The type of Dual Fuel system we are talking about here is an AIR TO AIR HEAT PUMP (ATAHP)used in conjunction with a NATURAL GAS or PROPANE FURNACE. Below are some or the reasons for this system.
1. An ATAHP is most energy efficient at warmer temperatures and becomes less energy efficient as it gets colder outside (see COP). Gas furnaces have a constant efficiency and therefore a consistent cost of operation if the costs of fuel remain constant. In most situations there is a point, we call the ECONOMIC BALANCE POINT, at which above that outdoor temperature it is cheaper to heat with the ATAHP and below that point it is cheaper to heat with the Gas furnace. This point must be calculated by several variables and then adjusted any time there are major changes in energy costs. This number is programmed into the thermostat which has an outdoor temperature sensor, and then the thermostat does the rest.
2. An ATAHP is also able to make more heat on mild days than it is on very cold days. There is a point at which the ATAHP can't keep up. This is called the BUILDING BALANCE POINT, and is often near 15 degrees. Consequently most ATAHPs have a need for some method of supplemental heat. Often Straight Resistance Electric Heat is used which is very expensive. A DUAL FUEL SYSTEM uses Natural or Propane for this period of time.
3. When an ATAHP goes into it's defrost cycle it needs to use a supplemental heat source to keep warm air in your ductwork. Often gas is a cheaper and better choice for this function.
4. As the price of Gas and Electricity change, the point at which you switch from one to the other will change. This gives you the power to choose who you buy most of your energy from.
Weber Refrigeration Heating and Air Conditioning