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EER vs SEER in Hot Climates

kurtgranroth | Posted in Energy Efficiency and Durability on

Years ago, an HVAC pro convinced me to get a 14 SEER variant of a unit rather than its 16 SEER variant since both were 10 EER.  According to him, SEER was based off a temperature of 83 degrees whereas EER averages around 95 degrees.  Since few people here in the Phoenix metro area would even turn on their A/C when it is 83 degrees out and 95 degrees is pretty close to the actual average daily temperature in the summer, he concluded that SEER was meaningless here.  Given that both units were 10 EER, it made no sense to “upgrade” to the more expensive model if both had identical performance in the end.

The thing is, I haven’t heard a single HVAC tech talk about SEER vs EER since then.  You never see EER advertised, even locally.  It’s hard to even find the EER rating of most units!

Yet my reading of the subject suggests that he might have been one to something.  He was bang-on on how the two ratings are calculated.  He was also right that EER doesn’t change anywhere near as dramatically as SEER does between model lines.

In fact, looking at the Carrier mini-split with a 42 SEER rating and compare it to a Mitsubishi unit with a 24 SEER rating and I see that both are rated at 15 EER.

Can I then read that as saying that in hot climates, the above Carrier and Mitsubishi units perform IDENTICALLY, even though they are advertised to be dramatically different?  If so, should I be completely ignoring SEER and exclusively comparing units on EER?

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  1. joshdurston | | #1

    Since the EER is measurement is specific to a single higher load condition (rather than a more complicated seasonal calculation), it won't necessarily show the benefit of some of the high efficiency tricks of a higher end units. Under part loads (usually less than 95F ambient), you reap the benefits of ECM indoor/outdoor fans, variable speed compressors, and high end expansion valve control. At higher daytime temps things tend to run a little more flat our hiding some of the benefits of modulation. I think the EER rating tends not to pickup the benefit of some of these features.

    That being said I think there is some cheating, or at least shady misrepresentation to ultra high SEER ratings. From what I've seen the units that have incredible, to good to be true SEER ratings also have dismal SHRs and low moisture removal rates. I think the high SEER is partially from running the evaporator coil at higher temperatures to reduce energy consumption. In Phoenix, or maybe a data center computer room this wouldn't be a problem. But where I live in Southern Ontario, we get many days that are just barely warm enough to justify the AC, but the humidity necessitates it.
    I can run it in dry mode or a separate dehum, but ultimately I suspect the real world energy efficiency would be higher if the split unit was configurable to supply colder (dehumidified) air without having to resort to dry mode where I give up temp control. Seems like it would be so easy to read a space humidity and reduce the coil temperature as the humidity goes up, even it means a little more cycling (since colder coil at the same airflow = more BTUs).

    I'm not convince there is a lot of real world difference once you get above about 20 SEER unless you have local conditions that play to the equipment's strengths. Which you probably do living in hot dry AZ.

    1. kurtgranroth | | #2

      Your reply is exposing some of the fringes of my knowledge and emphasizes why I need to ask these questions.

      Your implication in your first paragraph is that nearly all of the high efficiency gains are at temperatures under 95 degrees.

      Well, just picking a day at random, I see that September 4 had a high of 110 and a low of 88. It hit 95 at 9am and didn't go below until midnight. That's notably for a couple of reasons. First, it suggests that my A/C units are gaining no real high efficiency benefits for fully 2/3 of the day... and for the 1/3 that they do, I'm sleeping anyway and really care as much (set the thermostat higher at night, plus electricity is on off-peak rates). That's pretty much what it looks like for most of the summer here -- the notable majority of the day will be above the 95 degree threshold.

      You say that there may be real world difference in ultra high SEER rates in places like hot dry AZ... but is that true if the EER shows absolutely no difference?

      Could that mean that 100% of the high efficiency tricks that the Carrier unit employs to hit 42 SEER would never come into play 2/3 of the time and the 1/3 of the time that they start working is when the electrical rates are lowest, anyway?

      Hmm... I wonder if there's a way to calculate the energy cost difference. If we say that the Carrier and Mitsubishi models (just examples, BTW) consume exactly identical amounts of energy for 2/3 of the day and all of the on-peak times, then it would be a matter of figuring out how much less energy the Carrier unit would consume during the 1/3 of the day that its tricks manifest. Multiply that by the off-peak rate and by the number of days we run A/C at all and I guess it would tell me how much money the higher SEER unit saved me in a year. But... how to do that first calculation of energy usage given identical EER but differing SEER?

      1. joshdurston | | #5

        EER is a single datapoint taken under a fixed set of steady state conditions.
        SEER take a wider range of OA operating conditions into account both higher (104F) and lower (64F) than the EER point. So you can't necessarily assume that's it's equal when higher than 95F, just at 95F.

