Dual fuel (hybrid heat pump)
A system that pairs a heat pump with a gas furnace backup, switching to gas automatically below a set outdoor "balance point" temperature.
A dual-fuel (or hybrid) system runs a heat pump for cooling and most heating, then switches over to a gas furnace once the outdoor temperature drops below a balance point the installer sets — typically somewhere between 25°F and 40°F. It captures the heat pump's cheap, efficient shoulder-season heat while keeping a furnace's full-blast output for the coldest snaps, without oversizing the heat pump itself.
The trade-off is capital cost: buying both a heat pump and a furnace new is the priciest path, since you're paying for two complete heating systems instead of one. Dual fuel makes the most sense in cold-winter climates where you're keeping an existing furnace that still has years of life left, or where cheap gas makes cold-weather electric-only heating a bad bet.