A heat pump 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 just the way an air conditioner functions.
Instead of using electricity directly to heat the air, it uses electricity to move existing heat from the outside into the house. In the heating mode, the efficiency of a heat pump decreases as the outdoor air temperature decreases.
Heat flows naturally from a warm area to a cooler area. The 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 which contains very cold refrigerant (a liquid which boils at a very low temperature, as low as -15 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. As cool air passes over the coil, the vapor cools and 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 is cooled by releasing its heat into the house and 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.
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 may translate into lower utility bills. In more temperate regions of the country, installing a heat pump can reduce electricity bills as much as 35-45 percent in homes with conventional electric baseboard heating.
Heat pumps are most economical when they can be used year-round for both winter heating and summer cooling.
During the heating season, ice may form on the outdoor heat exchange coil when the temperature drops below 32F. To melt the ice, the heat pump has a defrost cycle in which the reversing valve periodically sends hot refrigerant through the outdoor coils to melt the ice. Heat pumps generally have defrost cycles lasting anywhere from two to ten minutes. During the defrost cycle supplemental electric resistance units heat the house; this reduces a heat pump's overall efficiency by three to ten percent. Steam, not smoke, rising from the evaporator (outdoor) coil can be disconcerting when experienced for the first time, but is part of the normal defrost cycle.
A heat pump may be economical where winters are relatively mild and the average temperature is above 25F. 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 25F, thus requiring frequent use of the backup heating system, it may not be economical to purchase a heat pump.
Finally, electricity costs more than fossil fuels in many parts of the country, and a heat pump costs 10 to 25 percent more to install than a conventional, fossil-fueled system with air conditioning. Where fossil fuels are comparatively expensive, a heat pump may be an economical way to heat and cool your home.
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 50F most of the year on the average.
Ground-coupled 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.
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 to the point where it becomes less expensive to run the fossil fuel system. At this point, the thermostat automatically turns off the heat pump and activates the furnace.
Heat pumps come in different types and sizes, ranging from window units to large commercial and industrial units. A utility company representative may be able to recommend sources and references on the subject of sizing a heat pump to heat and cool a house or building efficiently. 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. 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.
For air-to-air heat pumps currently on the market, the minimum HSPF rating is approximately 6.8. The minimum SEER is 10. In some areas, manufacturers, dealers and utilities offer incentives such as rebates or low interest loans for purchasing or installing high efficiency heat pumps. Some states also offer tax incentives for energy efficient products; contact your state energy office for information. Maryland currently does not offer any tax incentives.
A heat pump works best in a building that is weather protected. Therefore, seal and insulate 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 energy use goes up sharply every time it comes on. The desire to suddenly raise the thermostat is compounded by the relatively cool (90F-100F) air the heat pump produces.
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.
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 or adaptive recovery thermostats, which automatically raise the setting less than 2 at a time, can take advantage of setting back the thermostat at night or during the day. 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. Also make sure the vents are not blocked by furniture, drapes or other obstructions.
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. It is a good idea to contact a qualified, licensed contractor every 12-18 months to perform routine maintenance.
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. Variable speed, scroll, and screw compressors are recent and more efficient innovations.
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. As 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.
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