Heat Pumps: The Complete Guide
Heat pumps have become the dominant heating technology. According to the Federal Statistical Office, over 70% of all new builds in Germany in 2023 specified heat pumps as the primary heat source. The global market reached approximately 70 billion US dollars in 2024.
Several factors are driving this trend: rising fossil fuel prices, growing environmental awareness and attractive grant schemes. Technological advances have also made heat pumps more efficient and quieter than ever before.
This guide explains how heat pumps work, compares the various types, examines costs and available grants, and provides guidance on proper sizing. You'll also find links to our in-depth articles on specific topics.
What Is a Heat Pump?
A heat pump is a device that transfers heat from a lower temperature level to a higher one. The principle is identical to that of a refrigerator – only the objective is reversed: whilst a fridge extracts heat from its interior and releases it into the room, a heat pump extracts heat from the environment and delivers it to your heating system.
The Four-Phase Cycle
The heat pump cycle comprises four sequential phases:
| Phase | Component | Process | State of Refrigerant |
|---|---|---|---|
| 1 | Evaporator | Heat absorption from environment | Liquid → Gas |
| 2 | Compressor | Pressure and temperature increase | Gas (hot) |
| 3 | Condenser | Heat release to heating system | Gas → Liquid |
| 4 | Expansion valve | Pressure and temperature reduction | Liquid (cold) |
The refrigerant circulates continuously through this cycle. It absorbs heat at low temperature and releases it at higher temperature. The compressor is the only component requiring electrical energy.
The physics: Heat pumps don't contradict thermodynamics. The electrical energy powering the compressor enables the transfer of heat against the natural temperature gradient.
For a detailed explanation of the underlying physics, see our article The Reverse Fridge: How Does a Heat Pump Work?.
Components at a Glance
Every heat pump consists of the same fundamental components working together in a closed loop:
| Component | Function | Characteristics |
|---|---|---|
| Evaporator | Absorbs heat from environment | Heat exchanger, large surface area |
| Compressor | Compresses the refrigerant | Electrically driven, main energy consumer |
| Condenser | Releases heat to heating system | Heat exchanger, compact |
| Expansion valve | Reduces pressure and temperature | Throttling device, maintenance-free |
| Refrigerant | Transports the heat | Evaporates at low temperature |
The Shift in Refrigerants
Traditional refrigerants such as R410A have a high global warming potential (GWP). Modern heat pumps increasingly use R290 (propane) with a GWP of just 3 (compared to 2088 for R410A). R290 is flammable, which is why charge quantities are limited and safety distances must be observed.
For details on individual components, see our article The Components: Heat Exchangers, Compressor and Expansion Valve.
Comparing Heat Pump Types
Heat pumps are classified according to their heat source and the medium used to distribute heat. The three most common types for residential properties are:
Air Source Heat Pump (Air-to-Water)
The air source heat pump extracts heat from outdoor air and transfers it to the heating water. It is by far the most common type in Germany and increasingly popular across Europe.
Advantages:
- Lower installation costs
- No planning permission required
- Flexible positioning (indoor or outdoor unit)
- Quick installation
Disadvantages:
- Efficiency drops in very cold weather
- Outdoor unit produces some noise
- Higher running costs than ground source
Ground Source Heat Pump (Brine-to-Water)
The ground source heat pump harnesses the constant temperature of the earth. Heat is collected via horizontal ground loops or vertical boreholes.
Advantages:
- Highest efficiency (consistent source temperature)
- Silent operation (no outdoor unit)
- Can provide passive cooling in summer
- Lowest running costs
Disadvantages:
- Higher capital costs (drilling)
- Boreholes require planning permission
- Large garden area needed for horizontal loops
- Longer planning and installation time
Air-to-Air Heat Pump
The air-to-air heat pump heats room air directly without a water circuit. It is less common for whole-house heating in central Europe.
Advantages:
- Can heat and cool
- Lower capital costs
- Rapid response to temperature changes
Disadvantages:
- Cannot heat domestic hot water
- Requires ductwork or indoor units
- Less efficient than water-based systems
Heat Pump Type Comparison Table
| Criterion | Air Source | Ground Source | Air-to-Air |
|---|---|---|---|
| Capital cost | £8,000–£18,000 | £15,000–£30,000 | £6,000–£12,000 |
| SPF | 3.0–4.0 | 4.0–5.0 | 2.5–3.5 |
| Space requirement | Small | Large (ground works) | Small |
| Planning permission | No | Yes (boreholes) | No |
| Hot water | Yes | Yes | No |
| Cooling | Optional | Passive possible | Yes |
| Noise | Outdoor unit audible | Quiet | Indoor units audible |
| Ideal for | New build, retrofit | New build with garden | Supplementary heating |
For more on the different types and combining with solar, see our article Heat Pump Types and the Dream Team with Solar Panels.
