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  1. #1

    Ductless Tech: Higher SEER always with smaller capacities..?

    Im in the AC business...primary resale and distribution of ductless type systems and have a technical question regarding the relation between SEER (energy efficiency) and capacity. I don't know if there are similar patterns with central forced but I've noticed with ductless units, a smaller capacity will always correlate directly with a higher SEER value. This pattern repeats across all the single zone series of ductless brands in the market. It's always the same: your smallest capacity 9000 BTU systems will feature the highest SEER and as the capacity goes to 12k-18k-24k etc., the SEER value always drops. I have some makes that start off at 23 SEER for the 9000 BTU that will dip all the way to 16 SEER by the time that same brand and make gets into the 36000 BTU range. Does anybody know why this is?


    (Sorry if this isn't in the right place - couldn't find the right forum for this question)

  2. #2
    Join Date
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    Yup, happens on central systems too. The issues is loss of coil size vs. capacity. Look at EER ratings too. You will find that once you go over 15 SEER (which get about 13 EER) EER doesn't improve much as units go higher in SEER. A 21 SEER has about the same EER as a 15 SEER. The extra SEER comes fro reducing cycling losses during part load conditions. A 13 SEER unit gets about 11 EER because of the smaller coil size.

  3. #3
    Join Date
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    get your post count up & join as pro member
    you'll have access to areas like tech to tech
    & others that are closed to the public.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  4. #4
    Quote Originally Posted by 54regcab View Post
    Yup, happens on central systems too. The issues is loss of coil size vs. capacity. Look at EER ratings too. You will find that once you go over 15 SEER (which get about 13 EER) EER doesn't improve much as units go higher in SEER. A 21 SEER has about the same EER as a 15 SEER. The extra SEER comes fro reducing cycling losses during part load conditions. A 13 SEER unit gets about 11 EER because of the smaller coil size.

    Thanks for the reply 54regcab. From my understanding the "seasonal" - EER represents a more precise measurement of efficiency in terms of year-around performance in the field. I'm not well versed in how the mechanics plays into this terminology but from a marketing standpoint, the SEER is the primary promotional angle. Even contractors rarely inquire about a systems EER as opposed to SEER/HSPF. The minimum EER requirement for tier-2 energy star is 12.5 which isn't difficult for many ductless systems to achieve. Even the baseline non-inverter don't have a problem hitting 12 EER. Seems the HSPF and SEER is what holds alot of systems back.

    This question is kind of a precursor for another inquiry I wanted to address once I had a better understanding of this adverse correlation between capacity and SEER. I know there's still plenty I don't understand, however, in regards to following statement, "A 21 SEER has about the same EER as a 15 SEER"

    See YMGI AHRI data sheet

    See Mitshubshi Mr Slim AHRI data sheet

    Above links are screenshots from 3'rd party AHRI performance testing for 2 flagship ductless brands: YMGI and Mitsubishi. In the YMGI cut sheet, you can see the 9000 BTU model (WMMS-09-V2A) has a SEER of 22 and EER of 14.2. On the 12k capacity, the EER dips to 12.5 while SEER decreases to 20. On the 18k/24k sizes EER falls to 12.0 while SEER dips to 18.0 and finally goes all the way to 8.2 EER / 16 SEER on the 36k. This trend is repeated in the Mitsubishi Mr Slim cut sheet where we have 26 SEER / 15.5 EER on the 9k system and then 8.0 EER / 14.5 SEER on the 36k. It does seem the EER and SEER parallel each other to some respect.

    I think what you're saying (in laymen's terms) is the reason why SEER decreases as capacity increases is because larger systems still require a good amount of draw to maintain thermostat control even though set temp could be maintained at a fraction of the power consumption > capacity....? Thanks again for taking the time to take my question.

