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Are my sizing calculations correct? Two-zone v. single-zone?

B_S | Posted in Energy Efficiency and Durability on

Hello, GBA Community, 

About three years ago, you provided helpful insights as I tried to determine the most efficient HVAC unit for our 1400 square foot home. 

If you have time, I’d love to get your perspective on some additional analyses that I’ve done. (See attached.) Do the attached numbers seem accurate? 

(1) On the heating side, we’re currently using about 13.5 MMBTUs per year. However, when doing a crude heat load calculation paired with a 1.4x sizing factor, it looks like we only need a 1-ton unit. Do my numbers seem right? 

(2) On the cooling side, I used some equations referenced by PSU’s College of Earth and Mineral Sciences (, example 4) to compare the cost of our current system to new ones using different SEER ratings. When doing so, I kept the “time of usage” constant in this equation to make my comparisons (similar to keeping MMBTUs constant when looking at heating systems). Am I using the proper logic here? 

Finally, I’m curious about your thoughts of running a ducted mini-split air-handler combo (12,000 BTU/1-ton) to cover my upstairs (700 square feet, 3 rooms/1 full bath), and a non-ducted mini-split (9000 BTU/.75 ton) to cover my downstairs (700 square feet/2 large rooms). It seems like we might realize some cost savings by utilizing a multi-zone set-up; however, it would be slightly oversized in comparison to just using one single-zone ducted mini-split air-handler for the entire home (18,000 BTU / 1.5 ton). What would you say is better in this scenario? A two-zone system, or a system that’s more accurately sized? 

Thanks in advance for your time and insight. 


Richmond, VA

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  1. Expert Member
    Dana Dorsett | | #1

    The 1-ton PVA-A12AA7 air handler is overkill on both capacity and cfm for your 700' upstairs zone. The SEZ KD09NA/ SUZ KA09NAHZ single zone unit is probably a better fit:

    ... or the SEZ KD09NA married to an MXZ multi-zone compressor:

    The GL09 is also a bit overkill for the first zone, but fine since it has a reasonable modulation range:

    The FH06 (and FH09) would be an even better fit, since they have lower minimum-modulated output than the GL series. The minimum capacity of the GL09 is 4500 BTU/hr @ 47F, well above the likely load for that zone at 47F, whereas the FH06 & FH09 can pull all the way down to 1600 BTU/hr.

    But if going multi-split the GL06 half-ton head is probably a better fit, delivering longer, more comfortable on cycles and a higher overall duty cycle:

    In my area it's usually cheaper to go with two separate mini-splits than a 2-zone multi-split. Also, the minimum modulated output of the cold climate MXZ compressors is 7400 BTU/hr which is more than half your design load, so it'll be cycling the compressor almost all the time, only rarely modulating:

    The non-cold climate 2-zone unit has the same lower minimum modulation, and still covers your design load with very little margin, but the -NAHZ version would still have you covered during a Polar Vortex disturbance cold snap, and even when cycling, at about the same efficiency as the-NA2:

    BTW: The 1.4x upsizing factor recommended by ASHRAE is better suited for 1 or 2 speed fossil burners using set-back strategies for fuel economy. With modulating heat pumps it should be considered something of an upper bound rather than a minimum (though fairly hard to really meet for low load house split between two zones.)

  2. B_S | | #2

    Thanks, Dana,

    I appreciate your help. In a similar post, I saw you indicate the following for the Fujitsu ARU/AOU18RLFCD ( "Whether it will work with pre-existing ducts would require some analysis of your ducts. It's cfm specs are at lower backpressure water-inches than most full-sized air handlers are specified at, but more oomph than most competitors' mini-duct cassettes. If your ducts are oversized, designed for bigger than needed air handler cfm there is at least a good chance it can work."

    Assuming we'll need a similar analysis, I've attached an existing diagram of my current ductwork (or very close to it). The intent is to seal off the "green" (downstairs) ducts/duct lines and replace them with a wall-mounted system on the first floor. Then, as you suggest, we'd use the concealed horizontal duct for the ductwork running upstairs.

    As for concealed horizontal duct, the M-Series specifies short duct runs, but Bailes seems to question this approach ( However, if this holds true, I'm curious as to what "short" implies, and if our system has "short runs."

    Overall, it seems like we'll need some analyses done on our ductwork. If you have any insight that would point me in the right direction (given the above context), I'd be grateful.

    Thanks for your time,


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