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Air-Conditioning

Cooling Basics

Heat flows from a higher temperature to a lower one. In winter, houses lose heat; in summer, they gain it. Just as heating bills benefit by reductions in heat loss, cooling loads—and bills—benefit by reductions in heat gain. Most summer heat gain comes from three places: solar radiation, internal gains, and air leakage in the building and ducts.

Where does the heat come from?

In the winter, the largest energy loads in most homes come from conductive heat loss through walls, ceilings, and windows. Air leakage in the building enclosure and ducts are a close second. Radiation effects are minimal. In hot weather the tables are turned. Conduction plays a minor role; the source of most air-conditioning loads is solar radiation through windows and uninsulated roofs. The same solarheat gain that provides free heat in winter is a major driver of air-conditioning in summer. And it’s not just south-facing windows; in fact, east and west-facing glass contribute more to air conditioning per square foot than south-facing glass. Even conductive heat gains through a poorly insulated attic are largely driven by radiation, when the sun shining on the roof superheats the attic far above outdoor temperatures. Internal gain is the name for heat that is generated inside a building by lights, appliances, and people. It is significant and can exceed all other sources besides windows. Some things that we do indoors, such as cooking, dishwashing, showering, and bathing, are obvious sources of heat. Anything that uses electricity gives off heat: refrigerators, lighting, televisions, computer equipment, and their accessories. There are less-obvious sources too, such as standby loss from water heaters. In addition to solar and internal heat gains, duct leakage in a central air-conditioning system is also a large contributor to cooling energy use. Building-shell leakage is a smaller driver, but it’s still important to seal leaks in the humid northeast to reduce the humidity load on the air conditioning.

How air conditioners work

Virtually all residential air-conditioning systems use a compressor-driven refrigerant vapor cycle to cool house air. The compressor cycle controls refrigerant temperatures in the condenser and evaporator coils, which absorb heat from indoor air at a low temperature, then reject it outdoors at a higher temperature. Air conditioners, heat pumps, and refrigerators all use the refrigerant vapor cycle to essentially push heat uphill, from a cooler place to a warmer place. An air conditioner pushes heat from the cool indoors to the hot outdoors, cooling the interior. In addition to sending indoor heat outdoors, an air conditioner must also reject the heat generated by the compressor and fan motors. Another aspect of air conditioners is their ability to dehumidify, as well as cool, indoor air. This happens when water condenses on the cold fins of the evaporator coil as humid air passes across them. Dehumidification is an important component in maintaining healthy indoor air quality and comfort. Water that is evaporated in air contains quite a bit of heat energy, which must be accounted for in the system’s design. The energy required to remove excess humidity is called latent load, and it comes from moisture sources within the home and, in humid climates, outdoor air leaking indoors.

Reducing Loads

With cooling, just as with heating, as the loads are reduced you spend less to stay comfortable. Think of the air conditioner as removing heat that builds up, rather than “cooling down” the house. The better you are at preventing heat from building up, the less heat your air conditioner must remove and the less time it will have to run—saving you money. There are two approaches to reducing cooling loads: reducing your need for cooling and reducing heat gains in your house.

Reduce the need for cooling

When you buy a ceiling fan, always choose an Energy Star model. Hundreds are available, with or without built-in efficient lights, for $75 to $225 each. Photo Credit: Bruce Harley

One surefire way to cut cooling costs is to use your air conditioner less. A house with more insulation, better windows, and less air leakage is comfortable with no air-conditioning for more of the summer. Setting the thermostat higher, particularly when the house is empty, reduces the system’s run time and saves energy.

Fans can also help raise the temperature at which you feel comfortable. By installing an efficient ceiling fan and keeping the air moving in occupied rooms, most people can comfortably increase the temperature setting of their thermostat by 4°F to 6°F, saving 20 percent to 35 percent on their cooling bills. Fan flow should point downward to maximize the feeling of air movement in the room. Oscillating and box fans can also provide portable comfort. When using any type of indoor fan, remember that it will keep people cool only while they are in the room. Turn off fans when you are not using them, because they generate heat.

