Air Conditioning

For Central Systems, Look for High SEER Score

UPDATED 5/30/2012

Bird's-Eye View

Getting the size right makes it greener

Home air conditioners come in many sizes, but all of them work basically the same way. A closed loop of refrigerant picks up heat from indoor air in an evaporator coil and sheds the heat outside in a condenser coil. An electrically driven compressor turns the refrigerant from a gas into a liquid so it can once again absorb indoor heat.

An air conditioner cools and dehumidifies air at the same time, a major advantage.

See below for:
ABOUT MINI-DUCT SYSTEMS
ABOUT ROOM AIR CONDITIONERS
ABOUT MINI-SPLITS


Key Materials

Most residential systems are split

Central air conditioners can be one of two types:

Split systems place the compressor and condenser outside, usually on a small concrete slab, and the evaporator coil inside, in the furnace plenum or air handler. The same duct system used to distribute heat also handles air conditioning. Split systems are the most common type of central air conditioning system in U.S. homes.

Mini-split systems are a cross between central air conditioning and window units. The evaporator and air handler are located inside while the compressor and condenser go outside, just as they would with central air conditioning.


Design Notes

Right-sized systems are more efficient to install and to run

HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. system designers must calculate the home's cooling load and duct sizes correctly. The use of guidelines published by the Air Conditioning Contractors of America — Manual J for cooling loads and Manual D for duct sizing — are mandated by most building codes.

Many contractors, however, are still designing systems by rules of thumb — in other words, taking an educated guess. They may be tempted to install systems that are too large, an unnecessary expense. Oversize air conditioners also cycle on and off frequently, reducing the unit's ability to dehumidify indoor air and in some cases decreasing efficiency.


Builder Tips

Seal ducts, and keep in conditioned space

For top efficiency, ducts should be located within the conditioned space and have mastic-sealed joints. An uninsulated attic is the worst place to run air conditioning ducts. If they must be located there, ducts should be extremely well insulated as well as carefully sealed. The system should be charged with refrigerant exactly to the manufacturer’s specifications.

The Code

Calculations, not guesswork, to size equipment

Oil Tank FigureClick for slide show
The International Residential Code includes several sections with requirements for air conditioning equipment.

General requirements for air-conditioning equipment are found in section G2449.

Section M1403.3 requires that "Heating and cooling equipment shall be sized based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies."

Section M1411.2 requires that cooling coils installed in the supply plenum of a warm-air furnace shall be rated at not less than 0.5 inch water column (124 Pa) static pressure unless the furnace is listed for use with a cooling coil.

Section M1411.3 includes requirements for condensate pans and drains.

Section M1601.4 requires that "Duct systems serving heating, cooling and ventilation equipment shall be fabricated in accordance with the provisions of this section and ACCA Manual D or other approved methods."

According to Section N1101.8 of the IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code., the type and efficiency of any cooling equipment must be noted on a permanent certificate installed on a home's electrical distribution panel.


Illustration: Code Check HVAC 2nd Edition. click to buy

OTHER CONSIDERATIONS

Old timers have a point

Southerners of a certain age often say, "We didn't have any mold problems in the old days." They're right: without an air conditioner, a Southern house will have no cold surfaces inside walls or ceilings where condensation can form.

One easy way to design a mold-resistant house is to omit air conditioning. That's green.

SHADE MYTH

Study deflates popular advice

There is little factual basis for the oft-repeated advice to locate the condensing coil and compressor of a central air-conditioning system in the shade. A two-year study by the Florida Solar Energy Center found energy savings are minuscule or nonexistent.

The reason is simple: to shed heat, lots of air has to move over the condensing coil. Any temperature difference between air in the shade and air in direct sunlight quickly becomes irrelevant.

COMFORT FACTS

Getting the most out of AC

Setting the thermostat lower than needed doesn’t help the room cool off any faster and can reduce air-conditioner efficiency.

Window-mounted air conditioners should be operated with the AC fan on high for best efficiency on all but the most humid days. When humidity levels are high, however, use a low fan setting for improved dehumidification.

Using an interior fan in conjunction with air conditioning will increase the evaporation of perspiration from the skin, making it seem cooler.

