|
|
EnergySmart Guide to Cooling Systems
Energy Efficiency of Central Air Conditioning Systems
Efficiency Losses
Upgrading Central Air Conditioners
Zoning
When to Replace an Air Conditioning System
Factors to Consider When Replacing an Air Conditioning System
Sizing of Air Conditioning Systems
Cooling Load Calculations
Oversizing
Room Air Conditioners
Heat Pumps
Evaporative Cooling
System Maintenance
New Technologies
Central Air Conditioning
Central air conditioning uses a considerable
amount of electricity, particularly in warmer climates,
and is one of the most expensive home appliances to operate.
The difference between central air conditioning and a room (or window)
air conditioner is the distribution system. A central air conditioner
uses a blower and ductwork to distribute chilled, dehumidified air throughout
the entire house. In contrast, a room air conditioner,
typically installed in a window or, in some cases, through a wall,
will cool a small area, generally a single room or perhaps two to three adjacent rooms.
Central air conditioning is becoming much more common in this country, and is now present
in over 47% of existing homes and over 77% of new homes!
Standard central air conditioners consist
of three main parts: a condenser, an evaporator,
and a compressor. The compressor and condenser are often
integrated into a single unit that is typically installed
outside the home but occasionally in an attic or on the roof.
The evaporator is the cooling coil, normally installed in the
air handling system. A refrigerant is circulated between these components.
The air handler blower draws air from various parts of the home through
return air ducts. The air then passes over the evaporator coil which
has been chilled by the refrigerant. Heat is absorbed from the air,
and moisture is drained away. The conditioned air is then routed back
to the rooms through the supply air ductwork. The refrigerant,
which is converted to a vapor by the heat, is pressurized by
the compressor and moved to the outside coil, or condenser, where the heat
is dissipated. As the heat is removed by the condenser, refrigerant turns
back into a liquid and then is sent (by way of the copper pipes)
to a metering device (that is usually inside of the cooling coil cover).
And the process starts over again.
Energy Efficiency of Central Air Conditioning Systems
The efficiency of central air conditioners is measured in a Seasonal
Energy Efficiency Rating (SEER). Government standards currently require a minimum SEER of 10. Unfortunately, manufacturers can label such units "high efficiency", even though they are the minimum allowed by law. Units with a SEER of 11-12 are typically labeled "super high efficiency", while those with SEER ratings over 12 are called "ultra-high". In fact, central air conditioners are now made with SEER ratings of 16 or more.
Efficiency Losses
Unfortunately, the rating on the unit does not tell the whole story. Several factors can have a dramatic impact on the efficiency of (and hence the cost to operate) an air conditioning system. Duct leakage, particularly in the attic, garage or crawlspace, can lead to losses of 30% or more! Unless the duct system was specially sealed, it is very likely that it leaks. Even in brand new systems, the majority leak a significant amount of air where the pieces are assembled. Other problems include improper refrigerant charge, low air flow, dirty indoor and outdoor coils, and recirculation. See the Upgrade and Maintenance sections for more detail on solving some of these problems. Even something as simple as a system that is larger than needed can cost more to run: see the section on replacement systems for more on proper A/C sizing.
Upgrading Central Air Conditioners
Duct insulation and sealing:
Central air conditioning ducts often suffer from air leakage through cracks in the ducts where the parts are assembled. Older ductwork may not be insulated well. Insulating ducts with insulation wrap, and sealing air leaks in ducts, can be among the most effective energy saving strategies. Air leaks are sealed using duct mastic (not tape!) wherever leaks are accessible, in attics, garages, crawlspaces, etc, and should be sealed before adding insulation wrap. A special aerosol duct sealing system is also available in some areas to facilitate sealing leaks in inaccessible ductwork. Even newer systems, that may already be insulated, may be very leaky so don't assume they are tight just because they are insulated. It is best to have a weatherization or HVAC professional who has a duct leakage testing device test your ductwork to be sure.
Zoning
The addition of zones in central air conditioners is not usually recommended. This is because air flow across the indoor coil is critical to efficient operation, and zoned systems reduce airflow when only some zones are calling for cooling. Since the efficiency is reduced, there really is not much energy savings. If multiple zones are desired in the home, it is better to install two or more small central air systems to provide independent control of different areas.
When to Replace an Air Conditioning System
If you are the owner of an older air conditioning system, you may be wondering if it will last another year. You are concerned that it may stop working in the middle of the summer leaving you and your home hot and soggy. If this is the case, it's probably time to call on an expert to consider replacement or repair?
