Heat Pump Key Figures and Sizing
Introduction: Finding the Right Heat Pump
The time has come – a heat pump is to be installed. The only question is: which one?
To compare heat pumps and choose the right size, there are important key figures. In this article, we explain all the relevant values and provide practical rules of thumb for sizing.
The Most Important Key Figures
COP – Coefficient of Performance
Definition: The ratio of generated heat output to electrical power input – measured under standard conditions (laboratory conditions).
Formula: COP = Heat Output (kW) / Electrical Power (kW)
The meaning of different COP values can be illustrated clearly:
| COP | Meaning |
|---|---|
| 3.0 | For 1 kW electricity you get 3 kW heat |
| 4.0 | For 1 kW electricity you get 4 kW heat |
| 5.0 | For 1 kW electricity you get 5 kW heat |
Guideline: A COP of above 4 is considered good, above 5 very good.
Important: The COP is measured under laboratory conditions and is only a momentary value. The actual efficiency in everyday use may differ!
SCOP – Seasonal Coefficient of Performance
Definition: The seasonal COP – a weighted average value over a typical heating season.
The SCOP takes into account:
- Various outdoor temperatures
- Different operating states
- More realistic usage conditions
Advantage: More realistic than COP as it includes various conditions.
SPF – Seasonal Performance Factor
Definition: The actual efficiency of a heat pump over an entire year – measured in real operation.
Formula: SPF = Heat Generated (kWh/year) / Electricity Consumed (kWh/year)
Depending on the achieved SPF, the efficiency of the system can be assessed:
| SPF | Assessment |
|---|---|
| < 3.0 | Needs improvement |
| 3.0–3.5 | Acceptable |
| 3.5–4.0 | Good |
| > 4.0 | Very good |
The difference from COP/SCOP:
- COP/SCOP are manufacturer specifications under defined conditions
- SPF is the actual value of your specific system
Further Important Key Figures
Besides COP and SPF, there are other important metrics for selecting a heat pump:
| Key Figure | Meaning |
|---|---|
| Heating output (kW) | Maximum heat output |
| Flow temperature | Temperature of heating water (optimal: 35–50°C) |
| Return temperature | Temperature of returning water |
| GWP | Global Warming Potential of refrigerant |
| Sound power level (dB) | Volume of outdoor unit |
Flow and Return Temperature
Flow temperature: The temperature of the heating water flowing to the heating system.
The required flow temperature depends on the heating system:
| Heating System | Typical Flow Temperature |
|---|---|
| Underfloor heating | 30–40°C |
| Low-temperature radiators | 45–55°C |
| Old radiators | 60–70°C |
Important: The lower the flow temperature, the more efficiently the heat pump works!
Why? At high flow temperatures, the compressor must work harder, which reduces efficiency.
GWP – Global Warming Potential
The greenhouse potential of the refrigerant used:
Depending on the refrigerant, the environmental impact varies considerably:
| Refrigerant | GWP | Assessment |
|---|---|---|
| R290 (Propane) | 3 | Very good |
| R32 | 675 | Medium |
| R410A | 2088 | Poor |
The lower the GWP value, the more environmentally friendly!
Sound Power Level
The volume of the outdoor unit – important for neighbours and night-time rest:
To put the decibel values in context, a comparison with everyday sounds helps:
| Sound Level | Comparison |
|---|---|
| 40 dB | Quiet residential areas |
| 50 dB | Normal conversation |
| 60 dB | Office noise level |
| 70 dB | Road traffic |
Recommendation: Choose an outdoor unit with maximum 50 dB.
Rules of Thumb for Sizing
To choose the right heat pump, the heat demand must first be determined.
Step 1: Calculate Total Heat Demand
Formula: Heat demand (kW) = Floor area (m²) × Specific heat demand (W/m²) / 1000
Specific Heat Demand by Building Type
The specific heat demand varies greatly depending on the insulation standard of the building:
| Building Type | Specific Heat Demand |
|---|---|
| Passive house | 10–20 W/m² |
| KfW-40 new build | 25–35 W/m² |
| KfW-55 new build | 35–45 W/m² |
| Well-renovated old building | 50–70 W/m² |
| Partially renovated old building | 70–100 W/m² |
| Unrenovated old building | 100–150 W/m² |
Example: 150 m² well-renovated old building
- 150 m² × 60 W/m² = 9,000 W = 9 kW
Step 2: Consider Block-out Times
Energy suppliers often offer favourable heat pump tariffs – but with block-out times (e.g. 3 × 2 hours per day).
