Heat Pump vs Gas Boiler: Carbon Footprint & Cost Compared

Home heating accounts for approximately 14% of the UK’s total CO₂ emissions — more than all UK aviation combined. The dominant technology is the gas boiler, installed in roughly 23 million UK homes. Replacing these with heat pumps is central to the UK’s net zero strategy.

But is a heat pump actually better for your carbon footprint? And what about cost? This guide gives you the honest numbers.

How Heat Pumps Work

A heat pump doesn’t generate heat by burning fuel — it moves heat from one place to another using refrigerant. An air-source heat pump (ASHP) extracts low-grade heat from outdoor air, even in cold weather, and concentrates it to heat your home.

The key metric is the Coefficient of Performance (COP): the ratio of heat delivered to electrical energy consumed. A COP of 3 means the heat pump delivers 3 kWh of heat for every 1 kWh of electricity used.

Modern heat pumps achieve:

• Average seasonal COP: 2.5–3.5 (real UK installation data from BEIS Heat Pump Ready programme)
• Peak COP: up to 5 in mild weather
• Minimum COP: ~1.5 in very cold conditions (below -5°C)

By comparison, a condensing gas boiler has an efficiency of approximately 0.85–0.92 — meaning it converts 85–92% of gas energy to heat, with the rest lost as flue gas.

Carbon Footprint Comparison

This is the critical question: does switching from gas to a heat pump actually reduce emissions?

The answer depends on the electricity grid’s carbon intensity. Here’s the comparison:

Heating system	Energy source	CO₂ per kWh of heat
Gas boiler (92% efficient)	Gas (0.202 kgCO₂/kWh)	0.220 kgCO₂/kWh
Heat pump (COP 2.5)	UK grid (0.194 kgCO₂/kWh)	0.078 kgCO₂/kWh
Heat pump (COP 3.5)	UK grid	0.055 kgCO₂/kWh

For a typical UK home using 12,000 kWh of heat per year, annual CO₂ emissions from heating:

• Gas boiler: 12,000 × 0.220 = 2,640 kg CO₂
• Heat pump (COP 2.5): 12,000 × 0.078 = 936 kg CO₂
• Annual saving: 1,704 kg CO₂ (65% reduction)

And as the UK grid decarbonises further, heat pump emissions will fall automatically while gas boiler emissions remain fixed.

But Wait — Heat Pumps Use More Electricity

Here’s the practical complexity. A heat pump with COP 2.5 uses 4,800 kWh of electricity to deliver 12,000 kWh of heat.

Electricity currently costs approximately 24p/kWh in the UK, versus 6p/kWh for gas. So despite using less energy overall:

System	Energy used	Annual energy cost
Gas boiler	13,000 kWh gas	~£780
Heat pump (COP 2.5)	4,800 kWh electricity	~£1,150
Heat pump (COP 3.5)	3,430 kWh electricity	~£823

At current energy prices, a heat pump with COP 2.5 is slightly more expensive to run than a gas boiler. A well-installed heat pump achieving COP 3.5 is roughly cost-equivalent.

This is a known distortion in UK energy pricing — electricity is taxed heavily while gas is taxed lightly, which is the opposite of what makes sense for decarbonisation. The government has committed to addressing this but progress has been slow.

What Determines Heat Pump Performance?

The difference between a COP of 2.0 and 3.5 is significant — both in cost and carbon terms. Key factors:

1. Flow temperature. Heat pumps work most efficiently when delivering heat at lower temperatures (35–45°C). Radiators sized for gas boilers often run at 70–80°C. Replacing radiators with larger ones (or using underfloor heating) allows lower flow temperatures and dramatically improves COP.

2. Insulation level. A well-insulated home needs less heat, which means a smaller heat pump running at lower capacity and higher efficiency. Insulating before installing a heat pump significantly improves both performance and economics.

