No, they're talking about air source ones (which is what's popular these days): > At about 4 °C, most air-source heat pumps currently on the market start opera

No, they're talking about air source ones (which is what's popular these days): > At about 4 °C, most air-source heat pumps currently on the market start operating at less than their full capacity, and at some point (usually around -15 °C), they can no longer do the job at all. At that point, an auxiliary heat source kicks on, which is less efficient. > But advancements in compressor technology over the last five years have addressed that issue. By controlling the compressor motor’s speed, and improving the timing of when vapor is injected into the compressor, engineers have made heat pumps efficient in colder temperatures.