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Heat Pumps

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Homeowners seeking new ways to conserve energy and stretch their energy dollars are turning down their thermostats and adding insulation to their homes. To help homeowners achieve energy savings, architects, builders and manufacturers have been developing energy efficient heating and cooling equipment that offers long-term energy savings. One device that has been attracting attention is an efficient, economical alternative to conventional heating and cooling systems is the air-to-air heat pump.

Heat pumps are not new; electric heat pumps were first developed and marketed in the 1930's. Recent development in heat pumps marketed today have made them more reliable, and many applications can offer substantial energy and dollar savings.

What is a Heat Pump?

A heat pump is a device which extracts available heat from one area and transfers it to another. Even cold air contains some heat, and heat pumps can extract heat from the outside air on a cold day and transfer it indoors to maintain a comfortable temperature. A heat pump can also work in reverse during the summer, extracting heat from the indoors and transferring it outdoors much the way an air conditioner functions.

The heat pump doesn't really give you something for nothing, however. Instead of using electricity to heat the air, it uses electricity to move existing heat from the outside into the house. Below a certain outdoor temperature, the heat pump's efficiency also decreases because there is less available heat.

How Does a Heat Pump Work?

Heat flows naturally from a warm area to a cooler area. (A heat pump, like an air conditioner, works against this natural flow.) In its heating mode, a heat pump's fan blows cold air from outside across a coil (called the evaporator) containing refrigerant, a liquid which boils at a very low temperature (as low as 0 degrees F). When the refrigerant boils, it becomes a vapor, just as water becomes a vapor (steam) when boiled. This vapor is sucked into a compressor where it becomes a high pressure, high temperature vapor. It is then forced through a coil (called the condenser) within part of the heat pump located indoors. because the vapor is now hotter than room temperature, it turns back to a liquid, releasing heat which is blown through a duct system to heat the house.

The cycle begins again as the liquid refrigerant, cooled by releasing its heat into the house, is pumped back outside. On the way, it passes through an expansion valve, lowering the refrigerant's pressure and temperature again so it can boil more easily in the outdoor coil. In its cooling mode the heat pump works in reverse, extracting available heat from indoors and transferring it outside.

Is a Heat Pump for Your Home?

The efficiency of a home heating system is measured by the number of units of heat energy output obtained for each unit of energy input. In relatively mild climates, heat pumps can return the most heat per unit of energy consumed. The increased efficiency offered by heat pumps translates readily into lower utility bills. While actual savings depend upon factors such as climate and the price and availability of natural gas, oil, and electricity, certain heat pump allocations can offer substantial savings over conventional heating systems.

In more temperate regions of the country, installing a heat pump can reduce electricity bills as much as 35% - 45% in homes with conventional electric baseboard heating. To determine whether a heat pump is an economical alternative for your home, consider the following factors:

First, heat pumps are most economical when they can be used year-round for both winter heating and summer cooling. To get maximum use from a heat pump you will need to remove your existing furnace and/or air conditioner. Replacing them with a heat pump may not be cost-effective if your furnace or air conditioner is in good working order or relatively new.

Second, the efficiency of a heat pump varies significantly with the outdoor temperature. While a heat pump may be twice as efficient as a conventional heating system at 50 degrees F, it may be only slightly more efficient at 35 degrees F. Typically, when the outdoor temperature drops to less than 30 degrees F, the heat pump must be supplemented with a heating system such as electric heating (usually included in the system). At temperatures of 15 degrees F or less the heat pump shuts off and the conventional heating system takes over.

Furthermore, during the heating season, ice may form on the outdoor heat exchange coil when the temperature drops below 32 degrees F. Frost impairs the coil's ability to transfer heat, thus reducing the heat pump's efficiency. To melt the frost, the heat pump has a defrost cycle in which the reversing valve periodically sends hot refrigerant through the outdoor coils to melt the frost. Heat pumps generally have defrost cycles lasting anywhere from 2 - 10 minutes. During the defrost cycle supplemental electric resistance heating heats the house; this reduces a heat pump's overall efficiency by 3% - 10%. The two common defrost controls are: demand-defrost which uses sensors to determine when a cycle is needed; and time-temperature defrost which activates the defrost cycle at preset intervals when the outdoor temperature drops below a specified level.