3. Installation quality. Poorly designed systems (wrong-sized heat pump, insufficient pipework, incorrect refrigerant charge) can underperform badly. The MCS certification scheme for heat pump installers is the minimum quality benchmark.

4. Outdoor temperature. Heat pumps work well in typical UK weather (5–15°C). Performance drops in prolonged cold snaps, though modern cold-climate heat pumps (designed for Scandinavian conditions) maintain good COP even below 0°C.

Ground Source vs Air Source

Air-source heat pumps (ASHP):

• Extract heat from outdoor air
• Easier to install, no groundworks required
• Cost: £8,000–15,000 installed
• Typical COP: 2.5–3.5 in UK conditions

Ground source heat pumps (GSHP):

• Extract heat from underground pipework (horizontal loops or vertical boreholes)
• More complex and expensive installation
• Cost: £15,000–35,000 installed
• Typical COP: 3.5–4.5 (more consistent year-round as ground temperature is stable)

For most UK homes, an ASHP offers the better value proposition given the significant installation cost difference.

Financial Payback: Honest Assessment

Including the £7,500 Boiler Upgrade Scheme grant:

Cost scenario	After-grant cost	Annual running cost vs gas	Payback period
Well-insulated home, COP 3.5	~£5,500	-£40 (heat pump slightly cheaper)	Never (ongoing saving)
Typical home, COP 2.5	~£6,500	+£370 (heat pump more expensive)	~17 years
Poorly insulated home, COP 2.0	~£7,000	+£800 (heat pump more expensive)	Never financially

The financial case is clearest for well-insulated homes, and weakest for draughty, hard-to-heat homes. If you have poor insulation, insulate first.

Should You Switch?

Switch now if:

• Your boiler is approaching end of life (boilers last 10–15 years)
• Your home is well-insulated or you plan to insulate
• You have underfloor heating or can upgrade radiators
• You want to align with the UK’s net zero trajectory
• You have solar panels (maximise self-generated clean electricity)

Wait or consider alternatives if:

• Your boiler is new and working well
• Your home is poorly insulated and you can’t improve it in the near term
• You rent and can’t make structural changes

For renters and landlords: Minimum Energy Efficiency Standards (MEES) are being tightened. Landlords face requirements to upgrade EPC ratings, making heat pump installation increasingly relevant.

Calculate Your Heating Footprint

Use our free carbon footprint calculator to see exactly how much of your emissions come from home heating, and estimate your potential saving from a heat pump switch.

You might also want to read our guide on how to reduce your carbon footprint in the UK for the full picture on all available actions.

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FAQ

How much does a heat pump cost to install in the UK? Air-source heat pump installation typically costs £8,000–15,000 before grants. The government’s Boiler Upgrade Scheme (BUS) provides a £7,500 grant, bringing the after-grant cost to approximately £1,500–7,500. Grants are available for England and Wales.

Do heat pumps work in old UK houses? Yes, but with caveats. Older houses (Victorian, Edwardian) tend to be poorly insulated and were designed for high-temperature heating systems. Heat pump performance improves with insulation upgrades. Many older homes do successfully run heat pumps, but the system needs to be properly designed by an experienced installer.

Can a heat pump provide hot water as well as heating? Yes. Most ASHPs include domestic hot water capability, maintaining a tank at 50–55°C (with periodic legionella prevention cycles at 60°C). Some homes combine an ASHP with a solar thermal or solar PV system to reduce electricity costs for hot water.

Will gas boilers be banned in the UK? New gas boiler installations in new-build homes are being phased out. A ban on new gas boilers in existing homes has been discussed but no firm date is set as of 2025. The trajectory is clear: the gas boiler is a technology in terminal decline in the UK.

What is the Boiler Upgrade Scheme? The BUS provides grants of £7,500 for air-source heat pumps and £7,500 for ground-source heat pumps in England and Wales. Applications are made by registered MCS-certified installers on behalf of homeowners. The scheme runs until March 2028.