Table of Contents
Boiler vs Heat Pump: Complete Engineering Comparison
Quick Verdict
The boiler vs heat pump decision is fundamentally about efficiency, carbon, and installation constraints. Heat pumps are transforming building heating as the world decarbonizes, but boilers remain appropriate for specific situations.
Bottom Line: Heat pumps are the superior choice for new construction and well-insulated buildings, delivering 3-4× the heat output per unit energy and 50-75% lower carbon emissions. Boilers remain practical for older buildings with high heat loss, where their lower installation cost ($4,000-10,000 vs $12,000-25,000) and compatibility with existing high-temperature radiators outweigh efficiency disadvantages.
With many countries banning gas boilers in new construction and carbon pricing becoming common, heat pumps represent the future-proof choice. However, the transition requires appropriate building fabric and heat distribution—rushing to heat pumps in unsuitable buildings creates poor outcomes.
At-a-Glance Comparison Table
| Feature | Boiler | Heat Pump | Winner |
|---|---|---|---|
| Efficiency | 90-95% AFUE | 300-450% (COP 3-4.5) | Heat Pump |
| Installation Cost | $4,000-10,000 | $12,000-25,000 | Boiler |
| Running Cost | Higher (baseline) | 25-50% lower | Heat Pump |
| Carbon Emissions | 215 g CO2/kWh heat | 50-100 g CO2/kWh heat | Heat Pump |
| Flow Temperature | 70-80°C (easily) | 35-55°C (efficiently) | Boiler |
| Response Time | Instant | Slower (thermal mass dependent) | Boiler |
| Cold Climate Performance | Unaffected | Reduced below -10°C | Boiler |
| Cooling Capability | None | Reversible models available | Heat Pump |
| Best For | High heat loss, retrofits | New builds, insulated homes | — |
Efficiency: The Fundamental Difference
The efficiency difference between boilers and heat pumps is not incremental—it's a fundamentally different approach to heating.
Technical Note: Boiler efficiency is measured as AFUE (Annual Fuel Utilization Efficiency)—heat output divided by fuel energy input. Heat pump efficiency is measured as COP (Coefficient of Performance)—heat output divided by electrical input. These cannot be directly compared as percentages.
Boiler Efficiency
Modern condensing boilers achieve 90-95% AFUE by recovering latent heat from flue gases. The theoretical maximum is approximately 98% (lower heating value basis) or 108% (higher heating value basis accounting for condensation).
| Boiler Type | Typical AFUE | Condensing |
|---|---|---|
| Old non-condensing | 70-80% | No |
| Modern non-condensing | 80-88% | No |
| Condensing (standard) | 90-94% | Yes |
| Condensing (premium) | 94-98% | Yes |
Even the best boiler converts at most 98% of fuel energy into useful heat—energy is lost to flue gases and casing radiation.
Heat Pump Efficiency
Heat pumps don't create heat—they move it from outdoors (air, ground, or water) into the building. The electricity powers a compressor that concentrates low-grade ambient heat into high-grade heat for the building.
This allows COP values exceeding 1.0 (100%):
| Heat Pump Type | Typical COP | Equivalent Efficiency |
|---|---|---|
| Air-source (ASHP) at 7°C | 3.5-4.0 | 350-400% |
| Air-source (ASHP) at -7°C | 2.0-2.5 | 200-250% |
| Ground-source (GSHP) | 4.0-5.0 | 400-500% |
| Water-source (WSHP) | 4.5-5.5 | 450-550% |
A COP of 3.5 means 3.5 kWh of heat delivered per 1 kWh of electricity consumed—fundamentally superior to combustion.
Seasonal Performance
Real-world seasonal performance accounts for varying conditions:
- Boilers: AFUE already represents seasonal average, typically 90-94%
- Heat pumps: SCOP (Seasonal COP) typically 0.5-1.0 lower than rated COP; ASHP seasonal COP 2.5-3.5, GSHP 3.5-4.5
Verdict: Efficiency
Winner: Heat Pump — Delivering 3-4× the heat output per unit of input energy is a fundamental advantage. Even accounting for electricity's higher cost per kWh, heat pumps cost less to run in most scenarios.
Running Costs: The Economic Calculation
Running cost depends on efficiency, fuel prices, and heat demand. The calculation is straightforward but sensitive to local energy prices.
Cost Per Unit Heat Calculation
The cost to deliver 1 kWh of heat:
Gas Boiler:
Cost per kWh heat = Gas price / AFUE = $0.08 / 0.92 = $0.087/kWh heat
Heat Pump:
Cost per kWh heat = Electricity price / COP = $0.25 / 3.5 = $0.071/kWh heat
At these typical prices (US average), heat pumps are 18% cheaper per unit heat. The advantage increases with:
- Higher gas prices
- Lower electricity prices
- Better heat pump COP (well-insulated homes, UFH, mild climates)
Annual Running Cost Comparison
| House Type | Annual Heat Demand | Gas Boiler Cost | Heat Pump Cost | Savings |
|---|---|---|---|---|
| New build, 150m² | 8,000 kWh | $696 | $571 | $125 (18%) |
| 1990s house, 150m² | 15,000 kWh | $1,304 | $1,071 | $233 (18%) |
| Old house, 150m² | 25,000 kWh | $2,174 | $1,786 | $388 (18%) |
Price Sensitivity: Running cost comparison is highly sensitive to local gas and electricity prices. In some markets, gas is so cheap (or electricity so expensive) that boilers cost less to run. Always calculate with your local tariffs.
