Heat Pumps: Why Moving Heat Is 3x More Efficient Than Making It

A resistance heater converts exactly one unit of electricity into one unit of heat. A heat pump converts one unit of electricity into three to five units of heat. The difference is thermodynamics, not marketing.

## The Vapor Compression Cycle

All heat pumps operate on the **vapor compression cycle** — the same physics as your refrigerator, running in the heat-delivery direction rather than the heat-removal direction.

The four stages:
1. **Evaporation**: Low-pressure refrigerant absorbs heat from the cold source (outdoor air, ground, water) and evaporates at −5°C to +10°C
2. **Compression**: The compressor raises refrigerant pressure, which raises its temperature to 50–80°C — now hotter than the space you want to heat
3. **Condensation**: Hot refrigerant releases heat into your home through the indoor heat exchanger (the condenser), warming the space
4. **Expansion**: An expansion valve drops the refrigerant pressure, cooling it back to the evaporation temperature to restart the cycle

The **Coefficient of Performance (COP)** — units of heat delivered per unit of electricity consumed — is bounded by the Carnot limit:

```
COP_max = T_hot / (T_hot − T_cold)   [absolute temperatures in Kelvin]
```

At outdoor −5°C (268 K) and indoor 45°C delivery temperature (318 K):
COP_max = 318 / (318 − 268) = **6.36**

Real vapor compression systems achieve 60–70% of Carnot efficiency, giving COP 3.8–4.5 in mild conditions. In winter at −15°C, COP typically falls to 2.0–2.8. Still 2–3× better than resistance heat.

> ⚡ A resistance heater has COP = 1.0 by thermodynamic definition. There is no engineering path to COP > 1 using direct electrical resistance. A heat pump at COP 3.0 delivers the same warmth using one-third the electricity.

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## Refrigerant Selection

The refrigerant mediates the entire cycle. Its thermodynamic properties, global warming potential (GWP), and safety profile define the system.

| Refrigerant | GWP | COP (typical) | Status |
|---|---|---|---|
| R-410A (HFC blend) | 2,088 | 3.5–4.5 | Current North American standard; phaseout by 2036 |
| R-32 (HFC) | 675 | 3.7–4.8 | Transitional; most new EU residential systems |
| R-290 (Propane) | 3 | 3.8–5.0 | Near-zero climate impact; flammable |
| R-744 (CO₂) | 1 | 2.5–4.5 | Ultra-low GWP; high-pressure operation |

**R-410A** is being phased out under the Kigali Amendment to the Montreal Protocol. Its GWP of 2,088 means a single kilogram leaked during service has the same climate impact as 2 metric tons of CO₂.

**R-290 (propane)** has near-zero climate impact and excellent thermodynamic properties — in fact, better than R-410A in most temperature ranges. The limitation is flammability: installation requires leak-safe design, but modern residential units operate with only 150–300 g charge, well within established safety thresholds. The EU is mandating R-290 for new residential heat pumps.

**R-744 (CO₂)** operates at transcritical pressures up to 130 bar — significant mechanical engineering challenge — but enables very high delivery temperatures (up to 90°C) without efficiency loss. This makes CO₂ heat pumps ideal for retrofitting existing radiator-based heating systems that require higher water temperatures than conventional refrigerants can efficiently deliver.

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## Cold Climate Performance

The COP degrades as outdoor temperature drops — the temperature differential between source and delivery grows, pushing closer to the Carnot limit. Early air-source heat pumps became ineffective below −5°C. Modern designs have fundamentally changed this.

**Enhanced Vapor Injection (EVI)**: A second compression stage with intermediate flash gas injection allows the compressor to maintain efficiency at lower source temperatures. Mitsubishi's Hyper-Heating INVERTER (H²i) system and Bosch IDS units operate to −30°C while maintaining COP > 1.7.

**Variable-speed DC inverter compressors**: Traditional single-speed compressors cycle on and off, incurring efficiency penalties with every start. Variable-speed drives match compressor output precisely to heating load — this alone improves seasonal efficiency by 20–35% in cold climates.

**Defrost cycle management**: Below approximately +2°C outdoor temperature, ice forms on the outdoor coil. Defrost cycles — typically reversing refrigerant flow to melt the ice — reduce effective seasonal COP by 5–15%. Demand-defrost control (initiating defrost only when ice is actually detected) rather than timed defrost reduces this penalty substantially.

> ⚡ Properly sized cold-climate heat pumps installed in Maine (design temperature −20°C) demonstrate measured seasonal COP of 2.1–2.7. This is still 2–3× more efficient than the natural gas furnaces they replace, at equivalent cost at current New England energy prices.

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## Ground-Source vs Air-Source

**Ground-source heat pumps (GSHP)** extract heat from the earth, which maintains a stable 8–12°C year-round at depths of 1.5 m or more. This eliminates the cold-climate performance degradation entirely.

| System Type | Seasonal COP | Installation Cost (typical US) | Limitation |
|---|---|---|---|
| Air-source (ASHP) | 1.5–4.5 | $3,000–$8,000 | Degrades at low outdoor temps |
| Ground-source horizontal loop | 3.5–5.0 | $15,000–$30,000 | Requires land area |
| Ground-source vertical borehole | 4.0–5.5 | $20,000–$50,000 | High drilling cost |

The economic case for GSHP has weakened as cold-climate ASHP technology matured. Modern variable-speed ASHP units close most of the efficiency gap at a quarter of the installation cost. GSHP remains the right answer for district heating systems and buildings where drilling cost is amortized across many units.

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## Heat Pump Water Heaters

**Heat pump water heaters (HPWH)** represent the most economically compelling application. They extract heat from indoor or outdoor air and deliver it to a water storage tank, achieving COP 3.0–4.5 year-round regardless of outdoor climate.

Annual operating cost comparison (US, $0.15/kWh):
- **Resistance water heater**: 4,500 W element, COP 1.0 → ~$500/year
- **Heat pump water heater**: ~450 W average draw, COP 3.5 → ~$140/year
- **Payback period on $1,200 premium**: 3.5 years

The side effect: HPWHs extract heat from the room they're installed in, slightly cooling and dehumidifying the space. In hot climates, this is a benefit. In cold climates, it shifts some load back to the space heating system. Net seasonal impact is still strongly positive.

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## The Bigger Picture

Heating buildings and water accounts for roughly 40% of global energy consumption, almost entirely from combustion of natural gas, oil, and coal. Heat pump electrification, powered by a decarbonizing grid, is the only scalable pathway to deep decarbonization of this sector.

The technology is mature, reliable, and available at scale. The barriers are installation cost, a shortage of trained HVAC technicians, and the electricity-to-gas price ratio: in markets where electricity costs 3× gas per unit of energy (common in the southern U.S.), the COP advantage is neutralized by fuel price. In markets with low electricity prices or carbon-priced gas — most of Europe, the Pacific Northwest, Quebec — heat pumps are already the economically dominant choice without subsidies.

The physics is not the constraint. One unit of electricity, moved rather than burned, becomes three to five units of warmth. The constraint is price signals and installation capacity.

Heat Pumps: Why Moving Heat Is 3x More Efficient Than Making It

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