Understanding the Key Figures: COP, SPF, SCOP
Heat pump efficiency is expressed through various metrics. Understanding these values is essential for evaluating and comparing equipment.
COP – Coefficient of Performance
The COP is an instantaneous value measured under standardised laboratory conditions (e.g. A2/W35 = 2°C outdoor air, 35°C flow temperature).
Calculation:
COP = Heat output (kW) ÷ Electrical input (kW)
A COP of 4 means: 1 kW of electricity produces 4 kW of heat.
SCOP – Seasonal Coefficient of Performance
The SCOP accounts for various operating points across a heating season and is more meaningful than the COP. It is determined according to EN 14825 and shown on the EU energy label.
SPF – Seasonal Performance Factor
The SPF (in German: JAZ) is the real-world efficiency of an installed heat pump over an entire year. It accounts for all operating conditions, part-load operation and auxiliary energy.
Calculation (per VDI 4650):
SPF = Heat delivered (kWh/year) ÷ Electricity consumed (kWh/year)
Evaluating the SPF
| SPF | Rating | Typical Application |
|---|---|---|
| < 3.0 | Poor | Old systems, unfavourable conditions |
| 3.0–3.5 | Acceptable | Older property with high flow temperature |
| 3.5–4.0 | Good | Standard new build |
| > 4.0 | Excellent | New build with underfloor heating, ground source |
Grant requirement: In Germany, BEG grants require a minimum SPF of 3.0. Air source heat pumps must also achieve a sound power level of no more than 50 dB(A). UK schemes have similar requirements.
For detailed explanations of these metrics and their calculation, see our article Heat Pump Metrics and Sizing.
Choosing the Right Size
Correct sizing of a heat pump is crucial for efficiency and comfort. An oversized heat pump will cycle frequently (switch on and off), increasing wear and reducing efficiency.
Heat Load as the Basis
The heat load indicates how much heating capacity is required at the coldest expected outdoor temperature. It is calculated according to EN 12831.
Rule-of-thumb values for specific heat load:
| Building Type | Specific Heat Load |
|---|---|
| Passivhaus | 10–20 W/m² |
| New build to current regs | 25–35 W/m² |
| Post-2000 build | 40–50 W/m² |
| 1980s–1990s property | 60–80 W/m² |
| Pre-1980 uninsulated | 100–150 W/m² |
| Pre-1960 solid wall | 120–180 W/m² |
Rule of Thumb for Heat Load
Heat load (kW) = Floor area (m²) × Specific value (W/m²) ÷ 1000
Example: A modern new build of 150 m² at 45 W/m² requires: 150 × 45 ÷ 1000 = 6.75 kW heat load
Allowance for Hot Water
An additional allowance is added for domestic hot water:
- Average household: +0.25 kW per person
- If using a separate hot water heat pump: Not required
Complete example:
- 150 m² new build: 6.75 kW
- 4-person household: +1.0 kW
- Total: 7.75 kW → Select an 8 kW heat pump
Avoid oversizing: A heat pump 20% too large can reduce efficiency by 10–15%. It's better to size slightly conservatively and use a backup immersion heater during extreme cold.
For an accurate calculation, use our Heat Load Calculator.
Operating Modes
Different operating modes suit different buildings and requirements.
Monovalent Operation
The heat pump covers the entire heat demand alone. This is the most efficient operating mode.
Requirements:
- Well-insulated property (new build or retrofitted)
- Low-temperature heating system (max. 55°C flow)
- Heat pump sized to match heat load
Bivalent Operation
The heat pump works alongside a second heat source. Below a certain outdoor temperature (the bivalent point), the backup heating switches on.
Variants:
| Variant | Description |
|---|---|
| Bivalent parallel | Heat pump and backup operate simultaneously |
| Bivalent alternative | Below bivalent point, backup only |
| Bivalent part-parallel | Combines both strategies |
Hybrid Operation
A hybrid system combines a heat pump with a gas or oil condensing boiler in a single unit. The controls automatically select the most economical mode.
