Ground Source Heat Pump Efficiency vs Air Source
Both ground source and air source heat pumps are efficient renewable heating systems. But they are not equally efficient — and the gap between them widens at exactly the times you need heating most. Understanding where, when, and why ground source outperforms air source helps you decide which technology is right for your home.
This guide compares real-world efficiency figures, explains what drives the difference, and helps you work out whether the extra cost of ground source installation is justified by the efficiency gains.
Understanding Heat Pump Efficiency: COP and SCOP
Before comparing the two technologies, you need to understand how heat pump efficiency is measured.
COP (Coefficient of Performance)
COP measures how much heat a heat pump produces for each unit of electricity it consumes, at a specific moment in time. A COP of 3.5 means the heat pump produces 3.5 kW of heat for every 1 kW of electricity used. The remaining 2.5 kW comes from the environment — the ground or the air.
SCOP (Seasonal Coefficient of Performance)
SCOP is the average COP over an entire heating season. This is the more useful figure because it accounts for varying outdoor temperatures, defrost cycles, standby losses, and hot water production. SCOP gives you a realistic picture of what the system will deliver over a full year.
Use our heat pump calculator to estimate running costs based on realistic SCOP figures for your property.
The Numbers: Ground Source vs Air Source Efficiency
Ground Source Heat Pump Efficiency
Typical COP: 3.8 to 4.5 (at design conditions)
Real-world SCOP: 3.5 to 4.0
Winter COP (when it matters most): 3.5 to 4.2
Ground source heat pumps extract heat from the ground, where the temperature remains remarkably stable at 8-12°C throughout the year in the UK. Whether it is a scorching August day or a freezing January night, the ground loop is drawing heat from earth that sits at roughly the same temperature. This stability is the key to ground source efficiency.
Air Source Heat Pump Efficiency
Typical COP: 3.0 to 4.0 (at design conditions, typically 7°C air temp)
Real-world SCOP: 2.8 to 3.5
Winter COP (when it matters most): 2.0 to 3.0
Air source heat pumps extract heat from the outside air. When the air temperature is mild (10-15°C), they perform well — sometimes matching ground source. But as the temperature drops towards and below freezing, efficiency falls. The colder the air, the harder the heat pump has to work, and the less efficient it becomes.
Why Ground Source Is More Efficient
The efficiency advantage of ground source comes down to three factors:
1. Stable Source Temperature
The ground temperature at loop depth (1-2 metres for horizontal, deeper for boreholes) stays between 8°C and 12°C all year. Air temperature in the UK ranges from -5°C to 30°C. A heat pump extracting heat from a 10°C source will always be more efficient than one extracting heat from -2°C air.
This is basic thermodynamics. The smaller the temperature difference between the heat source and the heat output, the less work the compressor has to do, and the higher the COP.
2. No Defrost Cycles
Air source heat pumps must periodically reverse their refrigerant cycle to defrost the outdoor evaporator coil. This happens when air temperature is between roughly -3°C and 5°C and humidity is high — common UK winter conditions. During a defrost cycle, the heat pump is effectively running in reverse, consuming electricity without producing useful heat. This typically reduces seasonal efficiency by 5-10%.
Ground source heat pumps never need to defrost. The ground loop fluid never drops low enough to cause icing.
3. Less Extreme Operating Conditions
When an air source heat pump operates at -5°C ambient, the compressor is under maximum stress, working at the extremes of its operating envelope. When a ground source compressor operates on the coldest winter night, it is drawing from 10°C ground — comfortable operating conditions. Less stress means higher efficiency and longer component life.
When Does the Efficiency Difference Matter Most?
The efficiency gap between ground source and air source is not constant — it varies through the year:
Mid-Season (Autumn/Spring): Small Gap
When outdoor temperatures are 8-15°C, air source heat pumps perform well. The efficiency gap narrows to perhaps 10-15%. During these milder months, both technologies deliver excellent performance.
Deep Winter (December-February): Large Gap
This is where ground source pulls ahead significantly. On a -3°C January morning, a ground source heat pump is still drawing from 10°C ground, maintaining a COP of 3.5-4.0. An air source unit extracting heat from -3°C air while running defrost cycles might achieve a COP of only 2.0-2.5.
Since deep winter is when you use the most heating energy, this is when the efficiency difference has the biggest impact on your electricity bill.
Summer Hot Water: Small Gap
For hot water production in summer, both systems perform reasonably similarly. Air temperature is warm, so the air source unit is efficient. Ground temperature is stable. The difference is marginal.
Running Cost Comparison
How does this efficiency difference translate into actual running costs?
For a typical 3-bedroom semi-detached house with an annual heating demand of 12,000 kWh:
Ground source (SCOP 3.8): 12,000 ÷ 3.8 = 3,158 kWh electricity = approximately £790 per year (at 25p/kWh)
Air source (SCOP 3.2): 12,000 ÷ 3.2 = 3,750 kWh electricity = approximately £938 per year
Annual saving with ground source: approximately £148
For a larger, less well-insulated property with 20,000 kWh annual demand, the saving rises to roughly £245 per year. Over the 25-year life of the system, that compounds to £3,700-£6,100 in running cost savings.