    2. Jon_R | | #3

      > have dismal SHRs and low moisture removal rates

      I agree, although they seem to rig it to have a better SHR at some point and then list that as the SHR. In climates where humidity is an issue, consider Daikin Quaternity. Otherwise, some of your savings will to towards running a dehumidifier.

      As soon as a metric is developed, manufacturers start abusing it. Maybe they could have settings - a default of "marketing mode" and then a "reasonable mode".

      Kurt, In your case, 16 SEER/10 EER having little advantage over 14 SEER/10 EER is plausible. But identical isn't. It would be nice if manufacturers published enough data to accurately calculate efficiency for a specific location.

  2. Expert Member
    BILL WICHERS | | #4

    >”In Phoenix, or maybe a data center computer room this wouldn't be a problem.”

    Just to point something out since I work with mostly with datacenter systems, they aren’t good for comparison purposes to anything in a regular building. Cooling units for datacenters, commonly known as “CRACs” — computer room air conditioners — are concerned only with sensible load. Humidity is generally controlled around 40% or so, and kept there. The computers do nothing to the moisture levels in the room, so they contribute only to the sensible side of the heat removal ability of the CRAC. It is common to run the coils hotter, but the intake air temps are also usually hot (90-100 degrees is common), and the blower nevers shuts off — airflow is constant and only the compressor is modulated. Any modulation of the air blower is just to maintain a fixed static pressure in the cold air supply plenum.

    It’s a very different application from regular space cooling for residential or office applications.


  3. joshdurston | | #6

    You can look at the submittal data and calculate the COP/EER at different (than the standard 95F) OA conditions. If you run a lot above 100F I would crunch the power consumption versus capacity numbers under more extreme conditions and compare that efficiency too.

  4. kurtgranroth | | #7

    To followup on my own question... I ended up talking in depth with an HVAC pro that liked geeking out about Arizona-specific aspects of air conditioning.

    In short, he agreed that EER is the more important benchmark to go by in a hot dry climate like Phoenix. All else equal (or even close), it will always make the most sense to get a unit with the higher EER, regardless of SEER.

    But he disagreed that SEER was unimportant here. My assumption was that SEER achieved higher numbers largely based on the use of an inverter, which simply wouldn't come into play very often with the high ambient temperatures. That was wrong from two perspectives.

    First, even though the temps are below 95 for only 1/3 of the day at the peak of the summer, that is still 8 hours of running in lower-speed mode at the absolute worst and that alone will be notable in the total energy usage. Also, I purposefully picked the hottest days -- even in the summer, there are still numerous days that have more than 8 hours under 95. And that's not even counting the Spring and Fall months that are ramping up and ramping down. All in all, there would be far more hours where the temperature differential was close enough to justify the use of an inverter than I was originally thinking.

    But then there's the other issue, which is that I was strictly doing a differential between the desired inside temp (80) and the outside temp (up to 115) but that's ignoring the fact that my house is insulated. Experience has shown that even if the AC is entirely turned off (power was cut) when the temp hit 110 outside, the interior temperature never hit much above 90. So the differential is more often than not going to be closer to 90 minus 80 degrees and not 110 - 80 degrees. Such a small delta is another strong contender for lower speed with an inverter.

    All in all, he said that they've seen very notable differences in energy usage in houses that they've installed high-SEER (in addition to high-EER) mini-splits, suggesting that the efficiencies inherent in high SEER do come into play frequently even here in AZ. Really, the only reason mini-splits aren't more common is that they are egregiously more expensive than the "typical" units for a normal size home -- at least for a retrofit. But that's a different topic.

  5. d_barnes | | #8

    I know you’re talking about ductless in your example but it’s important to remember that these SEER and EER ratings happen in a test lab and that in the real world installation can make a bigger difference. Especially when you’re talking about central ducted systems where duct design and system airflow can make or break a high efficiency system. You can also adjust airflow per ton higher than the normal 350-400 cfm per ton when in a dry climate to gain a higher real world SEER and EER. But you need to keep your static pressure low enough not to cause higher watt draw on your ECM blower fan.

  6. theHeatPumpGuy | | #9

    I recently replaced my old ducted split 12 SEER air conditioners from 2004 with ducted mini-split heat pumps rated at 17 SEER. But they consume just as much power as my old AC did. It was only then did I discover that for a place like Las Vegas where I live, EER is what counts, not the SEER. It's very frustrating because almost nobody talks about why this is so important for people living in really hot climates like the desert southwest. I think one solution should be regional SEER raatings, instead of just one for the whole country. After all, all there is no such thing as a "typical cooling season". In fact the government has designated 8 climate zones.

    Below is the EERs I calculated for 115 degrees F for a few different systems, using data taken from submittals. Even this is hard to find. For full details, my website has more:

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