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Poor installation, duct losses, and inadequate maintenance are more of a problem for heat pumps than for combustion furnaces. A growing body of evidence suggests that most heat pumps have significant installation or service problems that reduce performance and efficiency. According to a report on research funded by Energy Star more than 50 percent of all heat pumps have significant problems with low airflow, leaky ducts, and incorrect refrigerant charge.

Increasing airflow in central heat pumps: The capacity and the efficiency of a heat pump depend upon adequate airflow. There should be about 400 to 500 cubic feet per minute (cfm) airflow for each ton of the heat pump's air-conditioning capacity. Efficiency and performance deteriorate if airflow is much less than 350 cfm per ton.

An ideal duct system has both a supply register and a return register for every room. Most homes, however, have only one or two return registers for the entire house. Air from other rooms must find its way back to these registers to be reheated or re-cooled. Obstructions in return air are a common air circulation problem, particularly from closed interior doors to rooms with no return-air register.

Blockage of supply or return air ducts and registers can pressurize or depressurize portions of the home, resulting in poor performance and increased air leakage through the building envelope. Restrictions to airflow have the greatest impact on the return-air side of the system, so repairs should start with the return ducts.

Air from every supply register must have an unobstructed pathway back to a return register. You can install louvered grilles through walls or doors, ducts between rooms, and/or additional return ducts and registers to improve air circulation.

Technicians can increase the airflow by cleaning the evaporator coil, increasing fan speed, or enlarging the ducts-especially return ducts. Enlarging ducts may seem drastic but in some cases, might be the only remedy for poor comfort and high energy costs.

Air-sealing ducts: Measurements of heat pump performance indicate that duct leakage wastes 10 to 30 percent of the heating and/or cooling energy in a typical home. It's one of the most severe energy problems commonly found in homes because the leaking air is 20° to 70°F warmer than indoor air in winter and 15° to 30°F cooler in the summer.

Duct leakage may cause some minor comfort problems when ducts are located in conditioned areas. But when leaky ducts are located in an attic or crawl space, the energy loss is often large. Some of the worst duct leakage occurs at joints between the air handler, and the main supply and return air ducts. Some main return ducts use plywood or fiberglass duct-board boxes. These boxes frequently leak because their joints are exposed to the duct system's highest air pressures. Heating and air-conditioning contractors often use wall, floor, and ceiling cavities as return ducts. These building-cavity return ducts are often accidentally connected to an attic, crawl space, or even the outdoors, creating serious air leakage. Fiberglass ducts and flex ducts are often installed improperly. These ducts may also deteriorate with age, leading to significant supply-duct leakage.

The best heating and cooling contractors have equipment to test for duct leakage. Testing helps locate duct leaks and indicates how much duct sealing is necessary. Do not use duct tape for sealing-its life span is very short, often less than 6 months.

Adjusting refrigerant charge: Room heat pumps and packaged heat pumps are charged with refrigerant at the factory. They are seldom incorrectly charged. Split-system heat pumps, on the other hand, are charged in the field, which can sometimes result in either too much or too little refrigerant.

Split-system heat pumps that have the correct refrigerant charge and airflow usually perform very close to manufacturer's listed SEER and HSPF. Too much or too little refrigerant, however, reduces heat-pump performance and efficiency.

For satisfactory performance and efficiency, a split-system heat pump should be within a few ounces of the correct charge, specified by the manufacturer. When the charge is correct, specific refrigerant temperatures and pressures listed by the manufacturer will match temperatures and pressures measured by your service technician. Verify these measurements with the technician. If the manufacturer's temperatures and pressure's don't match the measured ones, refrigerant should be added or withdrawn, according to standards specified by the EPA.

Refrigeration systems should be leak-checked at installation and during each service call. Manufacturer's say that a technician must measure airflow prior to checking refrigerant charge because the refrigerant measurements aren't accurate unless airflow is correct.

Operating a heat pump: Like combustion heating systems, you control heat pumps using thermostats. If you leave and return at regular times everyday, you'll save money by using automatic thermostats, which minimize energy use during the times the home is unoccupied. However, choosing an automatic thermostat's reactivation time requires considering the duration of heat-pump operation necessary to restore a comfortable temperature. During the heating season, some homeowners also set their thermostats back 10°F, manually or automatically, when they leave home or go to bed.

A two-stage thermostat controls the heating. The first stage activates the refrigeration system. If it's too cold outside for the refrigeration system to counteract the home's heat loss, then the thermostat's second stage activates the electric resistance coils. An outdoor thermostat will prevent the less efficient electric resistance heat from coming on until the outdoor temperature falls below 40°F. An outdoor thermostat also will prevent auxiliary heat from activating when an automatic thermostat is warming the house after a set-back period. Use setback thermostats that are only for heat pumps.

A defrost control tells the reversing valve when to send hot refrigerant outdoors to thaw the outdoor coil during the winter. During the 2-to-10-minute defrost cycle, auxiliary heat takes over, reducing the heat pump's overall efficiency up to 10 percent. The two most common types of defrost controls are time-temperature and demand-defrost. Time-temperature defrost controls activate defrost at regular time intervals for set time periods, whether there is ice on the outdoor coil or not.

A demand-defrost control senses coil temperature or airflow through the coil, and only activates defrost if it detects the presence of ice. Obviously, choosing a heat pump with demand-defrost will pay a significant efficiency dividend.

For greater efficiency, don't locate a thermostat near a heat source or cold draft because they can cause a heat pump to operate erratically. This includes shading thermostats from direct sunlight. Also, do not turn the thermostat beyond the desired temperature. It will not make the heat pump heat or cool your home any faster. It will only waste energy. Residents who duel one another over the thermostat settings, moving it up and down to suit their different comfort levels, cause heat pumps to operate erratically and inefficiently.

Maintaining and Servicing: Heat-pump performance will deteriorate without regular maintenance and service. The difference between the energy consumption of a well-maintained heat pump and a severely neglected one ranges from 10 to 25 percent.

  • Regular Maintenance: Either the homeowner or service technician can perform the following routine maintenance tasks:

  • Clean or replace filters regularly (every 2 to 6 months, depending on operating time and amount of dust in the environment).

  • Clean outdoor coils as often as necessary (when dirt is visible on the outside of the coil).

  • Remove plant life and debris from around the outdoor unit.

  • Clean evaporator coil and condensate pan every 2 to 4 years.

  • Clean the blower's fan blades.

  • Clean supply and return registers and straighten their fins.

  • Professional Service: You should have a professional technician service your heat pump at least every year. The technician can:

  • Inspect ducts, filters, blower, and indoor coil for dirt and other obstructions.

  • Diagnose and seal duct leakage.

  • Verify adequate airflow by measurement.

  • Verify correct refrigerant charge by measurement.

  • Check for refrigerant leaks.

  • Inspect electric terminals, and if necessary, clean and tighten connections, and apply nonconductive coating.

  • Lubricate motors, and inspect belts for tightness and wear.

  • Verify correct electric control, to be sure heating is locked out when the thermostat calls for cooling and vice versa.

  • Verify correct thermostat operation.

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