Therefore, a heat pump is economical where winters are relatively mild and the average temperature is above 25 degrees F. In this climate the heat pump alone will provide sufficient heat most of the day. In areas where the temperature in winter frequently drops below 25 degrees F, thus requiring frequent use of the backup heating system, it may not be economical to purchase a heat pump.

Finally, the cost of installing a heat pump is 10% - 25% higher than the cost of installing a conventional, fossil-fueled system, with air conditioning. Consequently, if a heat pump will not substantially increase energy efficiency, the long-term savings may not justify the initial expense.

What Types of Electric Heat Pumps are Available? (842, 924)

The air-to-air heat pump is the most commonly used electric heat pump, but there are three other types of units presently on the market.

Water-to-air heat pumps exchange heat with either groundwater, surface water, or water passed through cooling towers (for industrial or commercial use). Systems that use groundwater appear to have the greatest potential for efficient and economical operation because groundwater temperatures hover around 50 degrees F most of the year on the average.

Ground-source heat pumps are also efficient. Since ground temperatures below the frost line remain relatively constant throughout the year, it is a good heat source for heat pumps. The heat exchange loop can be either vertical, which entails drilling a very deep hole through which the coil passes, or horizontal, in which case the coil is laid in long, relatively shallow trenches. For optimal efficiency, the ground surrounding the coil should be moist, especially for cooling.

Another system is the dual-fuel heat pump which is an add-on unit to an existing oil, gas or propane furnace. The heat pump operates like other units until the temperature drops below the point where the heat pump can no longer meet the heating load. At this point, the thermostat automatically turns off the heat pump and activates the furnace. The strength of this arrangement is that the homeowner benefits from the efficiency of the heat pump during mild weather and the good performance of a fossil-fueled furnace during the cold weather. A dual-fuel heat pump may not be cost effective in all areas or for all heating systems, but it may be an option worth exploring.

How Should a Heat Pump be Selected?

Heat pumps come in different types and sizes, ranging from window units to large commercial and industrial units. Climate, building size and design are the most important factors to consider for determining the type and size heat pump that will operate most efficiently and economically.

A utility company representative can help determine the type and size of heat pump needed to heat and cool a house or building efficiently, and can estimate its operating costs. A dependable contractor can then help select and install the most efficient, reliable heat pump for that home or building.

You need not rely solely upon a contractor to help you select a heat pump; the following guidelines will help you compare brands. Manufacturers test and rate their heat pumps using U.S. Department of Energy testing standards. The ratings are the Heating Seasonal Performance Factor (HSPF) for heating, and the Seasonal Energy Efficient Ratio (SEER) for heating and cooling. The higher the rating, the more efficient the heat pump. Both ratings are a measure of a heat pump's output under a range of weather conditions considered typical for a particular area. If you live in a particularly hot climate, look for systems with a high SEER rating. Homeowners in particularly cool climates should look for a high HSPF rating. Choose a system which has ratings appropriate for your heating and cooling needs. Factors that diminish quality performance are coil frost, defrost cycling under part-load conditions and use of supplemental resistance heat.

For air-to-air heat pumps currently on the market, the average HSPF rating is approximately 6.5. The most efficient system have HSPF rating of 8.8 (36 percent more efficient than the average). The average SEER rating is 8.5 and the most efficient is 13.0 - 45% more efficient than the average. The high initial cost of purchasing a more efficient system should be weighed against the long-term savings on utility bills.