Maintenance Costs
- Boilers: Annual service mandatory ($100-200), typical repairs $200-500, replacement parts available but complex
- Heat pumps: Less maintenance required, refrigerant checks every 2-3 years ($100-150), major repairs can be expensive ($500-2,000)
Verdict: Running Costs
Winner: Heat Pump — In most markets, heat pumps cost 15-40% less to run. The advantage is greatest with high gas prices, low electricity prices, and high COP operation (mild climates, well-insulated buildings, UFH).
Carbon Emissions: The Environmental Factor
Carbon emissions are increasingly important due to climate policy, carbon taxes, and sustainability certifications.
Emissions Per Unit Heat
Gas Boiler:
- Natural gas: 184 g CO2/kWh fuel
- At 92% AFUE: 184 ÷ 0.92 = 200 g CO2/kWh heat
Heat Pump:
- Grid electricity: varies by location (50-500 g CO2/kWh)
- At COP 3.5 with 250 g/kWh grid: 250 ÷ 3.5 = 71 g CO2/kWh heat
Heat pumps emit 50-75% less CO2 in most locations. As grids decarbonize (more renewable energy), heat pump emissions approach zero.
Carbon Reduction Trajectory
| Year | UK Grid Carbon (g/kWh) | Heat Pump Emissions | vs Gas Boiler |
|---|---|---|---|
| 2020 | 230 | 66 g/kWh heat | -67% |
| 2025 | 150 | 43 g/kWh heat | -78% |
| 2030 | 75 | 21 g/kWh heat | -89% |
| 2040 | ~0 (target) | ~0 g/kWh heat | -100% |
Boiler emissions remain fixed; heat pump emissions decrease automatically as the grid decarbonizes.
Verdict: Carbon Emissions
Winner: Heat Pump — Even today, heat pumps emit 50-75% less CO2 than gas boilers. This advantage will increase to near-100% reduction as electricity grids decarbonize over coming decades.
Installation Cost: The Upfront Barrier
Higher installation cost is the primary barrier to heat pump adoption.
Cost Breakdown
| Component | Gas Boiler | Air-Source HP | Ground-Source HP |
|---|---|---|---|
| Equipment | $1,500-3,500 | $5,000-10,000 | $8,000-15,000 |
| Installation labor | $1,500-3,000 | $3,000-6,000 | $2,000-4,000 |
| Ground works/drilling | — | — | $5,000-15,000 |
| Electrical upgrade | — | $500-2,000 | $500-2,000 |
| Distribution modifications | — | $0-5,000 | $0-5,000 |
| Total | $4,000-10,000 | $12,000-25,000 | $18,000-40,000 |
Factors Affecting Heat Pump Cost
- Building insulation: Poor insulation requires larger, more expensive unit
- Existing distribution: UFH ready = no upgrade; radiators may need replacing/adding
- Electrical supply: May need supply upgrade for larger units
- Ground conditions: GSHP borehole vs horizontal loop costs vary significantly
- Refrigerant charge: Larger systems need more refrigerant
Government Incentives
Many governments offer significant incentives for heat pumps:
- UK Boiler Upgrade Scheme: £7,500 ($9,000) toward ASHP
- US Federal Tax Credit: 30% of installed cost (up to limits)
- Various state/local programs adding further incentives
After incentives, cost gap often reduces to $3,000-8,000.
Verdict: Installation Cost
Winner: Boiler — Heat pumps cost 2-3× more to install even before considering potential electrical and distribution upgrades. Government incentives reduce but don't eliminate the gap.
Application-Specific Recommendations
When to Choose a Boiler
Use a boiler when:
- Building has high heat loss (>80 W/m²) that would require oversized heat pump
- Existing radiator system designed for 70-80°C cannot be easily modified
- Installation budget is strictly limited ($4,000-10,000 available)
- Electrical supply upgrade would be prohibitively expensive
- No outdoor space available for ASHP unit
- Climate is extremely cold (regular temperatures below -15°C)
- Short-term ownership planned (less than 7 years, before payback achieved)
Typical Applications:
- Older, poorly insulated buildings awaiting renovation
- Period properties with heritage constraints
- Budget-constrained projects
- Buildings with limited electrical capacity
- Very cold climate regions
When to Choose a Heat Pump
Use a heat pump when:
- New construction (regulations increasingly mandate low-carbon heating)
- Building is well-insulated (heat loss less than 50 W/m²)
- Underfloor heating or low-temperature radiators available
- Carbon reduction is a project requirement (LEED, BREEAM, net-zero)
- Long-term ownership planned (10+ years to maximize payback)
- Government incentives significantly offset installation cost
- Future gas boiler phase-out is a concern
Typical Applications:
- New residential and commercial construction
- Deep energy retrofits (insulation + heating upgrade together)
- Buildings seeking sustainability certification
- Properties with high energy use seeking cost reduction
- Replacement of end-of-life boilers in suitable buildings
Installation Considerations
Boiler Installation
Boiler installation is a mature, well-understood process:
- Flue requirements: Balanced flue, typically through external wall
- Gas connection: Existing supply usually adequate
- Water connections: Connection to existing heating circuit
- Controls: Modern boilers integrate with smart thermostats
- Commissioning: Gas Safe registration, commissioning certificate
Installation typically complete in 1-2 days for like-for-like replacement.
