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Window-Mounted Air Conditioners Save Energy

Compared to homes with central air conditioning, homes with window units have lower cooling costs

Posted on May 25 2012 by Martin Holladay

Window-mounted air conditioners (also called room air conditioners) aren’t particularly efficient; the best available models have an 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. of about 10 or 11. Central air conditioners (also called whole-house air conditioners or split-system air conditioners) are significantly more efficient; it’s possible to buy one with an EER of 14 or even 15.

So if you care about energy efficiency, you should use a central air conditioner, not a window air conditioner — right? Well, not necessarily.

Air conditioners use a lot of electricity

According to the American Council for an Energy-Efficient Economy, the electricity used for home air conditioning represents almost 5% of all the electricity produced in the U.S. So if you are going to air condition your house, it would appear to make sense to buy the most efficient unit you can find.

There are two metrics used to rate the efficiency of central air conditioners: the Energy Efficiency Ratio (EER) and 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. ). The unit for both metrics is 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. /W•h.

EER is the cooling capacity of the appliance (in Btu/h) at an outdoor temperature of 95°F divided by the current draw of the appliance in watts.

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. is the total cooling output (in Btu) over the cooling season divided by the total electrical energy input (in watt-hours) over the cooling season. For typical residential air conditioners, EER equals about 0.875 SEER; however, this ratio varies somewhat for different air conditioner models.

EER and SEER ratings

Unlike central air conditioners — which are rated by both EER and SEER — room air conditioners (window units) are rated only by EER.

Room air conditioners that use at least 10% less energy than the federal standard are eligible 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. label. When shopping for a room air conditioner, look for an Energy Star unit with an EER of 10.7 or more.

The idea behind the SEER rating is to allow consumers to compare the seasonal efficiency (rather than the peak efficiency) of split-system air conditioners. If you are in the market for a new central air conditioner, look for an Energy Star unit with a SEER of at least 14.5 and an EER of at least 12.

Some critics have noted that air conditioner manufacturers design equipment to achieve a high score on the laboratory SEER test, even when some of the engineering solutions used to achieve high test scores result in poor performance or efficiency in the field. For more information on perceived flaws in the SEER standard and SEER testing protocols, see SEER Ratings Challenged.

John Proctor, the president of Proctor Engineering Group in San Rafael, California, has performed useful research into possible flaws in the SEER rating method. Proctor (with co-author Gabriel Cohn) has publshed a paper on the topic: Two-Stage High Efficiency Air Conditioners: Laboratory Ratings vs. Residential Installation Performance. Proctor and Cohn wrote, “The increased installation of high Seasonal Energy Efficiency Ratio (SEER) air conditioners along with utility program rebates for these units prompted a study of the measured performance of these systems. This project assessed the performance of these systems in the climate zones found in the mid-Atlantic region of the U.S. ... The data were analyzed to assess the relationship between laboratory testing and real world performance. This study found causes for concern including: actual seasonal energy efficiency ratios between 59% and 84% of the rated SEERs, constant fan operation substantially degrading seasonal efficiencies and reducing dehumidification, latent loads that exceed Manual J estimates, and sensible loads substantially lower than Manual J estimates.”

According to some experts, problems with the SEER rating system may make EER a more useful metric for comparing the efficiency of central air conditioners.

Window units versus central air conditioning

Even though central air conditioners are more efficient than window-mounted air conditioners, homes equipped with window-mounted air conditioners use less energy for cooling than homes with central air conditioning.

For some readers, this fact will be unsurprising. After all, homes with central air conditioning usually maintain lower temperatures in more rooms than homes with window-mounted air conditioners.

Other readers — including those who assume that equipment efficiency is the most important factor in energy use — may be surprised to learn that homes with window-mounted air conditioners are energy misers.

For those interested in the data behind this question, I recommend a blog by fellow energy nerd Tim Grejtak, “The Fascinating World of Air Conditioner Efficiency.” According to Grejtak’s analysis, “Window ACs use less energy on a per cooling degree day and per square footage basis by a factor of 1.75.”

So why do homes with window-mounted air conditioners use less energy for cooling? There are several reasons:

  • People living in homes with central air conditioning tend to keep every room in the house cool — even unoccupied rooms — while people living in homes with window-mounted air conditioners are more likely to just cool one or two rooms.
  • Central air conditioners often have ducts located in unconditioned attics. Research has shown that air conditioning systems with ducts in unconditioned attics have duct losses amounting to about 20% of the air conditioner’s cooling output.
  • Since window air conditioners are often noisy, homeowners remember to turn them off when they leave a room. By contrast, some homeowners with central air conditioning often leave their air conditioning system on all day, even when the home is unoccupied. As explained in an article by Brian Palmer, “The quiet and unobtrusive functioning of central air conditioners ... can lead to accidental overuse.”

