Heat Pump vs. Natural Gas Boiler: Carbon Emissions by Grid Source
⚙ Heat Pump vs. Natural Gas Boiler: Which Has the Lower Carbon Footprint?
When it comes to reducing carbon emissions in buildings, the choice between using a heat pump or a natural gas boiler isn’t just about equipment efficiency — it’s also about the source of electricity powering the heat pump.
This post compares both technologies under two electricity supply scenarios: one where the grid is powered by natural gas and another where it’s powered by coal.
↻ The Basic Comparison: Efficiency and Emissions
| System | Typical Efficiency | Emission Source |
|---|---|---|
| Heat Pump (air-source) | COP = 3.0 (300% efficient) | Grid electricity |
| Natural Gas Boiler | AFUE = 0.90 (90% efficient) | On-site combustion |
🔥 Scenario 1: Heat Pump Powered by Natural Gas–Fired Electricity
Assumptions:
- Heat pump COP = 3.0 → 1 kWh electricity → 3 kWh heat
- Electricity EF = 0.4 kg CO₂e/kWh
- Boiler EF = 5.3 kg CO₂e/therm ≈ 0.181 kg CO₂e/kWh heat (after 90% efficiency)
Results:
- Heat Pump: 1 kWh heat = 1/3 kWh electricity = 0.133 kg CO₂e
- Boiler: 1 kWh heat = 0.181 kg CO₂e
🏭 Scenario 2: Heat Pump Powered by Coal-Based Electricity
Assumptions:
- Electricity EF = 1.0 kg CO₂e/kWh (typical for coal)
Results:
- Heat Pump: 1 kWh heat = 1/3 kWh electricity = 0.333 kg CO₂e
- Boiler: 1 kWh heat = 0.181 kg CO₂e
⚡ What This Means
- If your grid is clean or average (≤0.5 kg CO₂e/kWh) → Heat pumps are typically lower carbon.
- If your grid is coal-heavy (≥0.8 kg CO₂e/kWh) → Natural gas boilers may be cleaner in the short term.
🧠 Bottom Line
Heat pumps aren’t always lower-carbon than natural gas. It depends on how clean the electricity is. As the grid decarbonizes, the carbon advantage of heat pumps will continue to grow.
Pro Tip: Before choosing your system, check your local emission factor:
Bob Fassbender graduated from the University of Wisconsin - Madison with a degree in Chemical-Engineering. Following graduation, he spent 3 years working as a Marketing Engineer for Trane C.D.S. In the C.D.S. group, Bob developed and supported design and analysis software, primarily TRACE 700™. In addition to his development work, Bob also traveled around the country as a TRACE 700™ and System Analyzer™ instructor. Bob is also an experienced user with eQUEST energy modeling software. Today, Bob continues training and energy modeling as a LEED accredited professional (with a focus on LEED EA credit 1).