A two stage heat pump heats your home by drawing in the heat from outdoors and forcing it into your home. It works the reverse of an air conditioning unit. The first stage is used to draw the heat from outside into your home. It works best when the outside temperature is in the 50-degree range. When the temperature drops much below this range, then the second stage goes into work. It is an additional heating unit that helps heat the air to the desired temperature you want. This is an effective way to heat your home and save money.