Decision guide:
| Situation | Recommended Mode |
|---|---|
| New build, underfloor heating | Monovalent |
| Retrofitted older property, low flow temp | Monovalent |
| Older property with radiators at 60°C | Bivalent or Hybrid |
| Uninsulated older property | Hybrid |
For details on operating modes, see our article Operating Modes: Monovalent, Bivalent and Hybrid.
Costs and Economics
Heat pump costs comprise purchase, installation and ongoing running costs.
Capital Costs (Including Installation)
| Heat Pump Type | Cost | Notes |
|---|---|---|
| Air source | £8,000–£18,000 | Varies by output and manufacturer |
| Ground source (horizontal) | £12,000–£22,000 | Including ground loops |
| Ground source (borehole) | £15,000–£30,000 | Including drilling (£60–100/m) |
| Water source | £12,000–£25,000 | Including well system |
Calculating Running Costs
Annual electricity costs can be estimated using this formula:
Electricity cost = Heat demand (kWh/year) ÷ SPF × Electricity price (£/kWh)
Example:
- Heat demand: 15,000 kWh/year
- SPF: 4.0
- Electricity price: £0.28/kWh
Electricity cost = 15,000 ÷ 4.0 × 0.28 = £1,050/year
Comparison of Heating Systems
| Metric | Heat Pump | Gas Boiler | Oil Boiler |
|---|---|---|---|
| Energy price | £0.28/kWh | £0.08/kWh | £0.07/kWh |
| Efficiency/SPF | 4.0 | 0.92 | 0.88 |
| Effective cost | £0.07/kWh | £0.087/kWh | £0.08/kWh |
| At 15,000 kWh/year | £1,050/year | £1,305/year | £1,200/year |
With an SPF of 4.0, the heat pump has the lowest running costs, even though electricity is more expensive than gas or oil.
Grants and Incentives
In Germany, the Federal Office of Economics and Export Control (BAFA) provides grants under the Federal Funding for Efficient Buildings (BEG):
| Component | Grant Rate |
|---|---|
| Base rate | 30% |
| Income bonus (household income < €40,000) | +30% |
| Climate speed bonus (replacing fossil heating) | +20% |
| Maximum grant | 70% |
In the UK, the Boiler Upgrade Scheme (BUS) offers grants of up to £7,500 for air source and £7,500 for ground source heat pumps (as of 2024).
Tip: Grants must typically be applied for before the work is commissioned. Check current terms on the relevant government website.
Heat Pumps in Older Properties
Installing a heat pump in an older property is entirely feasible but requires careful planning.
Challenges
- High flow temperatures: Older radiators often require 60–70°C
- Poor insulation: High heat load demands a larger heat pump
- Space constraints: Positioning the outdoor unit can be tricky
Solutions
| Measure | Effect |
|---|---|
| Insulation (walls, roof) | Reduces heat load by 30–50% |
| Window replacement | Cuts heat losses |
| Low-temperature radiators | Enable 45–50°C flow |
| Underfloor heating (partial) | Lowers flow temperature |
| Hybrid system | Supplements heat pump in extreme cold |
Realistic SPF Expectations in Older Properties
| Building Condition | Flow Temperature | Expected SPF |
|---|---|---|
| Uninsulated, old radiators | 60–70°C | 2.5–3.0 |
| Partially improved | 50–55°C | 3.0–3.5 |
| Insulated, new radiators | 45–50°C | 3.5–4.0 |
| Insulated, underfloor heating | 35–40°C | 4.0–4.5 |
Rule of thumb: Every 5°C reduction in flow temperature improves the SPF by approximately 0.3–0.5 points.
The Dream Team: Heat Pump + Solar PV
Combining a heat pump with a solar PV system offers particular advantages: self-generated solar electricity powers the heat pump, reducing running costs and improving the carbon footprint.
Synergies of the Combination
- Increased self-consumption: Surplus solar electricity drives the heat pump
- Lower electricity bills: Free electricity instead of £0.28/kWh
- Carbon-neutral heating: Renewable energy for warmth
- Greater independence: Less reliance on the grid
Sizing Recommendations
| Component | Sizing | Example (150 m²) |
|---|---|---|
| Heat pump | Matched to heat load | 8 kW |
| PV system | Standard size + 2–3 kWp | 10 kWp |
| Battery storage | Optional, 8–12 kWh | 10 kWh |
Worked Example
Starting point:
- 150 m² new build, 4 persons
- 8 kW heat pump, SPF 4.0
- Heat demand: 15,000 kWh/year → Heat pump electricity: 3,750 kWh/year
- Household electricity: 4,000 kWh/year
- Total: 7,750 kWh/year electricity demand
With 10 kWp PV and 10 kWh battery:
- PV yield: approx. 10,000 kWh/year
- Self-consumption: approx. 5,000 kWh/year (50%)
- Self-sufficiency: approx. 65%
- Grid import: only 2,750 kWh/year
- Savings: approx. £1,400/year
For more on this combination, see our article Heat Pump Types and the Dream Team with Solar Panels.
