- Heat pumps are devices which move heat from one area to another. They have been used for decades to power refrigerators and air conditioning. More recently, they have become popular as heaters or combination heating/air-conditioning units. Dual heat pumps function exactly like a normal heat pump most of the time. When the temperature gets too low, however, they use a gas furnace to provide some supplementary power.
- The heat pump is basically a simple series of tubes driven by a powerful pump called a compressor. The name of the compressor describes the first thing it does--compress a fluid called the refrigerant. When a fluid is compressed into a small space, it heats up. The super-heated refrigerant flows through a long coil called the condenser, where its heat gradually leaks out into the air or some other system designed to cool it. Then, it is sprayed out of a small nozzle into the low pressure evaporator. As the pressure of the refrigerant drops, it rapidly cools. It is then run through the evaporator coil, where it is gradually warmed up. Many modern heat pumps can actually run both ways; the evaporator becomes the condenser and vice-versa when the pump is run backwards. This allows a single heat pump to provide warmth during the winter and cooling during the summer.
- Dual heat pumps have one coil inside the house and one coil outside. The coil inside the house is hooked up to a blower fan. When the system is turned on, the fan blows air in it, spreading warm or cold air through the house. The coil outside is usually out in the air, although it can be in the ground. During the summer, the outside coil acts as a condenser, and the air cools it off. During the winter, the outside coil acts as an evaporator, and the air warms it. When it gets really cold, the gas system kicks in to provide a boost.
- Dual heat pumps are attached to a gas furnace that remains off most of the time. When the temperature gets cold enough, however, the heat pump becomes inefficient. The air doesn't provide enough warmth to heat up the evaporator properly, which means the system can't pump much air into the house. When this happens, the furnace switches on. The burners in the furnace heat refrigerant, which is pumped out to the evaporator coil. The heat from the refrigerant is transferred to the evaporator coil, keeping the heat pump working fairly efficiently. In emergencies, the furnace can also be used to heat the whole house, but it usually isn't. Typically, the furnace provides about 15 percent of the heat and the pump provides the other 85 percent.
