Heat Pump Efficiency in Cooling and Heating: What Degrades It and How to Track It

Key takeaways

• Heat pump coils swap roles by season, so both coils are working heat exchangers year round.
• NREL-cited research puts the seasonal COP penalty from frost and defrost at 1.5% to 12.7%.
• Rising auxiliary (strip) heat run time is the most expensive and most visible warning sign.
• ENERGY STAR cold-climate units must reach COP 1.75 at 5 degrees F and keep 70% of their 47 degree F capacity.
• Fix thermostat settings, defrost, charge and airflow before any refrigerant-side treatment.

A heat pump is an air conditioner that can run in reverse. In summer it moves heat out of the building; in winter it pulls heat from outdoor air (or the ground, or water) and delivers it indoors. Because it moves heat rather than making it, a heat pump can deliver much more heat than the electricity it uses. That efficiency advantage is why heat pumps are central to building electrification, and also why their maintenance matters in both seasons. This page covers air-source heat pumps in homes and commercial buildings, including packaged heat pump rooftop units, and explains how they lose efficiency, how to measure them, and where an internal oil-film treatment fits.

How a heat pump works

A heat pump has the same main parts as an air conditioner (compressor, two coils, expansion device) plus a reversing valve that swaps the roles of the coils. In cooling, the indoor coil is the evaporator and the outdoor coil is the condenser. In heating, the indoor coil becomes the condenser, releasing heat into the supply air, and the outdoor coil becomes the evaporator, absorbing heat from outdoor air even when it is well below freezing.

When outdoor air is cold and humid, frost forms on the outdoor coil. The heat pump then runs a defrost cycle, typically by reversing to cooling mode briefly so hot refrigerant melts the frost. Most systems also have auxiliary or backup heat, often electric resistance strips, for very cold weather and for tempering air during defrost. FEMP advises that because resistance heating elements are inefficient, "their use should be kept to a minimum" (FEMP).

Ratings: SEER2, EER2, HSPF2 and COP

• SEER2 and EER2 describe cooling efficiency, as for air conditioners.
• HSPF2 (heating seasonal performance factor) describes seasonal heating efficiency for residential-size units. DOE's nationwide minimum from January 1, 2023 for split-system heat pumps is 14.3 SEER2 and 7.5 HSPF2 (DOE).
• COP (coefficient of performance) is heat delivered divided by electricity used at a specific condition. A COP of 3 means three units of heat per unit of electricity. Commercial heat pumps are rated at 47°F and 17°F; ENERGY STAR's criteria for a small commercial unitary heat pump call for 3.5 COP at 47°F and 2.4 COP at 17°F (ENERGY STAR).

For cold climates, ENERGY STAR's cold-climate designation for residential heat pumps requires a COP of at least 1.75 at 5°F and heating capacity at 5°F of at least 70% of capacity at 47°F, along with a controls verification procedure (ENERGY STAR). As of October 2026, ENERGY STAR is being transitioned from EPA to DOE, with full transition planned by July 2027 (EPA); check current criteria and any incentive program's own requirements before buying.

How heat pump efficiency degrades

Installation and service faults, in both modes

NIST Technical Note 1848 is specifically about heat pumps. It found that duct leakage, refrigerant undercharge, an oversized heat pump with normal-size ductwork, low indoor airflow and refrigerant overcharge have the most potential to degrade performance and increase annual energy use, and that some faults combine to more than the sum of their parts, for example duct leakage together with undercharge (NIST). NIST considered energy use increases of 30% from poor installation practices plausible. It also noted that an undersized cooling-mode expansion valve had little effect in heating-dominated climates but could significantly increase energy use in cooling-dominated climates, a reminder that the same fault behaves differently by region.

Frost and defrost

Frost on the outdoor coil blocks airflow and heat transfer, and each defrost cycle uses energy and briefly takes heat from the building. An NREL project presented at DOE's 2024 peer review cites published work showing a seasonal COP reduction of 1.5% to 12.7% from frost growth and its removal, and notes the added electric power and grid impact (NREL / DOE BTO). Faulty defrost controls, failed outdoor temperature or coil sensors, and outdoor coils sitting in snow or under a dripping roof edge all make this worse.

Auxiliary heat creep

The single most expensive heat pump problem is often resistance heat running when it does not need to: a thermostat configured for the wrong equipment type, a lockout temperature set too high, an aggressive recovery setting, or a heat pump with low capacity because of charge or airflow faults that forces strip heat to cover the gap.

Coils, filters and refrigerant-side oil

As with any DX system, dirty indoor coils and filters reduce airflow, and dirty outdoor coils raise condensing temperature in cooling and reduce heat absorption in heating. On the refrigerant side, oil that circulates with the refrigerant can accumulate on internal coil surfaces in either coil; because a heat pump's coils swap roles, both are working heat exchangers year round. See oil fouling explained.