Ceiling-mounted whole-house fans that exhaust into the attic are another popular method to reduce the need for cooling, but they must be used properly to be effective. Air leaks between the attic and house must be sealed, and the attic must be well vented to the exterior, to ensure that the fan exhaust goes outdoors rather than back into the house.

Duct insulation and sealing

Air-conditioning ducts have the same leakage and insulation issues as those of heating ducts. In fact, leaks and missing insulation in attic ductwork hurt air-conditioning systems more than heating systems, because during peak cooling times attics are usually much hotter than outdoor ambient temperatures. All air conditioning ducts in attics, crawlspaces, and garages should be thoroughly sealed and insulated

Maintenance and Upgrades

Like most other equipment, air conditioners need regular maintenance and service. Some things you can probably do yourself, whereas others require a professional technician.

Airflow

Like heat pumps and furnaces, air-conditioning systems depend on adequate airflow to remove heat efficiently from your house. Whether you have a central air conditioner or individual room units, check the filters regularly and clean or replace them when they are dirty—once or twice a season should be adequate. Because airflow is important, do not shut off any of the supply registers in the system—especially if the ducts are located in an attic, garage, or crawlspace.

Cleaning air coils

When filters don’t work properly or air gets around them, the evaporator coil can become clogged with dirt and dust. Depending on the system, you may need a service technician to access the coil. If you can reach it yourself, lift the dirt from behind with a stiff brush. Be careful not to drive dirt further in or bend the fins. The outdoor condenser coil should also be kept clean and free of dirt and debris. Don’t spray water directly at the coil, which may drive dirt deeper into the space between the fins. Instead, using a long-bristle brush with flexible bristles, carefully insert the bristles between the fins, and lift the dirt from behind. If you are not completely comfortable doing this, get a service technician – they have specialized tools for coil cleaning.
Window- or wall-mounted air conditioners have lower efficiency ratings than central systems, but there are no ducts to waste the cold air. And cooling only one or two rooms can use much less energy than cooling an entire house. Photo Credit: John Curtis

When filters don’t work properly or air gets around them, the evaporator coil can become clogged with dirt and dust. Depending on the system, you may need a service technician to access the coil. If you can reach it yourself, lift the dirt from behind with a stiff brush. Be careful not to drive dirt further in or bend the fins. The outdoor condenser coil should also be kept clean and free of dirt and debris. Don’t spray water directly at the coil, which may drive dirt deeper into the space between the fins. Instead, using a long-bristle brush with flexible bristles, carefully insert the bristles between the fins, and lift the dirt from behind. If you are not completely comfortable doing this, get a service technician – they have specialized tools for coil cleaning.

Regular service

Regular service calls—typically every two to three years—should include testing the controls, checking and cleaning the blower, condenser, and evaporator coils as needed, checking the insulation on the refrigerant lines, and checking the condensate drain. An initial service appointment should include testing and fixing any airflow problems, and then carefully measuring and correcting the refrigerant charge. Like a heat pump, once that has been done, service technicians should not attach refrigerant gauges to the system unless there is evidence that something is wrong. Like a heat pump, an over- or undercharged air conditioner is less efficient (particularly older models), and systems are often improperly charged. Airflow must be in the recommended range of 400 CFM to 425 CFM per ton before the refrigerant charge can be measured.

New Air-Conditioning Systems

If you have central air conditioning that is 20 years old or more, replacing your system with a new, energy-efficient model should pay for itself in a reasonable time. High-efficiency systems on the market today are more than twice as efficient as systems from the 1980s. But before running out and replacing your system, you will first need to do two other things: Finish any upgrades on the building enclosure, and become familiar with air conditioning products and installation criteria. Of course, if you have a fairly new system (less than 10 years old), it is probably more cost effective to maintain it and keep it running as efficiently as possible until it wears out. As always, energy-efficient upgrades always cost the least when you are fixing or replacing something anyway. If your old air conditioner is on its last legs or not operating, replace it with the most efficient unit you can find.

