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More Than You Ever Wanted To Know About Heat Pumps... How does a heat pump work? If you understand how air conditioners work, then you are familiar with the basic process that allows an air conditioner to concentrate cold in one set of coils (inside the house) and heat in another set (outside the house). Imagine that you took an air conditioner and flipped it around so that the hot coils were on the inside and the cold coils were on the outside. Then you would have a heater. It turns out that this heater works extremely well. Rather than burning a fuel, what it is doing is "moving heat." A heat pump is an air conditioner that contains a valve that lets it switch between "air conditioner" and "heater." When the valve is switched one way, the heat pump acts like an air conditioner, and when it is switched the other way it reverses the flow of Freon and acts like a heater.Heat pumps can be extremely efficient in their use of energy. But one problem with most heat pumps is that the coils in the outside air collect ice. The heat pump has to melt this ice periodically, so it switches itself back to air conditioner mode to heat up the coils. To avoid pumping cold air into the house in air conditioner mode, the heat pump also lights up burners or electric strip heaters to heat the cold air that the air conditioner is pumping out. Once the ice is melted, the heat pump switches back to heating mode and turns off the burners. Selecting a Heat Pump When selecting an air-source heat pump, consider the following three characteristics carefully: the energy efficiency rating, sizing, and the system's components.
You'll find the Energy Star® label—sponsored by
the U.S Department of Energy (DOE) and the U.S. Environmental Protection
Agency (EPA)—on heat pumps with an HSPF of at least 7 and a SEER
of at least 12. Many new heat pumps exceed these ratings, but looking
for this label is a good way to start shopping for one. Sizing When selecting a new heat pump, it's important that you determine the proper size needed for your home. Bigger is not better. Oversizing causes the heat pump to start and stop more frequently, which is less efficient and harder on the components than letting it run for longer cycles. A properly sized heat pump also will provide you with better comfort and humidity control than an oversized one. The heating and cooling capacity of heat pumps is measured in Btu per hour. The cooling capacity is commonly expressed in "tons" of cooling capacity—each ton equaling 12,000 Btu per hour. Correct sizing procedures involve complex calculations, which are best performed by an experienced contractor, who uses sizing methods accepted by the heat pump industry. Don't employ a contractor who guesses the size of the heat pump needed. Rule-of-thumb sizing techniques are generally inaccurate, often resulting in higher than necessary purchase and annual energy costs.
You and your contractor should discuss options that will
help improve your home's comfort and the economy of your heat pump. Regarding
ducts, for example, it's important to carefully consider their design
and materials, as well as the proper amount of space they require. Check
your home's blueprints to see if the architect and builder have planned
adequate space for ducts and fans. Heating and cooling contractors complain
that they often have to squeeze heating and cooling systems into spaces
that are too small, resulting in constricted ducts and inadequate airflow.
Except for packaged systems, you'll also need to select the proper type
of indoor coil for adequate summer moisture removal. Improving Performance 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.
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. 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. 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.
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