Home air conditioning systems can be expected to last for 15 years or more. Good maintenance practices will, of course, provide an even longer service life. Deciding when to replace an old cooling system is not easy. Unless your present system is old and in very poor working condition, it may be hard to justify a new high efficiency system on energy savings alone. If your system doesn't seem to be working as well as it used to, you should have a service person look at it to determine the likely cause of the problem. If there is a significant problem, simply comparing the price to repair the system with the cost of replacement will give you a good idea of what you should do. Keep in mind, however, that, due to improvements in efficiency, a new system will have the added benefit of reducing your operating cost. If your system is over 10 years old, you may want to consider a new unit. Especially if you have a large home with a high cooling load, the annual dollar savings from installing a new system may pay for itself in a short time.
Home energy analyzer will help you to determine how much you will save if you improve the efficiency of your air conditioning system.
Factors to Consider When Replacing an Air Conditioning System
Before you purchase a new system you should consider improving the efficiency of your home by adding insulation and sealing air leaks. This may have an impact on the size (and cost) of the new air conditioning system.
When planning for the replacement of your central air conditioning system, you will have an opportunity to make some efficiency improvements. You must first consider the tradeoff between the efficiency and cost of the new system. Furthermore, if your outdoor (condenser) unit is mounted on the roof or in an attic, you may want to move it to a cooler location when you install a new unit. That alone can make a large improvement in efficiency. If you are thinking of just changing the outdoor unit; keep in mind that the indoor unit may be just as old. Changing the outdoor unit to a newer high efficiency style may not give you the results you are paying for due to the mismatching of SEER ratings and age of equipment between units.
Sizing of Air Conditioning Systems
Central air conditioning system sizes are measured in tons. One ton of cooling is 12,000 Btu's per hour, which is equivalent to the rate of cooling produced by one ton of ice melting over the course of a day. The larger the home, the larger the air conditioning unit that will be needed. Correct sizing is very important; an air conditioner that is too large will cycle on and off and be much less efficient. On the other hand, if it is too small it may not produce sufficient cooling.
Cooling Load Calculations
If you are installing a new air conditioning system or replacing an existing one, any contractor who is bidding on the job should perform a detailed cooling load calculation in order to select the proper system. They should not base their recommendation on simple "rules of thumb" which tend to oversize the equipment, and they should not simply replace the system with a new one of the same size. This is because all houses are different, the existing air conditioning system may have been oversized to begin with, and the house may have had insulation added or new windows installed since the original system was put in.
The contractor should measure your home to determine size of windows, the direction they face, the type of glass , the size of each room, the areas of walls, ceilings and the amount of insulation. Preferably, they should do a room-by-room calculation to determine whether the existing ducts are adequate for the new system. In most cases, the time it takes them to perform these calculations should be offset by the savings in a smaller installed system.
Oversizing
Bigger is NOT necessarily better! Oversizing is common, because it is a way of compensating for potential distribution problems such as uninsulated or leaky ductwork. A new system will be much more efficient to operate if those problems are fixed first. An air conditioner that is too big will cycle on and off much more often, resulting in "blasts" of cold air, reducing efficiency, and increasing stress on components. In humid climates, oversized air conditioning will do a poor job of lowering the humidity, which is just as critical to comfort as the temperature setting in the room! In fact, a slightly under-sized air conditioner will be just as comfortable-if not more-than an oversized air conditioner.
Room Air Conditioners
Room air conditioners are generally sized in BTUs (this actually means Btu's per hour). One ton is equivalent to approximately 12,000 Btu's per hour. Rough guidelines for sizing a unit are presented below. Energy efficiency of room air conditioners is measured as an Energy Efficiency Rating (EER). New room air conditioners must have a minimum EER of about 8.6, depending on the size of the unit..
|
|
Rough Sizing of Room Air Conditioners
|
Area to be cooled
(sq.ft.) |
Air conditioner size
(BTUs) | |
100-250 | 5,000-6,000 | |
250-400 | 6,000-8,500 | |
400-550 | 8,500-11,000 | |
550-900 | 11,000-15,000 | |
|
|
Heat Pumps
A heat pump is basically a central air conditioner that can be reversed in the winter to heat the home. All of the efficiency, upgrade and maintenance issues that apply to air conditioners apply equally to heat pumps, except that the operating cost impact is larger for a heat pump because it is supplying both heat and cooling to the house year round, rather than just one or the other. Heat pumps also use electric heaters to help the system when outdoor temperatures are very low.
