The Rheem RXPF fossil fuel kits are intended to be used where the heat pump may provide heat at a lower cost than oil, natural gas or LP gas within a certain outdoor temperature range. The kit permits the heat pump to operate at temperatures (usually from 30 - 35 deg F and above) where the heat pump is most efficient. For the colder periods, the heating will be provided by a standard furnace which has approximately the same efficiency at all temperatures. The RXPF fossil fuel kits are for use with Rheem heat pumps and most fossil-fueled warm air furnaces. They operate by turning the heat pump off at a pre-determined outdoor temperature permitting the furnace to heat as demanded by the wall thermostat. When the outdoor temperature warms up, it will lock out the furnace and let the heat pump do the heating.