Heat Pump COP and SCOP: Cold-Climate Selection Explained
A high catalogue COP does not guarantee winter capacity. Read performance at the actual outdoor design temperature and plan supplemental heat deliberately.
COP = Qout ÷ Win
Heat Pump COP and SCOP: Cold-Climate Selection Explained
Original MEPFlow engineering guide
Prepared to help mechanical designers understand the calculation, assumptions and review checks. Examples are original and educational; verify the governing code, project criteria and equipment data before using a result for construction or permit documents.
Coefficient of performance describes useful heat delivered divided by electrical energy consumed at a particular operating condition. A COP of 3 means three units of heat are delivered for each unit of electricity at that test point. Seasonal metrics summarize performance over many conditions, but equipment still has to meet the building load on the selected winter design day.
Cold-climate selection therefore requires two curves: building heat loss rises as outdoor temperature falls, while available heat-pump capacity and COP generally decline. Their interaction matters more than one headline efficiency number.
COP, SCOP and seasonal ratings
COP is an operating-point ratio, not a fixed property. It changes with outdoor temperature, supply temperature, compressor speed and defrost. SCOP or region-specific seasonal metrics combine performance across a defined temperature bin distribution.
Use seasonal metrics to compare annual performance under the relevant rating procedure. Use manufacturer capacity and input-power tables at design conditions for peak sizing.
Engineering visual
What changes as outdoor temperature falls
Building
Heating load rises
Envelope and outdoor-air losses increase with ΔT
Heat pump
Capacity and COP may fall
Check certified low-ambient performance and defrost
COP = useful heating output ÷ electrical input
If a heat pump delivers 12 kW of heat while drawing 4 kW, its operating COP at that condition is 3.0.
Capacity matters as much as efficiency
Two units can have similar seasonal ratings but very different low-temperature capacity retention. Compare the net heating output at the project's winter design temperature with the calculated building load. Do not use only the nominal capacity listed at a mild rating condition.
For air-to-water systems, higher leaving-water temperature usually reduces capacity and COP. Emitters, coils and reset schedules should be designed to use the lowest practical water temperature.
Balance point and supplemental heat
Below the thermal balance point, the building requires more heat than the heat pump can supply. A bivalent design uses auxiliary heat to cover the difference. The auxiliary source may be electric resistance, a boiler or another staged system.
The control sequence should define when supplemental heat is enabled and whether the heat pump continues to operate. Electrical-service capacity, demand charges and resilience can affect the preferred approach.
1. Calculate the design heat loss
Use room and block loads at the selected winter design condition.
2. Plot the load line
Estimate how building load changes with outdoor temperature.
3. Add manufacturer data
Use capacity and power at the relevant outdoor and supply temperatures.
4. Locate the balance point
Identify where available capacity and building demand intersect.
5. Size and control backup heat
Cover the remaining load and document the staging sequence.
Cold-climate review questions
A heat-pump schedule should make the design basis auditable.
- What outdoor design temperature was used for the building load?
- What is the unit's net capacity and input power at that temperature?
- What water or supply-air temperature is required?
- How are defrost and crankcase or pan-heater energy represented?
- What capacity is available after one unit is unavailable, if redundancy is required?
- How and when does supplemental heat stage?
Use a capacity-versus-load balance point, not COP alone
A cold-climate selection can be visualized with two curves. Building heat loss rises as outdoor temperature falls, while available heat-pump capacity usually falls or changes with compressor staging and defrost. Their intersection is the thermal balance point. Above it, the heat pump can meet the load without supplemental heat. Below it, another heat source or deliberate indoor-temperature drift is required unless the unit was selected to cover the full design load.
Suppose a building loses 60,000 Btu/h at -13°F and approximately 30,000 Btu/h at 20°F. A candidate heat pump delivers 42,000 Btu/h at 5°F and 34,000 Btu/h at -13°F. The unit may have an attractive rated COP, but it leaves a 26,000 Btu/h shortfall at the winter design point. That shortfall must be covered by staged electric heat, a boiler, a second compressor or a larger unit. The electrical service and controls must be designed for the actual supplemental strategy.
The economic balance point can differ from the thermal one. In a dual-fuel system, controls may switch sources where operating cost, electrical demand or emissions favour the alternate heat source even though the heat pump still has capacity. A design narrative should distinguish capacity balance, economic balance, lockout temperature and emergency-heat operation.
| Outdoor condition | Building load | Heat-pump capacity | Design implication |
|---|---|---|---|
| 20°F | 30,000 Btu/h | Above 42,000 Btu/h | Heat pump carries load with modulation |
| 5°F | About 45,000 Btu/h | 42,000 Btu/h | Near thermal balance point |
| -13°F | 60,000 Btu/h | 34,000 Btu/h | 26,000 Btu/h supplemental capacity required |
A defensible heat-pump selection workflow
Start with an hourly or temperature-bin building load and the manufacturer's extended performance data. Match indoor and outdoor units exactly and use the design airflow or water temperature. Interpolate cautiously between published points. Nominal catalogue tonnage is not a substitute for low-ambient capacity, and a standard rating point does not describe defrost or auxiliary-heat operation.
Next check modulation. A unit selected for winter capacity may be oversized for shoulder-season and cooling loads. Minimum compressor capacity, zone diversity and the active water volume or air-zone load determine whether it can run steadily. Short cycling reduces comfort and efficiency and can require a buffer tank, additional zoning strategy or different equipment combination.
Finally coordinate electrical demand, condensate and defrost drainage, sound, snow clearance, wind exposure and controls. State when supplemental heat stages on, whether the compressor remains enabled, and how the system recovers from setback. Commissioning should verify sensor placement, staging, airflow or water flow and the actual changeover sequence under representative weather.
- Use certified matched-system data at the project's outdoor and indoor design points.
- Check both maximum low-ambient capacity and minimum mild-weather capacity.
- Include defrost and supplemental heat in electrical demand and operating-sequence review.
- Compare seasonal performance using the regional climate and operating temperatures, not one COP value.
- Document the thermal balance point, lockout logic and source of backup heat.
Frequently asked questions
Is a higher COP always better?
At the same operating condition, yes, but compare capacity and COP across the temperatures and supply conditions the project will actually experience.
Can a heat pump work below -20°C?
Some cold-climate models can operate at very low temperatures, but available capacity and COP vary substantially. Use certified model-specific data and provide supplemental heat when the building load exceeds capacity.
What is the heat-pump balance point?
It is the outdoor condition where the heat pump's available heating output equals the building heating load under the assumed operating conditions.
Primary references
Use the edition and method accepted for your project. These authoritative resources provide further context; this article is educational and is not a code-compliance determination.