Understanding Performance Curves: The Essential Guide to Choosing a Heat Pump for Northern Winters

Introduction

Installers and distributors often compare heat pumps based on nominal capacity (BTU) and published low-temperature output. Yet one of the most important tools for evaluating a system is still too often overlooked: the performance curve.

This graph reveals the true personality of a heat pump. It shows how the unit reacts in dry cold, humid cold, windy conditions, high heat-loss situations, or during rapid temperature transitions.

1. Nominal Capacity vs. Real Capacity

Nominal capacity (e.g., 18,000 BTU) is only a standardized laboratory measurement. It does not predict how a system will actually behave in winter.

A performance curve, on the other hand, shows:

• capacity at different outdoor temperatures

• modulation stability

• capacity loss as temperatures drop

• defrost dynamics

2. Why the Curve’s Slope Matters More Than the Lowest Point

Two units may both show:

• 18,000 BTU nominal

• 12,000 BTU at –15 °C

But one may start losing capacity very early (for example at –2 °C), while the other maintains a performance plateau much longer.

The shape of the curve is far more revealing than the minimum number alone.

3. Willis Curves: Designed for Stability

Willis units are engineered to:

• maintain stable system pressure

• delay capacity drop-off

• preserve a broad performance plateau

• reduce the effects of humid cold

This stability is achieved through:

• large outdoor coils

• high fin density

• precise modulation

• intelligent defrost strategies

• integrated electronic control

4. How to Read a Performance Curve Properly

A. Observe the initial plateau

Indicates capacity during fall and early winter.

B. Observe the slope in moderate cold (–2 °C to –10 °C)

This is where differences between units become most apparent.

C. Observe performance retention in extreme cold (–15 °C to –25 °C)

Capacity differences of 20% to 40% between brands are common.

5. Practical Example: Interpreting a Willis Curve

A unit may show:

• 18,000 BTU nominal

• yet maintain 14,000 BTU at –10 °C

• and still deliver 12,000 BTU at –20 °C

A lower-performing unit will exhibit steeper drops caused by:

• a less efficient compressor

• a smaller outdoor coil

• poorly timed defrost cycles

Conclusion

In northern climates, choosing a heat pump is not simply about comparing BTU numbers on a datasheet. It requires careful analysis of the heating performance curve.

This graph reveals real capacity at various temperatures, modulation stability, resistance to humid cold, and the impact of defrost cycles on heat output.

A well-engineered curve—with a long plateau and a gradual slope—indicates a system capable of delivering consistent comfort, higher energy efficiency, and lower operating costs throughout the winter.

Understanding and interpreting this data allows installers, distributors, and homeowners to make informed decisions and select a heat pump truly suited to North American conditions.