The bottom line

There are several valuable lessons arising from this analysis:

  • Efficient equipment is no bargain if it encourages waste.
  • Two or three window-mounted air conditioners cost less to install than central air conditioning.
  • Bad ductwork is the Achilles’ heel of central air conditioning systems. If you have central air conditioning, be sure to seal all of the seams in your ductwork and insulate ducts well.
  • If you are designing a new home, be sure to locate all ductwork inside of the home's conditioned envelope.
  • If you aren’t home, turn off your air conditioner.
  • When you’re sleeping, you may only need a 6,000 Btu/h air conditioner to cool your bedroom. It’s wasteful to use a 2-ton central air conditioner for such a small load.
  • If you use one or two window-mounted air conditioners instead of central air conditioning, pat yourself on the back. You probably have a lower carbon footprint than your neighbors who have high-SEER central air conditioners.
  • The most efficient way to air condition your house is probably with ductless minisplit units. The only problem with this approach is the high cost of the equipment. However, if you have a new superinsulated house, and if ductless minisplit units can provide both heating and cooling, then these units may be a good, low-cost solution.
  • If you live in a dry climate, consider using an evaporative cooler to cool your house. The cost to operate an evaporative cooler is only a fraction of the cost to operate an air conditioner.

Remember: you don’t need a slash

I’ll end with a gentle reminder: the correct abbreviation for “air conditioner” is AC (or A.C.). It is not A/C. There is no slash.

It isn’t a fraction. No one is dividing A by C. It’s an abbreviation for two words. So, please — don’t include a slash.

Last week’s blog: “Choosing an Energy-Efficient Refrigerator.”

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Mar 21, 2017 9:56 AM ET

Advances in Mini AC
by Trevor Bowden

Martin, Dana,

I'm far from an expert on HVAC yet I'm trying to hone in on a good recipe for efficient HVAC for homes. I've done two whole-home installations in central Florida, one new and one retrofit, with LG ductless minisplits. One area I lack research is looking at the data beyond SEER. As Dana pointed out, at the lowest fan speeds ductless units can lower their capacity quite a bit. I've read about some Mitsubishi products that are pretty amazing.

If you look at the room loads (attached), especially for the new home, you'll see that they're relatively small. On both the new and retro job there were two spare bedrooms and a spare bathroom all sharing a hallway and both times I installed a ducted indoor unit in the spare hallway to supply each area. I don't like the ducted unit because I've already had to replace the blower in one and it's a hassle ripping apart your false ceiling. Also if the people in each bedroom prefer different conditioning, you're now compromising comfort. Another home I'll be building soon will have a third spare bedroom all along the northern exposure of the building. This setup is the best for energy load, but a challenge for equipment capacity and cost (lots of units potentially). So I got to thinking, Googling, and finding that Martin is typically a few steps ahead of me.

I'll digress here a bit, but my thinking led me to a market for units similar to window units in that they contained everything in one box, but like wall-mount ductless in that it's efficient. Imagine a hole in the wall with a cube inserted that was small enough capacity to condition a room but was reliable and efficient enough to compete with a minisplit.

Anything like that on the horizon that you guys see? Any advice for tackling situations with lots of smaller cooling loads? I've looked a little at room-to-room fans, but I'm skeptical. It's been almost five years since this article was written so I hope there's some new advice! Thanks.

load_summary_new.jpg load_summary_retro.jpg

Mar 21, 2017 11:18 AM ET

Lots of smaller loads calls for ducts (or nothing at all).
by Dana Dorsett

An adult sleeping human puts out between 200-250 BTU/hr and can "self heat" very low heating load rooms such as windowless closets, or small -window bathrooms, etc. Ventilation of those rooms with conditioned air will usually keep up with the cooling loads.

IIRC the LG Art Cool Premier units can throttle back to about 1000 BTU/hr in both heating & cooling modes, so even if it's nominal capacity is ridiculously oversized for the loads, at minimum speed (and highest efficiency) it may not be.

Rooms such as hallways & foyers with reasonably good air communication with adjacent to rooms with ductless heads don't need their own head but the load number should be covered by the capacity of the head in the adjacent room. There may be sensible temperature differences at design condition, but it's not usually enough to be a comfort problem. It's a judgement call.

When the individual doored-off room loads are under 3000 BTU/hr at design condition it's worth considering ducted mini-split solutions rather than another Art Cool Premier. A ducted Fujitsu xxRLFCD series only throttles back to about 3000 BTU/hr, but if it's serving 4- rooms with a combined load of 8000 BTU/hr or higher it's not a bad solution.

The Mitsubishi half-ton FH06NA can be a good fit for smaller loads, but it's minimum modulation is about 1600-1700 BTU/hr, so it's less than ideal for design loads less than about 4000 BTU/hr.

Mar 22, 2017 6:30 AM ET

Response to Trevor
by Martin Holladay

Dana has given you some good suggestions.

For the "box through the wall" approach, the usual answer (for cooling only) is to use a room air conditioner (also called a window-mounted air conditioner) as described in this article. The higher the EER, the more efficient the unit.

If you want a "box through the wall" that provides heating as well as cooling, the usual solution is a PTAC (Packaged Terminal Air Conditioning), also known as a PTHP (Packaged Terminal Heat Pump).
Anyone who has ever stayed in an American hotel or motel is familiar with PTACs. Until recently, PTACs were noisy and inefficient (especially when operating as a heater); however, the PTAC industry is improving the noise ratings and efficiencies of its units. Better equipment may be on the horizon.

-- Martin Holladay

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