-climate models perform better

How Heat Pumps Work And Why Temperature Matters

Heat pumps move heat from one place to another using a refrigerant cycle and a compressor. In heating mode, an outdoor coil extracts heat from outside air and the compressor raises its temperature before releasing it indoors. The colder the outside air, the less heat is available to extract and the compressor must work harder.

Performance Depends On The Temperature Differential between outdoor air and desired indoor temperature. Larger differentials force higher compressor pressures and energy use, lowering the unit’s coefficient of performance (COP).

Key Efficiency Metrics: COP, HSPF, And SEER

Understanding these metrics helps interpret how temperature affects efficiency.

• COP (Coefficient Of Performance): Ratio of heat output to electrical input at a given operating point. Higher COP means better efficiency. COP varies with outdoor temperature.
• HSPF (Heating Seasonal Performance Factor): Seasonal measure of heating efficiency for heat pumps in the U.S. It averages performance across variable conditions and is used for comparing models.
• SEER (Seasonal Energy Efficiency Ratio): Rating for cooling efficiency; relevant if the unit also cools.

Typical Temperature Thresholds Where Efficiency Drops

While specifics vary by model and refrigerant, commonly observed thresholds for air-source heat pumps are:

• Above 40°F: Heat pumps operate near peak efficiency. COP often exceeds 3, meaning one unit of electricity delivers three units of heat.
• Around 32–40°F: Efficiency begins to decline as the outdoor coil works with colder air. COP typically falls to the 2–3 range.
• 20–32°F: Noticeable efficiency loss. Auxiliary resistance heat (electric strip) or a hybrid furnace may engage more frequently. COP may be between 1.5 and 2.
• 0–20°F: Significant drop in performance. Many standard air-source systems will switch to supplemental heat; COP may approach 1 or slightly higher.
• Below 0°F: Traditional systems become much less efficient and often cannot meet demand without backup heat. Cold-climate heat pumps and ground-source systems are designed to perform better in this range.

Examples Of COP Changes With Temperature

The following examples represent typical behavior for a modern air-source heat pump; exact numbers depend on manufacturer and model.

Outdoor Temp	Example COP	Implication
50°F	3.0–4.5	Very efficient; electric heating cost low
35°F	2.0–3.0	Good efficiency; minimal backup use
20°F	1.3–2.0	Supplemental heat increasingly likely
5°F	~1.0–1.5	Backup heat often engaged; operating cost rises
-10°F	<1.0 (unless cold-climate model)	Standard models costly; cold-climate or ground-source better

Air-Source Versus Ground-Source Heat Pumps

Not all heat pumps respond the same to cold.

• Air-Source Heat Pumps (ASHP): Most common and least expensive to install. Efficiency drops as outdoor air temperature falls. Many standard ASHPs start losing significant efficiency below 30–40°F.
• Cold-Climate Air-Source Heat Pumps: Designed with improved compressors, refrigerants, and controls to maintain higher COPs at low temperatures, sometimes operating effectively down to -15°F or lower.
• Ground-Source (Geothermal) Heat Pumps: Use relatively stable ground temperatures, keeping COPs steady year-round. Ground-source systems typically maintain higher efficiency at low outdoor air temperatures, but with higher upfront costs.

Factors That Accelerate Efficiency Loss

Several operational and environmental factors can cause a heat pump to lose efficiency faster than temperature alone would predict.

• Poorly Maintained Coils: Dirt or ice on the outdoor coil reduces heat transfer.
• Frequent Defrost Cycles: Repeated defrosting uses energy and momentarily reduces heat output.
• Undersized Equipment: Underperforming units run longer and less efficiently.
• Improper Refrigerant Charge: Too much or too little refrigerant lowers COP and can damage the compressor.
• Poor Building Envelope: Leaks and weak insulation increase heating load, making the heat pump work harder.