Formula for additional output:
Additional output = Base load × (Block-out time in hours / 24)
Example: 9 kW base load, 6 hours block-out time
- 9 kW × (6/24) = 9 kW × 0.25 = 2.25 kW
- Total output: 9 + 2.25 = 11.25 kW
Step 3: Consider Hot Water
If the heat pump is also to prepare hot water:
Rule of thumb: ~0.25 kW per person
Depending on household size, an appropriate allowance must be planned:
| Household Size | Additional Demand |
|---|---|
| 2 people | 0.5 kW |
| 4 people | 1.0 kW |
| 6 people | 1.5 kW |
Complete Calculation Example
A concrete example shows how all factors interact:
| Item | Calculation | Result |
|---|---|---|
| Base load | 150 m² × 60 W/m² | 9.0 kW |
| Block-out time | 9 kW × 0.25 | 2.25 kW |
| Hot water | 4 people × 0.25 kW | 1.0 kW |
| Total | 12.25 kW |
→ A 12–14 kW heat pump would be suitable.
Avoid Oversizing!
Caution: A heat pump that is too large is not better!
Problems with Oversizing
A heat pump that is too large can cause various problems:
| Problem | Explanation |
|---|---|
| Cycling | Constant on/off switching |
| Wear | Higher stress on compressor |
| Inefficiency | Heat pump rarely operates at optimum |
| Higher costs | Unnecessarily high purchase costs |
The Golden Rule
Better to size slightly smaller and switch on an immersion heater during peak loads than to oversize!
Comparing Key Figures
When selecting a heat pump, you should compare:
When buying a heat pump, some values can be compared while others can only be determined during operation:
| Can be compared beforehand | Determinable after operation |
|---|---|
| COP (manufacturer specification) | SPF (your actual efficiency) |
| SCOP | |
| Sound power level | |
| GWP of refrigerant | |
| Heating output (kW) |
What to Pay Particular Attention To?
- SCOP is more realistic than COP
- Low flow temperature enabled (underfloor heating!)
- Low GWP value for environmental protection
- Appropriate volume for your living environment
- Suitable sizing – not too large!
Economic Calculation
Calculate Annual Electricity Costs
Formula: Electricity costs = Heat demand (kWh) / SPF × Electricity price (€/kWh)
Example:
- Heat demand: 15,000 kWh/year
- SPF: 4.0
- Electricity price: €0.30/kWh
→ 15,000 / 4 × 0.30 = €1,125/year
Comparison with Other Heating Systems
A cost comparison shows the advantages of the heat pump over other heating systems:
| System | Cost for 15,000 kWh heat |
|---|---|
| Heat pump (SPF 4) | ~€1,125/year |
| Gas heating | ~€1,800/year |
| Oil heating | ~€2,100/year |
| Direct electricity | ~€4,500/year |
The heat pump is most economical in the long term!
Conclusion
Core Message: The most important metrics for heat pump selection are SCOP/COP (efficiency - the higher the better), heating output (matching the calculated demand), flow temperature (the lower, the more efficient), sound level (for good neighbourly relations) and GWP (for environmental protection). With the rules of thumb, you can roughly estimate your heat demand. For precise planning, a professional heating load calculation according to DIN EN 12831 is recommended.
Whether your heat pump should heat alone or work together with a second heat generator is clarified in the article Operating Modes: Monovalent, Bivalent and Hybrid.
The Complete Article Series "Heat Pumps"
- The Anti-Refrigerator: How Does a Heat Pump Work? – Fundamentals
- The Components: Heat Exchanger, Compressor and Expansion Valve – Components
- Heat Pump Key Figures and Sizing – You are here
- Operating Modes: Monovalent, Bivalent and Hybrid – Operating modes
- Heat Pump Types and the Dream Team with Solar Systems – Air-water, ground-source & solar