Highest efficiency heat pump

An air-source heat pump can provide efficient heating and cooling for your home, especially if you live in a warm climate. When properly installed, an air-source heat pump can deliver one-and-a-half to three times more heat energy to a home than the electrical energy it consumes. This is possible because a heat pump moves heat rather than converting it from a fuel like combustion heating systems do.

Although air-source heat pumps can be used in nearly all parts of the United States, they do not generally perform well during extended periods of sub-freezing temperatures. In regions with sub-freezing winter temperatures, it may not be cost-effective to meet all your heating needs with a standard air-source heat pump.

Systems with gas heating as a backup can overcome this problem, however. Heat pumps specifically designed for cold climates also show promise, and reverse cycle chillers may operate efficiently at below-freezing temperatures.

HOW THEY WORK

A heat pump’s refrigeration system consists of a compressor and two coils made of copper tubing (one indoors and one outside), which are surrounded by aluminum fins to aid heat transfer. In heating mode, liquid refrigerant in the outside coils extracts heat from the air and evaporates into a gas. The indoor coils release heat from the refrigerant as it condenses back into a liquid. A reversing valve, near the compressor, can change the direction of the refrigerant flow for cooling as well as for defrosting the outdoor coils in winter.

When outdoor temperatures fall below 40°F, a less-efficient panel of electric resistance coils, similar to those in your toaster, kicks in to provide indoor heating. This is why air-source heat pumps aren’t always very efficient for heating in areas with cold winters. Some units now have gas-fired backup furnaces instead of electric resistance coils, allowing them to operate more efficiently.

The efficiency and performance of today’s air-source heat pumps is one-and-a-half to two times greater than those available 30 years ago as a result of technical advances such as:

• Thermostatic expansion valves for more precise control of the refrigerant flow to the indoor coil
• Variable speed blowers, which are more efficient and can compensate for some of the adverse effects of restricted ducts, dirty filters, and dirty coils
• Improved coil design
• Improved electric motor and two-speed compressor designs
• Copper tubing, grooved inside to increase surface area.

Most central heat pumps are split-systems — that is, they have one coil indoors and one outdoors. Supply and return ducts connect to a central fan, which is located indoors.

Some heat pumps are packaged systems. These usually have both coils and the fan outdoors. Heated or cooled air is delivered to the interior from ductwork that passes through a wall or roof.

SELECTING A HEAT PUMP

Every residential heat pump sold in this country has an EnergyGuide label, which displays the heat pump’s heating and cooling efficiency performance rating, comparing it to other available makes and models.