Practical Strategies To Maintain Efficiency In Cold Weather

Several actions can reduce the impact of cold temperatures on heat pump efficiency.

• Choose A Cold-Climate Model when winters are severe; these models keep higher COPs at lower temperatures.
• Use A Hybrid System that pairs a heat pump with a high-efficiency gas furnace to switch to the most economical heat source as temperatures fall.
• Optimize Controls: Smart thermostats and outdoor temperature sensors can minimize backup heat use and optimize defrost scheduling.
• Regular Maintenance: Keep coils clean, ensure proper refrigerant charge, and verify defrost controls are working.
• Improve Home Insulation And Air Sealing to lower heating load and reduce run times for the heat pump.
• Supplement With Passive Solar And Zoning to reduce demand on the heat pump during the coldest hours.

When To Expect Backup Heat And How It Impacts Bills

Most systems have auxiliary electric resistance heat or a fossil-fuel furnace as backup. Auxiliary heat is significantly less efficient than the heat pump and can be expensive if used often.

Trigger Points For Backup Heat depend on system settings and climate. In many homes, backup heat begins to supplement or replace heat pump output when outdoor temperatures fall below 20–30°F. Users should review thermostat and manufacturer settings to control when backup heat engages.

Sizing And Installation Considerations

Correct sizing and proper installation are critical for maintaining efficiency across temperature ranges.

• Avoid Oversizing: Oversized heat pumps short-cycle, increasing wear and lowering seasonal efficiency.
• Ensure Proper Airflow: Indoor and outdoor airflow must meet manufacturer specifications for rated COPs to be achieved.
• Professional Commissioning: Proper refrigerant charge and control calibration ensure the unit performs according to published temperature performance curves.

Selecting The Right Heat Pump For Cold Climates

When winter temperatures regularly drop below freezing, prioritize models with proven low-temperature performance.

• Look For Low-Temp Performance Data: Manufacturers sometimes publish COP and capacity curves down to -13°F or lower.
• Check HSPF And Cold-Climate Certification: Higher HSPF values and third-party cold-climate ratings indicate better seasonal cold performance.
• Consider Ground-Source If Long-Term Efficiency Is Priority: Although more expensive upfront, geothermal systems offer stable COPs and lower operating cost in very cold climates.

Common FAQs

At What Outdoor Temperature Should A Heat Pump Stop Being The Primary Heat Source?

There is no universal cutoff, but many installers recommend supplemental heat below 20°F to 30°F for standard units. Cold-climate models can remain primary heat sources at much lower temperatures.

Can A Heat Pump Keep A Home Warm At 0°F?

Some modern cold-climate air-source and most ground-source heat pumps can maintain indoor comfort at 0°F, though the unit may run continuously and electricity use will be higher. Backup heat may still be needed during extreme cold or to respond quickly to sudden heat loss.

How Much More Expensive Is Heating With Backup Heat?

Electric resistance backup heat has a COP of about 1, making it 2–4 times more expensive than a heat pump operating with COP 2–4. Hybrid systems can reduce cost by switching to gas when it is cheaper than electric resistance heat.

Monitoring Performance And When To Upgrade

Owners can track energy use and indoor comfort to determine if a heat pump is losing too much efficiency in cold weather.

• Monitor Electricity Consumption: Sudden increases during cold spells may indicate excessive backup heat use or a failing component.
• Watch Run Times And Cycling: Longer continuous run times at lower output suggest reduced COP.
• Consult Technicians For Diagnostics: A professional can check refrigerant levels, defrost cycles, and compressor performance to recommend repair or replacement.

Summary: Practical Takeaways

Heat pumps begin to lose noticeable efficiency below about 40°F, with significant drops below 20°F and major reliance on backup heat in subzero conditions for standard air-source models. Cold-climate air-source units and ground-source systems mitigate this loss. Proper model selection, correct sizing, regular maintenance, and improving the building envelope are the most effective ways to maintain efficiency and control operating costs during cold weather.