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Energy efficiency of multi zone mini splits vs. multiple single zone?

gerexu | Posted in General Questions on

Having a dispute with my HVAC contractor, hoping the smart people on here can shed some light. For a given heating load size, would 3 higher efficiency single zone mini splits use more or less electricity than 1 multi zone system?

For example, we are contrasting a system that uses 3 Gree Sapphires (9k, 9k, 12k). These supposedly have COPs of 4.47 (for the 9k) and 3.8 (for the 12k) whereas a 30k 3 zone system has a COP of about 3.3.

My thought is that I have a certain amount of heating load and I want to meet that load using the most efficient equipment possible at rated capacity and also have the most flexibility to run mixed loads – thus I conclude that the 3 single zone systems would be more efficient.

My HVAC guy insists that the modern multi zone systems operate at variable speeds and that powering one outdoor unit would save electricity versus powering three.

Which one is right? 3 heads on 3 condensers or 3 heads on 1 condenser?

Thank you!

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Replies

  1. Matt F | | #1

    Dana will probably be along with a real answer, but I will swing, as much for my own education as yours.

    Most multi split system compressors do modulate, but only to turn on and off each indoor unit. The individual indoor units don’t modulate.

    What is the house layout, sf and location? Who determines the load? If you can get down to two single head systems, maybe using a mini duct, that would be most efficient.

  2. Jon R | | #2

    > These supposedly have COPs

    The problem is that they all have a range of COPs depending on load and temperatures. And both of these vary significantly. A correct full year answer needs all of this data. That being said, my guess is the 3 units.

  3. FluxCapacitor | | #3

    Dana will have the answer and I think it’s going to be that three individuals will be more efficient if you have the room for them.

    I read here that multisplits don’t modulate as low as some individual units, and that the refrigerant demand to individual heads are more of a on/off per head scenario. However, with that said, my multi splits heads definitely change their fan speeds as demand goes up and down. So as long as the total demand for the house is above the multi split outdoor units minimum modulation Im thinking it’s close between multi and singles.

    Low demand days is probably when the singles are more efficient because multi split outdoor unit is more likely turn compressor off and on...At least that’s how it was explained to me.

  4. Expert Member
    Dana Dorsett | | #4

    I'm pretty sure I DON'T have the answer, votes of others notwithstanding.

    A LOT depends on the individual heat loads each would be serving, and whether there are 6-7K heads available for the multi-split. A 9K head puts out a LOT of heating/cooling. The minumum modulated output of the 9K Sapphire SAP09HP230V1A at 47F is 2388 BTU/hr , and the min-output of the 12K SAP012HP230V1A is 3,071 BTU/hr

    https://www.greecomfort.com/wp-content/uploads/2019/05/GREE_SAPPHIRE_A_SUBMITTAL_09MBH_230V_080118.pdf

    https://www.greecomfort.com/wp-content/uploads/2019/05/GREE_SAPPHIRE_A_SUBMITTAL_12MBH_230V_080118.pdf

    Add it all up and it's a fairly substantial 7,847 BTU/hr, possibly more than your total heat load @ +47F. That is higher than the minimum modulated output of many multi-splits, but do you even NEED a 30KBTU unit (I suspect not). Most 2 ton multi-splits can support 3 zones, and may be more optimally sized for your actual loads (or not.) Most multisplits can support more than 1.25x of head capacity than compressor capacity (1.24x 24K= 30K). Gree's MULTI24CFLR301 2- tonner can handle a pair of 9K heads and a 12K head. The MULTI24HP230V1CO can too, with a nominal HSPF efficiency of 11.0, and a minimum modulation of 7500 BTU/hr @ +47F, which is lower than a 2x 9K + 12K Sapphires, but higher than three 9K Sapphires.

    http://www.greecomfort.com/wp-content/uploads/2017/07/GREE_MULTI21_C_SUBMITTAL_24MBH_230V_062217.pdf

    https://resource.comfortup.com/is/content/Watscocom/gree_multi24cflr301_article_1487315148113_en_cc1?fmt=pdf

    One would have to look at the minimum output and nameplate efficiency of the multi-split in question AND your actual load numbers to take a WAG at it.

    Without the heating and cooling load numbers (zone by zone) and design temperatures and the model number of the multi-split there is no way to make intelligent guesses.

    If the HVAC contractor ran room by room load numbers that's maybe a good thing, but counting on those numbers for accuracy would be taking a chance. The industry track record is pretty terrible, and HVAC contractors are incentivized by both fear and profit motive to have at least a pinky if not a fat thumb on the scale. It's usually better to have a disinterested third party run the numbers.

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