In some areas, manufacturers, dealers and utilities offer incentives such as rebates or low interest loans for purchasing or installing heat pumps. Some states also offer tax incentives for energy efficient products; contact your state energy office for information.

Operating and Maintaining a System (804)

A heat pump works best in a building that is weather protected. Therefore, seal your home heating system's ducts and stop air leaks around windows. Also avoid frequently adjusting the thermostat upward. It takes longer to heat a house with a heat pump than an oil or gas furnace, and a sudden upward adjustment of the thermostat will activate the backup heater to meet the jump in demand. The resistance heater is two to three times less efficient than the heat pump, which means your energy use goes up sharply every time it comes on. The desire to suddenly raise the thermostat is compounded by the relatively cool (90 degrees F - 100 degrees F) air the heat pump produces.

Hence, one of the keys to an efficient heat pump is the proper selection and operation of a thermostat. Usually a two-stage thermostat is used for heating, and a one-stage thermostat is used for cooling. During the heating cycle, one stage of the thermostat controls the compressor and fan, while the other stage activates the supplemental heater when necessary. Some systems are equipped with an outdoor thermostat separate from the room thermostat. This limits supplemental heating and minimizes electricity demand, particularly when the room thermostat is suddenly turned up.

In case of compressor or general system failure, many thermostats have an emergency heat switch that bypasses the thermostat and activates the supplemental heater. Following a power outage, the supplemental electric resistance heater should run for a time equal to the outage to allow the heat pump's crankcase oil to reheat the liquid refrigerant.

While setting back thermostats at night reduces energy consumption in oil or gas furnaces, it is usually not recommended for heat pumps with two-stage thermostats. The sudden upward adjustment of the thermostat in the morning would activate the supplemental heater, negating overnight energy savings. Two-stage units with an outdoor thermostat can take advantage of setting back the thermostat at night. In any case, check the owner's manual for specific guidelines.

Closing air registers and vents to conserve heat is not recommended with heat pumps. The heat pump system is sized to meet the entire house's heating requirements and blocking off vents can reduce mechanical performance and efficiency.

Filters should be checked monthly for dirt build-up and cleaned or replaced as needed. The manufacturer's instruction booklet should indicate when and how to lubricate fan motors, and how to adjust the blower unit and drive belts. Indoor heat exchanger coils should be cleaned periodically with a vacuum or brush, and outdoor coils can be washed with a garden hose. Do not surround the outdoor coil with shrubs, tall grass or enclosures that would impede air flow around the coils. If the outdoor coil is exposed to the summer sun, shading it with an awning or overhang will improve the heat pump's cooling efficiency.

Defining Components

Compressor - a heat pump's central component. It pressurizes the gaseous refrigerant, raising the temperature and causing it to flow through the rest of the system. The two most common types of compressors are the reciprocating and the rotary. Both raise the pressure and temperature of the refrigerant by squeezing it with a piston.

Heat Exchangers - usually called coils, transfer heat from two physically separated fluids with different temperatures. All systems have at least two coils for circulating the refrigerant - one for condensing the hot refrigerant and one to evaporate the refrigerant when it is cool. The coils are usually arranged in snake-like fashion with fins or other protrusions to increase surface area and thereby increase heat transfer capacity.

Expansion Valve - reduces the pressure of liquid refrigerant which cools it before it enters the evaporator coil. Cooling the refrigerant allows it to absorb more heat.

Refrigerant - a fluid that boils at a very low temperature, enabling it to evaporate and absorb heat. When the refrigerant is exposed to heat, it absorbs the heat and becomes a vapor. When the refrigerant is exposed to cool air it gives up its heat and condenses into a liquid. Compressing the refrigerant makes it hotter; reducing the pressure allows it to cool.

Reversing Valve - reverses the refrigerant's direction of flow, allowing the heat pump to switch from cooling to heating or heating to cooling.

Accumulator - stores liquid and keeps it from flooding the compressor. The accumulator takes the strain off the compressor and improves the reliability of the system.