However, a ground source installation typically costs £8,000-£15,000 more than an air source system. So the running cost savings alone do not always recoup the extra capital outlay — though the longer lifespan of GSHP systems shifts the equation further in ground source's favour.
When Ground Source Efficiency Justifies the Extra Cost
Ground source is most likely to be worthwhile when:
You have high heating demand. Larger, older, or poorly insulated properties use more energy, so the percentage efficiency gain translates to bigger absolute savings. A property using 25,000+ kWh per year will see the efficiency payback sooner.
You are replacing oil or LPG. The running cost savings compared to oil or LPG are substantial regardless of whether you choose ground or air source, but the comparison against gas is tighter. When displacing expensive oil heating, ground source's higher efficiency adds meaningful extra savings.
You plan to stay in the property long-term. The efficiency advantage compounds over time. If you expect to live in the house for 20+ years, ground source's higher initial cost is spread over enough years for the efficiency savings to accumulate.
You have suitable land. If your garden can accommodate a ground loop without enormous expense, the installation cost premium is lower, and the payback period shortens.
You live in a colder region. Properties in Scotland, northern England, or at altitude experience more heating degree days and colder winter temperatures. The efficiency gap is wider here, making ground source's advantage more pronounced.
Use our suitability checker to see which type suits your property.
When Air Source Is Efficient Enough
For many UK homes, air source heat pumps are efficient enough that the extra cost of ground source is not justified:
Well-insulated homes with low heat demand. If your property is already well insulated and only needs 8,000-10,000 kWh of heat per year, the absolute running cost difference between ground and air source is perhaps £80-£120 annually. That will never recoup a £10,000+ installation cost premium.
Mild climate areas. In the south of England, winter temperatures are milder, and air source heat pumps maintain better efficiency. The gap narrows.
Limited budgets. If the budget does not stretch to ground source, a well-installed air source system with a SCOP of 3.0-3.5 is still vastly more efficient than any gas or oil boiler. Do not let the perfect be the enemy of the good — heat pumps are worth it in either form.
No garden access. If ground works are impractical, air source is the only option. Its efficiency, while lower than ground source, is still excellent compared to any combustion heating system.
Real-World Data from UK Installations
The Energy Saving Trust's Renewable Heat Premium Payment (RHPP) field trial monitored hundreds of UK heat pump installations. Key findings on efficiency:
Ground source average SCOP: 3.2 (with a wide range from 2.3 to 4.0+, largely driven by installation quality)
Air source average SCOP: 2.8 (range from 2.0 to 3.5+)
The important takeaway: installation quality matters as much as technology choice. A well-installed air source heat pump outperforms a poorly installed ground source one. The technology sets the ceiling — but the installer determines how close you get to it.
Get quotes from MCS-certified installers experienced in your chosen technology through our free quotes service.
Improving Efficiency for Either System
Regardless of which type you choose, these steps maximise efficiency:
Insulate first. Reducing your home's heat loss means the heat pump delivers at lower flow temperatures, which increases COP. Every £1 spent on insulation improves the heat pump's performance for its entire life.
Use low-temperature emitters. Underfloor heating or correctly sized radiators operating at 35-45°C allow the heat pump to run at its most efficient. See our guide on radiators for heat pumps.
Get the design right. Proper heat loss calculations, correct system sizing, and appropriate weather compensation settings are essential. Read our installation guide for what to expect.
Maintain regularly. Annual servicing keeps the system running at peak efficiency.
Frequently Asked Questions
How much more efficient is a ground source heat pump than air source?
On a seasonal basis, ground source is typically 15-25% more efficient than air source. In absolute terms, a GSHP might achieve an SCOP of 3.5-4.0 versus 2.8-3.5 for an ASHP. The gap is widest during the coldest winter months, when ground source can be 40-60% more efficient.
Is ground source always more efficient than air source?
In mild weather (above 10°C), a good air source heat pump can match or even slightly exceed ground source efficiency because the air temperature is similar to the ground temperature. Ground source's advantage is consistency — it maintains high efficiency regardless of weather conditions.
Does the higher efficiency of ground source offset its higher installation cost?
It depends on your heating demand, fuel being replaced, and how long you stay in the property. For high-demand properties replacing oil or LPG, the payback on the extra investment can be 12-18 years. For low-demand properties on mains gas, it may never fully pay back through efficiency savings alone — though the longer lifespan of GSHP tilts the equation.
What COP should I expect from a ground source heat pump?
A well-installed system should achieve an SCOP of 3.5 to 4.0. At peak conditions (mild weather, low flow temperature to underfloor heating), instantaneous COP can exceed 5.0. If your system is consistently below 3.0, something is wrong and needs investigation.
Do ground source heat pumps work well with radiators?
Yes, but efficiency is highest with underfloor heating or oversized radiators that allow low flow temperatures (35-45°C). Standard radiators may need the flow temperature raised to 50-55°C, which reduces COP by roughly 10-15% compared to underfloor heating.
Will climate change affect ground source vs air source efficiency?
Warmer winters would narrow the efficiency gap slightly, as air source units would spend less time at very low temperatures. However, ground source would remain more efficient overall, and warmer summers increase cooling demand — where ground source also has an efficiency advantage if cooling is added.