DOING DOUBLE DUTY

Ductless systems for heat, too

Designers of near-zero-energy homes are beginning to specify ductless mini-splitA type of small-capacity heat pump (as little as a ton or even less) with a closely-associated outside compressor and inside evaporating coil (often through-the-wall in design). These heat pumps often come with variable-speed compressors and blowers,giving them excellent modulation for thermal comfort. These features also contribute to COPs of around 4 for ductless min-split heat pumps. They are also well-suited for ultra-high performance, small-volume homes. systems for both heating and cooling.

In Townsend, Mass., builder Carter Scott recently completed a near-zero-energy house with a peak heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. of only 10,500 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h. The load was easily satisfied by a Mitsubishi Mr. Slim ductless mini-split unit at a total cost of only $5,250.

AIR-CONDITIONED HOMES MAY NEED A DEHUMIDIFIER

In hot, humid climates, a stand-alone dehumidifier keeps indoor air dry

Builders of energy-efficient homes in hot, humid climates are often surprised to learn that improvements to a home’s thermal envelope can make the interior of the home more humid. Although the explanation for the phenomenon is somewhat convoluted, the solution is simple: High-performance homes in hot, humid climates need a stand-alone dehumidifier in addition to an air conditioner.

A home with above average levels of insulation and low-solar-gain windows will require less cooling than a conventional home. In hot, humid weather, the air conditioner runs so infrequently that it doesn’t have a chance to wring any moisture from the interior air, so interior moisture levels remain high.

The problem was first noticed in a cluster of Pulte homes in Houston. Built to exacting Building America standards, the homes had mechanical ventilation systems (central-fan-integrated supply ventilation), thick insulation and excellent windows. Yet indoor humidity levels were higher than in comparable new homes without high performance features.

Called in to investigate the problem, Armin Rudd, principal engineer at the Building Science Corporation, conducted a study to compare proposed solutions. The least effective equipment tested was a $1,450 energy-recovery ventilator (ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.), which had only a minimal effect on indoor humidity. The most effective solution was the least expensive — the installation of a stand-alone dehumidifier in a hall closet equipped with a louvered door. Rudd estimated that the incremental cost associated with installing the dehumidifier and a dedicated drain was only $500.

A report on Rudd’s research appeared in the January 2003 issue of Energy Design Update. According to EDU, “The study laid to rest the theory that the humidity problems were caused by ventilation equipment which introduced added moisture along with ventilation air. ‘All of the systems with dehumidification of recirculated air separate from the cooling system exhibited much better humidity control than those with dehumidification of ventilation air only and those with dehumidification only as part of the cooling system,’ noted Rudd. ‘Therefore, the problem does not lie with mechanical ventilation, and the solution does not lie with the cooling system. The problem of elevated humidity in energy-efficient homes in hot humid climates is a result of interior moisture generation and lowered sensible heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss.. The solution, for now, is to employ dehumidification separate from cooling for hot humid locations.’ ”

For more information on Rudd’s study, visit www.buildingscience.com/documents/reports/rr-0215-dehumidification-syste....

DRAIN CONDENSATE PANS

Standing water can lead to bacteria and mold growth

When drains are set at the lowest point of the pan, water should be able to drain freely away as the air conditioner condenses it out of the air. Drain lines should be kept clear of obstructions. Waterless traps are available that block the passage of air in either direction but don't impede the flow of condensate.

GREEN POINTS
LEED-H Cooling approach potentially affects up to 4 points in EA1 (Energy & Atmosphere) or EA6.

NGBS Under Chapter 7 — Energy Efficiency: up to 24 points for cooling efficiency of central air conditioning system (703.4.5).

ABOUT CENTRAL AIR CONDITIONING

Cooler, drier air and better energy performance

Air conditioners are significantly more efficient than they were in the 1970s and are now capable of producing the same amount of cooling with as much as 50% less energy, the government’s energy office says.