Reversing valve and component wear

A reversing valve that leaks internally lets hot discharge gas bypass to the suction side, reducing capacity in both modes. Compressors in heat pumps also run more hours per year than in cooling-only equipment.

How to measure a heat pump's performance

Smart thermostats and most commercial controllers can log compressor, auxiliary heat and defrost run time. For small buildings, those three numbers alone catch most of the expensive problems.

Maintenance that matters

1. Verify thermostat configuration, auxiliary heat lockout temperature and staging.
2. Test defrost operation and the sensors that control it before winter.
3. Clean indoor and outdoor coils; keep the outdoor unit raised above snow line and away from roof drip.
4. Verify charge and airflow; FEMP notes that regular maintenance, including "charging refrigerant and replacing filters," is necessary to maintain peak performance, and recommends installation to the ANSI/ACCA quality installation specification (FEMP).
5. Check the reversing valve for internal leakage by comparing temperatures across it.
6. Follow the ENERGY STAR checklist items for electrical connections, motor current, condensate drains and controls (ENERGY STAR).

Where an internal oil-film treatment fits

CryogenX4 has a dedicated heat pump solution page. According to the company, its treatment lifts insulating oil film from internal coil surfaces and returns it to the compressor sump, conditions the metal for better heat transfer, and improves the lubricity of the existing oil, reducing friction and compressor heat. Because a heat pump's coils work in both directions and the compressor runs in both seasons, any improvement in internal heat transfer would apply across more operating hours than in cooling-only equipment. The company reports a one-time application, installed while the system runs, usually within one day and with no modifications; it states energy savings of up to 30% and a typical treatment payback of 12 to 36 months. Results vary by equipment condition.

A heat pump is a reasonable candidate when thermostat and auxiliary heat settings are correct, defrost works, charge and airflow are verified, coils are clean, and the unit still runs longer or uses more strip heat than it should for the weather.

Questions to ask before treating a heat pump

1. Is the thermostat configured for a heat pump, and what is the auxiliary heat lockout setting?
2. How many hours of auxiliary heat ran last winter, and at what outdoor temperatures?
3. Has defrost been tested, and are the sensors reading correctly?
4. Has charge been verified in cooling mode against manufacturer targets?
5. Is there a leak history, and what refrigerant and oil does the system use?
6. How will results be measured across both heating and cooling seasons? See how HVAC energy savings are measured.

Next step

Pull last winter's auxiliary heat and defrost run times from your thermostats or controls. If strip heat is running more than expected after controls and service issues are fixed, contact us about measuring a treated heat pump through a full heating and cooling season.

Frequently asked questions

Why does my heat pump blow cool air for a few minutes in winter?

It is probably in defrost. During defrost the system briefly reverses to melt frost on the outdoor coil. Many systems turn on auxiliary heat to temper the air. Frequent or very long defrost cycles are worth a service call.

What is a good COP for a heat pump?

It depends on outdoor temperature. ENERGY STAR criteria for small commercial heat pumps call for 3.5 COP at 47 degrees F and 2.4 at 17 degrees F; cold-climate residential units must reach at least 1.75 at 5 degrees F. Anything above 1.0 delivers more heat than electric resistance for the same electricity.

How do I know if auxiliary heat is running too much?

Check thermostat or controller logs for auxiliary heat hours and compare them with outdoor temperatures. Strip heat running in mild weather usually points to a configuration problem, a low lockout setting being ignored, or a heat pump that has lost capacity.

Can a heat pump be charged in winter?

Manufacturers usually specify charge verification in cooling mode at suitable outdoor temperatures. Winter checks rely on manufacturer heating-mode charts or weighing in the charge. Ask your technician which method the manufacturer requires.

Sources

1. 2023 Central Air Conditioner and Heat Pump Standards FAQ — U.S. Department of Energy
2. Heat Pump Equipment Key Product Criteria — ENERGY STAR
3. Light Commercial HVAC Key Product Criteria — ENERGY STAR
4. ENERGY STAR (program transition to DOE) — U.S. Environmental Protection Agency
5. Purchasing Energy-Efficient Residential Air-Source Heat Pumps — U.S. DOE Federal Energy Management Program
6. Sensitivity Analysis of Installation Faults on Heat Pump Performance (NIST Technical Note 1848) — National Institute of Standards and Technology
7. BTO Peer Review: Quantifying and Reducing Defrost Energy Use (NREL, 2024) — U.S. DOE Building Technologies Office / NREL
8. Maintenance Checklist (heating and cooling) — ENERGY STAR