Advantages and Disadvantages at a Glance
Advantages
| Advantage | Explanation |
|---|---|
| High efficiency | SPF 3–5: 1 kWh of electricity yields 3–5 kWh of heat |
| Environmentally friendly | No direct CO₂ emissions; carbon-free with green electricity |
| Low running costs | Cheaper than gas/oil with a good SPF |
| Long lifespan | 15–25 years, low maintenance |
| No fuel storage | No oil tank or gas supply required |
| Cooling capability | Many models can cool in summer |
| Grants available | Substantial government support |
Disadvantages
| Disadvantage | Explanation |
|---|---|
| High capital cost | £8,000–£30,000 depending on type |
| Electricity dependent | No heating during power cuts |
| Efficiency in cold | Air source loses output at –15°C |
| Noise | Outdoor unit is audible (35–50 dB) |
| Low flow temperature | Not suitable for all heating systems |
| Planning required | Careful sizing is essential |
Frequently Asked Questions (FAQ)
Is a heat pump worthwhile in an older property?
Yes, under certain conditions. The key factors are achievable flow temperature and heat load. Where flow temperatures can be kept below 55°C and an SPF of at least 3.0 is achievable, a heat pump makes economic sense. A hybrid system may be the better choice if high flow temperatures cannot be avoided.
How noisy is a heat pump?
Modern air source heat pumps achieve sound power levels of 35–55 dB(A). For comparison: a refrigerator produces around 40 dB(A), and normal conversation is about 60 dB(A). Installation should observe minimum distances from neighbours and bedrooms.
How long does a heat pump last?
With regular servicing, a heat pump typically lasts 15–25 years. The compressor is the component most susceptible to wear. Frequent cycling (short on/off periods) shortens lifespan, which is why correct sizing matters.
What is the optimum flow temperature?
The lower the better for efficiency. Typical values:
- Underfloor heating: 30–35°C
- Low-temperature radiators: 45–50°C
- Conventional radiators: 55–60°C
Every 5°C reduction in flow temperature improves the SPF by approximately 0.3–0.5 points.
Can a heat pump also provide cooling?
Many heat pumps can operate in reverse to provide cooling in summer. Air source units offer active cooling; ground source systems can provide passive cooling via the ground. Cooling capacity is limited and won't match a dedicated air conditioning system.
Conclusion
Core Message: Heat pumps harness ambient heat and, with SPF values of 3 to 5, operate far more efficiently than fossil heating systems. The technology is ideal for new builds and can also work well in older properties – provided flow temperatures can be kept below 55°C. Combined with a solar PV system, a heat pump enables virtually carbon-neutral heating.
The right heat pump type depends on your property, plot and budget. Air source heat pumps offer the best balance of cost and efficiency, whilst ground source systems deliver the highest efficiency where space permits.
The Complete Heat Pump Article Series
- Heat Pumps: The Complete Guide – You are here
- The Reverse Fridge: How Does a Heat Pump Work? – The physics explained
- The Components: Heat Exchangers, Compressor and Expansion Valve – Components in detail
- Heat Pump Metrics and Sizing – COP, SPF, SCOP
- Operating Modes: Monovalent, Bivalent and Hybrid – Operating modes explained
- Heat Pump Types and the Dream Team with Solar Panels – Types & solar combination
Sources
- DESTATIS: Heat Pumps in New Builds 2023
- BAFA: Federal Funding for Efficient Buildings (BEG)
- VDI 4650: Calculation of the Seasonal Performance Factor for Heat Pump Systems
- VDI 4645: Planning and Sizing of Heat Pump Systems
- EN 14511: Testing and Rating of Heat Pumps
- German Heat Pump Association (BWP)
- Mordor Intelligence: Heat Pumps Market Report
Calculate Your SPF Now
With our free Heat Pump Calculator you can determine the seasonal performance factor of your heat pump according to VDI 4650 – including running costs and CO₂ balance.