Watch the numbers. An air conditioner's seasonal energy efficiency ratioSeasonal Energy Efficiency Ratio (SEER) is the total cooling output (in BTU) of an air conditioner or heat pump during its normal annual usage period divided by its total energy input (in Watt-hours) during the same period. The units of SEER are Btu/W·h. SEER measures how efficiently a residential central cooling system operates over an entire cooling season. The relationship between SEER and EER depends on location, because equipment performance varies with climate factors like air temperature and humidity. (SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. ) is key. That's the total cooling capacity (in BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. ) compared with total electrical input (in watt-hours) over the course of a cooling season.

The higher the SEERSeasonal Energy Efficiency Ratio (SEER) is the total cooling output (in BTU) of an air conditioner or heat pump during its normal annual usage period divided by its total energy input (in Watt-hours) during the same period. The units of SEER are Btu/W·h. SEER measures how efficiently a residential central cooling system operates over an entire cooling season. The relationship between SEER and EER depends on location, because equipment performance varies with climate factors like air temperature and humidity., the higher the energy efficiency. Minimum federal standards were increased in 2006 to SEER 13. Minimum SEER for an Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. rating is 14, but units with ratings of over 20 are available.

Get the right size. Output is measured by the ton, equal to 12,000 Btu/h. Residential systems are typically sized between 1 and 5 tons.

Some HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractors oversize air-conditioning equipment just to make sure the house stays cool. But calculating the cooling load and buying the right capacity lowers purchase and operating costs.

Help the ozone layer. Chlorofluorocarbons, or CFCs, were widely used as a refrigerant until scientists discovered how much damage the chemicals did to the Earth’s ozone layer. They have been replaced by hydrochlorofluorocarbons (HCFCs) like R-22, which have substantially less ozone-damaging potential.

These, too, are gradually being phased out in favor of refrigerants that have no ozone-damaging potential, such as R-410a, which can be specified in some AC equipment.

More information. For an in-depth technical introduction to the nuts and bolts of air conditioners, see Air Conditioner Basics.

ABOUT MINI-DUCT SYSTEMS

Small ducts ideal for retrofits

Mini-duct air-conditioning systems distribute cool air through small-diameter ducts at high velocity. They’re designed especially for retrofits where it would be difficult to install full-size ductwork.

Mini-duct systems are essentially the same as conventional air-conditioning systems. But ducts as small as 2 inches in diameter are much easier to snake through walls and floors than full-size ductwork, minimizing demolition and repairs.

More effective cooling. Manufacturers say mini-duct systems thoroughly mix air for even cooling (or heating) and claim they remove 30% more moisture from the air than conventional systems. That may allow higher thermostat settings for the same level of comfort.

Mini-duct systems are available in capacities up to 5 tons (60,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. ). The refrigerant R-410a, which is not harmful to the Earth’s ozone layer, can be specified.

Lower efficiencies. The most efficient air-conditioning units are unlikely to be available for use with mini-duct systems.

SpacePak, a company specializing in mini-duct systems, says the best SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. rating now available is 13, which became the government’s minimum standard in 2006 and is well short of the SEERSeasonal Energy Efficiency Ratio (SEER) is the total cooling output (in BTU) of an air conditioner or heat pump during its normal annual usage period divided by its total energy input (in Watt-hours) during the same period. The units of SEER are Btu/W·h. SEER measures how efficiently a residential central cooling system operates over an entire cooling season. The relationship between SEER and EER depends on location, because equipment performance varies with climate factors like air temperature and humidity. 23 rated systems that are currently available.

According to the Air Conditioning, Heating and Refrigeration Institution, air conditioning systems must have a minimum SEER rating of 14 to quality for an Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. rating.

ABOUT ROOM AIR CONDITIONERS

Window units may be more economical

If cooling only a few rooms is enough, running one or two window-mounted air conditioners is less expensive than operating a central air conditioner.

Energy efficiency ratings. Room air conditioners are rated by an energy efficiency ratio, or EEREnergy-efficiency rating or energy-efficiency ratio. As most commonly used, EER is the operating efficiency of a room air conditioner, measured in Btus of cooling output divided by the power consumption in watt-hours; the higher the EER, the greater the efficiency.. This is the output in BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. divided by the power it takes to operate the device in watts.