  5. #5
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    I've not looked at a lot mini-split data in detail. For central systems EER is determined in large part by coil size vs compressor ratio. This is effective only up to a certain point, I've not seen EER's above 14 for air cooled units. Increasing the spread between SEER and EER requires reducing off cycle losses. Unfortunately 3/4's of high SEER equipment is oversized, negating most of the SEER advantage vs. a system with the same EER but a lower SEER.

    SEER is a test done at 82F outside and 80F inside, then deducting cycling losses. EER is done @ 95F outside and 80F inside, no cycling losses. Equiptment sizing and ductwork selection is VERY important to obtaining the rated capacities and SEER.

  6. #6
    Quote Originally Posted by energy_rater_La View Post
    get your post count up & join as pro member
    you'll have access to areas like tech to tech
    & others that are closed to the public.
    I'm working om that quota energy_rater_La. I've researched these topics extensively online before addressing them on this forum so my inquires are going to be rather intermediate. I'll probably need to ask them again once I get access to the pro forums. I'm not much of a small talker so 15 posts is going to take some time. I hate to be somebody who only takes and doesn't contribute - unfortunately my knowledge in the HVAC field is primarily marketing based specific to ductless air conditioners so won't have much to contribute outside that niche arena. I would be inclined to make a small donation so I could atleast show my support to the community in some respect.

  7. #7
    Quote Originally Posted by 54regcab View Post
    I've not looked at a lot mini-split data in detail. For central systems EER is determined in large part by coil size vs compressor ratio. This is effective only up to a certain point, I've not seen EER's above 14 for air cooled units. Increasing the spread between SEER and EER requires reducing off cycle losses. Unfortunately 3/4's of high SEER equipment is oversized, negating most of the SEER advantage vs. a system with the same EER but a lower SEER.

    SEER is a test done at 82F outside and 80F inside, then deducting cycling losses. EER is done @ 95F outside and 80F inside, no cycling losses. Equiptment sizing and ductwork selection is VERY important to obtaining the rated capacities and SEER.

    So just trying to follow up to understand the relationship between energy savings and SEER/EER. Say we have a 36' x 40' continuous space totaling 1440 square foot that requires 3 tons or 36000 BTU to cool. Would you save on energy costs if instead of cooling this area with a single-zone 36000 BTU system @ 8.2 EER / 16 SEER; rather divided that space into 4 separate 18' x 20' partitions and cooled it with 4 individual 9000 BTU systems @ the 14.2 EER / 22 SEER?

    Once again thanks for your help 54regcab - I don't expect anybody to know all the answers...anything you can offer it appreciated. Also, do you know how the condenser of a central system differs from that of a ductless condenser in operational principle? The fan spins vertically with ductless AC's rather than horizontally with central but unclear as to why.

  8. #8
    Join Date
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    Splitting the load between 2 or more units would offer significant advantages even if the space is kept whole. On moderate days only 1 or 2 units would need to be turned on to keep the space cool. Longer cycles = increased real world efficiency. Only during the peak of the summer during the afternoon would all 4 units need to run. The other 90% of the summer 3 or less units would do the job.

    A central system and ductless condenser work the same in theory, I do think central systems tend to have higher condenser surface areas per BTU. Minisplits don't have any duct losses, this gives them a HUGE real world advantage. Condenser discharge direction is irrelevant to efficiency, central units have top discharge so multiple units can be placed side by side. Minisplit manufacturers are more concerned with keeping a slim profile against a wall since it's not as common to have multiple Minisplit condensers side by side.

    Central units seem to be practical for homes with several small rooms, minisplits just aren't conducive to cooling several small spaces efficiently at a reasonable 1st cost. Where mini splits really shine are house add-ons. Somebody adds a 20x20 room, which may put too much additional load on the existing HVAC system. Tapping into and/or upgrading the existing system could be costly, and even then not have a good tempature balance with the existing structure. Minisplit to the rescue, especially if the addition is a single room. Smaller size is perfect for the 400sqft addition, installation costs cheaper than upgrading/redoing/adding another HVAC system. No ductwork to deal with, no gas lines to run. Being quieter than a central system and having a remote is just icing on the cake.

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