A heat pump’s balance point — capacity falls as the heating load rises

What this diagram shows

A line graph of heat output versus outdoor temperature for a heat pump. As it gets colder (moving left) the building’s heating demand rises in a straight line, while the heat pump’s capacity falls — a standard air-source unit drops to roughly 55% of nominal at 17°F and 35% at 5°F. The two lines cross at the balance point, near 33°F in this example. Warmer than that, the heat pump carries the whole house; colder than that, the building needs more heat than the pump can make and the shaded gap must be covered by supplemental backup — an electric strip or, in a dual-fuel system, a gas furnace. A cold-climate heat pump holds its capacity longer, so its line crosses further left and leaves a smaller gap.