The seasonal energy efficiency ratioSeasonal Energy Efficiency Ratio (SEER) is the total cooling output (in BTU) of an air conditioner or heat pump during its normal annual usage period divided by its total energy input (in Watt-hours) during the same period. The units of SEER are Btu/W·h. SEER measures how efficiently a residential central cooling system operates over an entire cooling season. The relationship between SEER and EER depends on location, because equipment performance varies with climate factors like air temperature and humidity. (SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. ) used to describe central air conditioning equipment is similar. But EER measures performance at a high peak load rather than as a seasonal average. The higher the EER, the more efficient the appliance.

Improving performance. National standards require room air conditioners to have a minimum EER of 8.0. The American Council for an Energy-Efficient Economy recommends buying a model with an EER of at least 10.8. Replacing a 1970s-era air conditioner with one that efficient will cut energy consumption in half.

A room air conditioner can have a lower EER (efficiency) than a central air conditioner and still save energy if fewer rooms are kept cool.

Sizing a unit. Based on room size alone, the Department of Energy recommends an output of 20 Btu for every square foot of floor space to be cooled. But ceiling height, shading, window size and insulation also should be considered and may affect a sizing decision.

An air conditioner that runs for extended periods of time is more efficient, and will do a better job of dehumidification, than an oversize air conditioner that runs in short bursts.

Wiring considerations. Smaller window-mounted units, those drawing 7.5 amps or less, probably can be plugged into a standard 115V circuit. Some larger models require 230V.

It's best to plan ahead for window-mounted air conditioners since larger units may need a dedicated electrical circuit.

ABOUT MINI-SPLITS

Air conditioning without the ducts

Mini-split systems are a cross between central air conditioning and window units. The evaporator and air handler are located inside while the compressor and condenser go outside, just as they would with central air conditioning.

But instead of dumping cool air into ductwork, the air handler distributes it directly into the room where the unit is installed. A single compressor and condenser can serve a number of evaporators in individual rooms.

Like central air-conditioning systems, mini-split efficiency is rated by its seasonal energy efficiency ratioSeasonal Energy Efficiency Ratio (SEER) is the total cooling output (in BTU) of an air conditioner or heat pump during its normal annual usage period divided by its total energy input (in Watt-hours) during the same period. The units of SEER are Btu/W·h. SEER measures how efficiently a residential central cooling system operates over an entire cooling season. The relationship between SEER and EER depends on location, because equipment performance varies with climate factors like air temperature and humidity. (SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. ).

No duct losses. In a mini-split system, cold refrigerant is piped to each indoor unit through insulated copper tubing. When a room thermostat calls for cooling, the indoor unit's fan kicks on and pulls air over the cold coil.

In most U.S. systems, the cold coil is located in a central air handler. Some the cool air escapes through duct leaks before reaching its destination. Energy losses can hit 30%.

Benefits over conventional systems. Small-diameter refrigerant tubing is easier to snake through tight spaces than bulky ducts. That's an advantage in a retrofit.

Mini-splits are ideal for homeowners who want the benefits of air conditioning but don’t have a ducted heating system and don’t want to give up window space for an air conditioner.

Flexible installation. Air handlers can be suspended from the ceiling, hung on a wall, or mounted flush to a wall or ceiling. The units are less obtrusive than window-mounted units, though not as unobtrusive as conventional AC.

Outdoor units can be up to 50 feet away from the air handlers. An insulated conduit 3 inches or less in diameter carries power, a refrigerant loop and a drain between the compressor and the air handler.

But at a higher cost. The major drawback to mini-splits is higher cost, ranging from about $1,500 to $2,000 per ton of capacity. That's about 30% more than central systems and as much as twice a much as window units, according to the government’s energy office.

FURTHER RESOURCES

Consortium for Energy Efficiency

Air Conditioner Basics

Calculating Cooling Loads

Keeping Ducts Indoors

Window-Mounted Air Conditioners Save Energy


Image Credits:

  1. Don Mannes/Fine Homebuilding
  2. Dan Morrison/Fine Homebuilding
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1.
Jun 16, 2014 3:42 PM ET

SEER Score
by Mark Gadala

SEER Score is crucial in deciding the efficiency of an air conditioning unit. Smart consumers should definitely be looking for units with a SEER score of 15 or higher


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