Evaporative Cooling
Evaporative coolers can be excellent, highly efficient air conditioning units in some situations. They have a large blower which draws outdoor air through a wetted pad or media. The air is cooled as it flows through the pad and some of the water in the pad evaporates. The cooling takes place because the energy of evaporation takes heat out of the air. Evaporative coolers only work in hot, dry climates, because they add moisture to the air. In a humid climate there is too much moisture in the air already so there would be little benefit to evaporative cooling. Also, in hot, humid weather, dehumidification (removing moisture) is equally or more important than chilling of the air, so evaporative coolers don't work.
Evaporative coolers are less expensive than air conditioners, and much more efficient, but they do not cool the house as well. Some homes in hot, dry climates have both an evaporative cooler for most of the summer, with a central or room A/C for the really hot days.
System Maintenance
Central Air Conditioners:
Proper maintenance of an air conditioner will improve its efficiency and extend its life. This should be done at least every other year and it will require the services of a professional. The service call will involve a general tune-up, inspection, and cleaning of the system. Every tune-up should include an air filter change, inspection of the inside and outside coils (and cleaning if necessary), a check for proper air flow, and a check of all electrical connections.
In addition, the first time your air conditioner is serviced, the technician should check the refrigerant levels by testing something called "superheat" or "subcooling" (depending on the type of unit). Once they verify the proper refrigerant charge, no service person should attach refrigerant gauges again, unless you notice a decrease in performance or a sudden increase in your electric bills.
Also have the technician show you how to change the air filter. This should be done every month during which the system is operating. Fiberglass filters are the least expensive option. As an upgrade from this, we recommend a 1" pleated type filter for most applications. High efficiency filters do not need to be replaced as often, but still should be checked at the beginning of the cooling season and replaced if dirty. Always follow the manufacturers instructions when replacing filters.
When choosing an air filter, you have to be concerned about both filtering effectiveness and resistance to air flow. "Washable" electrostatic air filters create a lot of resistance which can reduce air flow and operating efficiency. On some systems, depending on the size of the ductwork, even a pleated filter can cause air flow problems. This can be checked by observing air flow with one and then the other filter in place.
Always keep the air vents open in a central air conditioning system, and don't obstruct them with furniture or draperies. In addition, do not attempt to cover or shade the outdoor coil; even a deck built above the unit, with open sides, can cause recirculation and reduce efficiency dramatically. Keeping leaves and dirt from accumulating on the sides of the outdoor unit helps to maximize heat transfer and minimize cooling bills.
Room Air Conditioners:
Regular maintenance of a room air conditioner not only improves efficiency but also extends the useful life of the unit. As warm weather approaches, the two things to check are the air filter and the cooling fins.
- Air Filter : Replace or clean if it is dirty and continue to check it every month. Replace or clean as necessary.
- Cooling Fins : Found on the back and front of the air conditioner, the cooling fins are a series of thin metal sheets with a very small gap between them. The cooling fins should be evenly spaced, but often they get damaged and rub up against one another. To separate the fins a fairly inexpensive but essential tool is a fin comb, available at most refrigeration parts stores and some building supply stores.
If you notice the performance dropping off and the filter and fins are okay, then take the unit to a repair shop for service.
Heat pumps:
Proper maintenance should be performed by a professional service technician at least every other year. The service call will involve a general tune-up, inspection, and cleaning of the system. Every tune-up should include a filter change, inspection of the inside and outside coils (and cleaning if necessary) and a check for proper air flow. In addition, the first time your heat pump is serviced, the technician should check the refrigerant levels by testing something called "superheat" or "subcooling" (depending on the type of unit). Once they verify the proper refrigerant charge, no service person should attach refrigerant gauges again, unless you notice a decrease in performance or a sudden increase in your electric bills.
In addition, have the technician show you how to change the filter. This should be done every month when the system is operating. We recommend a 1" pleated type filter for most applications, as an upgrade from the cheapest fiberglass filters. "Washable" electrostatic air filters create a lot of resistance which can reduce air flow and operating efficiency.
New Technologies
Gas-fired Heat Pumps:
Gas-fired heat pumps use thermal energy from a natural gas burner rather than an electric compressor motor to drive the refrigerant cycle that provides heating and air conditioning in buildings. This technology may save 20-50% or even more over conventional air conditioners and heat pumps, but is not widely available at this time.
|
|