High-efficiency air-conditioning

New central air conditioners are rated for energy performance with a Seasonal Energy Efficiency Rating (SEER). SEER ratings are measurements under standardized test conditions of Btu per hour of cooling output, divided by input watts. The higher the SEER, the more efficient the system. New central air conditioners are required to have a SEER rating of at least 13; the most efficient units have SEER ratings from 16 to 23. Some of the best high efficiency systems have variable-speed compressors and fans; the lower speeds provide better, more efficient cooling and dehumidification under mild, part load conditions. When you are replacing a central air conditioning unit with a more efficient one, it’s very important to match the new outdoor unit and the indoor evaporator coil; high-efficiency variable-speed units will likely require changing the entire indoor unit as well as the outdoor unit. If you get a proposal to replace only the outdoor unit, the contractor should be able to demonstrate that the existing indoor coil is listed for use with the new outdoor unit; don’t accept “don’t worry, it’s fine the way it is” as an answer.

If you have an older central AC unit, or you currently use window air conditioners, consider installing a high-efficiency cold-climate heat pump. One or more ductless heat pumps can give you some of the highest cooling efficiencies available; they can bypass leaky, inefficient ductwork; and they can also provide very efficient heating to help reduce your existing heating bill. In some cases a central unit might make sense, if you are replacing your entire heating and cooling system and have ductwork that is in good shape (or can be fixed).

Sizing a system

Proper sizing has been emphasized as a key efficiency issue for years in the energy-efficiency world. Duct sealing and insulation in unconditioned spaces are actually more important, especially with modern, high efficiency equipment.

Oversized air conditioners do cost more to buy and maintain and require more airflow and larger ductwork. Dehumidification, which depends on air flowing through the evaporator coil for enough time to extract condensed moisture, is one important reason to carefully size an air conditioning system in humid climates. When an oversized system runs for only a few minutes, the moisture may just start to collect on the coils before the thermostat is satisfied. Once the unit shuts down, the moisture re-evaporates into the house. An air conditioner that is too large by 50 percent to 100 percent will be unable to control humidity effectively, so people may tend to set the thermostat lower to compensate. Many new, efficient air conditioners have multiple speeds or special dehumidification modes that can help compensate for oversizing; New England is a humid climate, so it’s worth considering a variable-speed AC or heat pumps system.

Correct load calculations are based on your home’s window size, the glass type, and the size and R-values of the walls and roof. The proper size also varies with the direction the home faces: Sometimes a 90-degree rotation can cut the cooling load in half. If a contractor gives you a price for an air conditioner on the phone, or simply offers to replace the existing unit with a new one of the same size, get a bid from someone else, especially if you have already made energy improvements in your house. The money you save by buying a smaller unit may pay for an efficiency upgrade.

When installing a new air conditioner, shop for value, not just for price. Other things to look for when assessing bids on new air-conditioning equipment include the warranties (from both the manufacturer and the installer), service contracts, longevity of the business, customer references, and technician knowledge. Try to find a contractor who is an Energy Star partner and/or NATE certified, or regularly attends manufacturer’s trainings.

Ductless mini-split air conditioners are another great alternative to window units. They are more efficient and quieter, and they don’t use up a window (or allow air to leak). The most efficient units on the market are also heat pumps that can save money on your heating bills, too. A mini-split costs much more than a window unit, but does much more and may cost a third to a half as much as retrofitting central air into an existing home.

All new air conditioners have an Energy Guide label that shows its efficiency rating, as well as a comparison with the ratings of other units of the same type. Room units have an EER, while central systems have a SEER. Photo Credit: John Curtis
All new air conditioners have an Energy Guide label that shows its efficiency rating, as well as a comparison with the ratings of other units of the same type. Room units have an EER, while central systems have a SEER. Photo Credit: Bruce Harley
New air-conditioning systems are more than twice as efficient as the minimum standards of 20 years ago—the best ones have almost triple the efficiency. Photo Credit: Bruce Harley

Material adapted from Build Like a Pro: Insulate & Weatherize, published by The Taunton Press, 2012; used by permission. 

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    Mansfield, MA 02048
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