11.2 Heat Pump Systems & Water Heating Heat Pumps
Key Takeaways
- A reversing valve lets an air-source heat pump switch which coil acts as condenser vs. evaporator, providing both heating and cooling from one system.
- Heat pumps run periodic defrost cycles when outdoor coil frost builds up in cold, humid conditions, typically energizing auxiliary heat during the cycle.
- Heat pump water heaters use the refrigeration cycle to move heat into stored water, achieving a COP of roughly 2.0-3.5 versus about 0.9-1.0 for standard electric-resistance water heaters.
- The C-20 classification explicitly lists water-heating heat pumps in its scope of work because they are refrigeration appliances, not just plumbing fixtures.
- Split-system heat pumps carry a national minimum efficiency standard of 14.3 SEER2 / 7.5 HSPF2, unlike cooling-only air conditioners, which have three separate regional minimums.
Heat Pump Systems & Water Heating Heat Pumps
Quick Answer: An air-source heat pump is a standard vapor-compression refrigeration system with one added component -- a reversing valve -- that lets it move heat in either direction, providing both cooling and heating from a single outdoor unit. Because the outdoor coil must extract heat from cold outside air in heating mode, heat pumps periodically run a defrost cycle to clear frost from the outdoor coil. The same refrigeration technology also powers heat pump water heaters (HPWHs), which the C-20 license explicitly covers. Heating performance is rated by HSPF/HSPF2; cooling performance by SEER/SEER2.
How an Air-Source Heat Pump Works
A heat pump uses the exact same four-stage vapor-compression cycle covered in the previous section, but adds a reversing valve (also called a four-way valve) between the compressor and the two coils. The reversing valve is an electrically actuated solenoid valve that redirects the flow of hot, high-pressure refrigerant vapor leaving the compressor, determining which coil functions as the condenser and which functions as the evaporator at any given moment.
- Cooling mode: the outdoor coil acts as the condenser (rejecting heat outside) and the indoor coil acts as the evaporator (absorbing heat from indoor air) -- functionally identical to a standard air conditioner.
- Heating mode: the reversing valve energizes and flips the refrigerant flow, so the outdoor coil becomes the evaporator (absorbing heat from the outside air, even when it is cold) and the indoor coil becomes the condenser (releasing that heat into the home).
This works because even cold outdoor air still contains usable heat energy; the refrigerant inside the outdoor coil is kept colder than the outdoor air (through the pressure drop at the metering device) so it can still absorb heat from it. As outdoor temperatures fall, a heat pump's heating capacity and efficiency decline, which is why many heat pump systems include supplemental electric-resistance heat strips or a dual-fuel furnace backup for the coldest days.
The Defrost Cycle
When a heat pump operates in heating mode with outdoor temperatures at or below roughly 40 degrees Fahrenheit and high humidity, moisture in the outside air can freeze onto the cold outdoor coil, just as frost forms on a freezer's cooling fins. A layer of frost restricts airflow across the coil and steadily degrades heating capacity, so the system must periodically reverse into a brief defrost cycle:
- The reversing valve temporarily switches the system back into cooling-mode refrigerant flow, sending hot compressed refrigerant to the outdoor coil to melt the frost.
- The outdoor fan shuts off during defrost so heat is not blown away before it melts the ice.
- Because the indoor coil is now acting as an evaporator, it would blow cold air into the house -- so the system's auxiliary or electric-resistance heat typically energizes during defrost to temper the supply air.
- Once frost is cleared (sensed by a defrost thermostat/sensor or a timed cycle), the system reverts to normal heating mode.
Two defrost control strategies are common in the field: time-temperature defrost (initiates defrost on a fixed timer, such as every 30, 60, or 90 minutes, if a coil sensor confirms it is cold enough) and demand defrost (uses coil temperature and airflow or pressure sensors to initiate defrost only when frost buildup is actually detected), which is more energy-efficient and standard on most modern control boards.