Heat Pump Installation
Heat pump installation requires more planning:
- Heat loss assessment: Accurate calculation essential for sizing
- Electrical supply: 20-40A dedicated circuit, potential supply upgrade
- Outdoor unit positioning: Noise regulations, clearances, aesthetics
- Distribution assessment: May need UFH addition or radiator upgrades
- Hot water strategy: Heat pump may provide DHW or require separate cylinder
- Commissioning: F-Gas registered installer, MCS certification for incentives
Installation typically 2-5 days depending on modifications required.
Field Tip: Don't undersize heat pumps. Unlike boilers that can temporarily boost output, heat pumps must meet peak load without backup (unless hybrid system). Undersizing leads to cold complaints, auxiliary heating use, and poor efficiency. When in doubt, go one size larger.
Performance in Different Climates
Mild Climates (Design temp >-5°C)
Both systems perform well. Heat pumps excel with high COP (3.5-4.5) due to warm ambient temperatures. ASHP is the default heat pump choice.
Cold Climates (Design temp -5°C to -15°C)
Heat pump COP drops but remains viable (COP 2.0-3.0). Cold-climate ASHP models recommended. Consider:
- Slightly larger heat pump to compensate for reduced capacity at cold temps
- Backup resistance heating for extreme cold days
- GSHP for stable ground temperature regardless of air temp
Very Cold Climates (Design temp below -15°C)
ASHP efficiency drops significantly (COP below 2.0). Options:
- Ground-source heat pump: Ground at 8-12°C regardless of air temperature
- Hybrid system: Heat pump for mild weather, boiler for cold snaps
- Cold-climate ASHP: Specialized models operating to -25°C or below
Scandinavian countries successfully use heat pumps despite severe winters, proving technical viability—but system design must account for climate.
Common Mistakes to Avoid
| Mistake | Impact | Prevention |
|---|---|---|
| Undersizing heat pump | Cold building, backup heater use | Accurate heat loss calculation, don't undersize |
| Installing ASHP in poorly insulated building | Low COP, high running costs | Insulate first (fabric-first approach) |
| Keeping high-temp radiators | Heat pump runs at elevated temps, poor efficiency | Upgrade to UFH or low-temp radiators |
| Ignoring DHW requirements | Insufficient hot water, legionella risk | Size system for DHW and heating |
| Placing ASHP in enclosed space | Reduced efficiency, noise reflection | Open location with good airflow |
| Not planning for defrost cycles | Cold air during defrost | Account for defrost in sizing, consider buffer tank |
Related Tools
Use these calculators to assess heat pump suitability:
- Heat Loss Calculator - Determine if building suits heat pump operation
- Radiator Selection Calculator - Size radiators for low-temperature operation
- Expansion Tank Calculator - Size expansion vessel for heat pump system
Key Takeaways
- Efficiency: Heat pumps achieve COP 3-4.5 (300-450%) vs boilers' 90-95%—fundamentally superior
- Carbon: Heat pumps emit 50-75% less CO2, improving as grids decarbonize
- When to choose boilers: High heat loss buildings, budget constraints, existing high-temp systems
- When to choose heat pumps: New builds, insulated homes, carbon targets, long-term ownership
- Cost trade-off: Heat pumps cost 2-3× more upfront but 25-50% less to run—7-15 year payback
Further Reading
- Understanding Heat Loss Calculations - Foundation for system sizing
- Radiator vs Underfloor Heating - Distribution system comparison
- Understanding Circulation Pumps - Pump selection for hydronic systems
References & Standards
- EN 14825: Air conditioners, liquid chilling packages and heat pumps—Testing and rating at part load conditions
- EN 14511: Air conditioners, liquid chilling packages and heat pumps—Performance testing
- ASHRAE Handbook—HVAC Systems and Equipment: Chapter 9, Applied Heat Pump and Heat Recovery Systems
- BS EN 15450: Heating systems in buildings—Design of heat pump heating systems
- MCS 020: Heat pump standard for MCS certification
Disclaimer: This comparison provides general technical guidance. Actual performance depends on specific installation conditions, building characteristics, climate, and energy prices. Always consult with qualified professionals and verify compliance with local regulations before making final decisions.