A fossil fuel kit is a control system that employs an outdoor thermostat to shift between the heat pump and fossil fuel furnace. Upon the demand for heat, the control system will energize the heat pump, which will continue to function until a thermostat set point is reached and the demand for heat is satisfied. If the heat demand is not satisfied, the fossil fuel kit turns the heat pump off and energizes the furnace.
Installation of a fossil fuel kit typically requires a discrete control panel and two temperature sensors. The controls are mounted next to the furnace and act as the central hub of the system. The furnace, outdoor unit, thermostat and other components are then wired into that hub.
Compared to an all-electric system, a system that uses a fossil fuel kit to switch between heat sources is inefficient. During the switch, the heat pump shuts off while the more expensive back-up heat has to energize and bring the household up to the desired temperature.