Heat Pump Water Heaters (HPWHs)
A heat pump water heater applies the same refrigeration technology to domestic hot water: instead of heating water directly with a gas burner or electric-resistance element, a small heat pump unit (integrated into or mounted above the tank) extracts heat from the surrounding air and transfers it into the stored water. Because HPWHs move existing heat rather than generating it directly from electricity, they are dramatically more efficient than resistance-element electric water heaters:
| Water Heater Type | Typical COP | Relative Efficiency |
|---|---|---|
| Standard electric-resistance | Approximately 0.9-1.0 | Baseline (about 1 unit of heat per unit of electricity) |
| Heat pump water heater | Approximately 2.0-3.5 | Roughly 2-3.5x more efficient than resistance electric |
Coefficient of performance (COP) is the ratio of heat delivered to electrical energy consumed; a COP of 3.0 means the unit delivers three units of heat energy for every one unit of electrical energy it consumes. Consumer water heater efficiency labels use the Uniform Energy Factor (UEF) rather than COP for comparison shopping, but the underlying physics is the same. Because HPWHs pull heat from the surrounding air, manufacturers require adequate air volume around the unit -- commonly cited around 450 to 700 cubic feet per ENERGY STAR program guidance, though the exact figure varies by model and must always be confirmed against the specific installation manual -- and the units produce cooler, drier exhaust air as a byproduct, a useful side effect in a hot garage but a liability in an already-cold utility closet. Most HPWHs include backup electric-resistance elements that engage automatically during high-demand periods (a hybrid operating mode) when the heat pump alone cannot keep up.
Why C-20's Scope Explicitly Includes Water-Heating Heat Pumps
California's C-20 classification, codified in the California Code of Regulations (CCR), Title 16, Division 8, Article 3, defines the license to cover a contractor who fabricates, installs, maintains, services, and repairs warm-air heating systems and water heating heat pumps. That explicit inclusion matters because it resolves a potential trade-boundary question: conventional gas and electric-resistance water heaters are core C-36 (Plumbing) work, but a heat pump water heater is fundamentally a refrigeration appliance -- it has a compressor, refrigerant charge, metering device, and coils, the same components a C-20 contractor already services on air conditioners and heat pumps. By writing water heating heat pumps directly into the C-20 scope of work, the California Contractors State License Board (CSLB) confirms that installing, servicing, and repairing HPWHs falls squarely within C-20 licensure, alongside (and often overlapping) C-36 plumbing work for the water piping and code-required connections.
Efficiency Ratings: HSPF/HSPF2 and SEER/SEER2
Since testing procedures changed industry-wide on January 1, 2023, heating performance for air-source heat pumps is rated using HSPF2 (Heating Seasonal Performance Factor 2), and cooling performance using SEER2 (Seasonal Energy Efficiency Ratio 2) -- updated versions of the older HSPF/SEER metrics that use more realistic duct-static-pressure test conditions. Current federal minimum standards for most split-system heat pumps are 14.3 SEER2 and 7.5 HSPF2 (roughly equivalent to the pre-2023 ratings of 15.0 SEER and 8.8 HSPF); single-package heat pumps have a slightly lower federal minimum of 13.4 SEER2 and 6.7 HSPF2. Unlike cooling-only air conditioners -- which have three separate regional minimum-efficiency tiers (North, Southeast, Southwest) -- split-system and packaged heat pump minimums are set at a single national standard regardless of region, since federal heating-performance regulation is not tied to a cooling-climate zone.
Official Resources
- ENERGY STAR: How Heat Pump Water Heaters Work -- official explanation of HPWH operation and COP ratings
- AHRI 2023 Energy Efficiency Standards -- official SEER2/HSPF2/EER2 minimum standards summary
What single component allows an air-source heat pump to provide both heating and cooling from one outdoor unit?
During a heat pump's defrost cycle, why does the system typically energize auxiliary or electric-resistance heat?
A heat pump water heater has a coefficient of performance (COP) of 3.0. What does that COP value indicate?
Why does the C-20 license classification explicitly list 'water heating heat pumps' as part of its scope of work, separate from conventional water heaters?