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  1. #1
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    Question Need help with mini-split AC strategy

    I've had a few proposals for multi-split systems for our 2 1/2 storey pre-war colonial home in North Jersey/NYC suburbs. None of the companies did a Manual-J calculation, and they've recommended a whole range of different sized systems.

    I'm considering this one:
    • Daikin 36,000 BTU, 4-port condenser
    • 2x 12,000 BTU wall units for the main floor (750 sf open floor plan)
    • 2x 7,000 BTU wall units for bedrooms (186 sf + 110 sf. The separate bedroom zones are attractive)


    I question the sizing, though. We currently have two 12,000 BTU thru-the-wall AC's on our main floor and they are just barely adequate on the hottest summer days, but they are 25 years old. I wonder if one 18,000 BTU mini-split wall unit at the back of the house would cool the whole floor evenly? If so, then we could use a smaller 2-ton, 3-port condenser. It's 36 ft from the front of the house to the back wall with a pretty much open floor plan. There's no attractive location in the middle of the room to mount a ductless wall unit so most proposals have been for two 12k units at the front and rear of the house instead, based on what we have now with the through the wall ACs.

    I ran LoadCalc which told me that my whole-house cooling load is 28,165 BTU. However that includes the 175 sf finished attic which has its own through-the-wall AC and is not part of the mini-split plan. I'm really just cooling 1350 sf on the first and second floors combined, with the mini splits.

    The contractor/salesman says don't worry about the larger condenser, the inverter technology will drop to as little as 15% rated capacity to match the demand and should not be a problem. Set up the temperature and we'll get the exact results we desire. He also says the larger condenser produces more heat in the winter, but I'm not relying on this system for heat.

    Should I try to scale this system back and use one wall-hung unit on the main floor (but at the far end)? Alternately, I could put two 9,000 BTU wall units on the main floor and lose the second bedroom to get down to a 2-ton, 3-port system. Or am I just overthinking this, and 36k should be fine?

  2. #2
    Join Date
    Nov 2006
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    He may be concerned that one unit will not have enough throw to cover 36’. It is correct that units can modulate down to a fraction of their full capacity. And it may be that he wants the larger unit for the heating capacity.
    AOP Rules: Rules For Equipment Owners.

    Free online load calculator: http://www.loadcalc.net/


    There = not here. Their = possessive pronoun. They're = they are
    It's = contraction of it is. Its = the possessive form of it
    Too = also. To = expressing motion. Two = 2
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    Questions should end with a question mark "?" Statements end with a period "."

  3. #3
    Join Date
    Sep 2016
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    Saint Augustine, Florida
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    With variable speed technology that comes with most ductless systems the system will modulate down to the lowest possible capacity to effectively and efficiently remove the heat and humidity from your home. All the systems i have installed and maintained develop a memory from taking temperature samples and determining what is most efficient for a structure. They are basically designed to operate as long as possible at the lowest possible energy use. I am not a fan of over sizing equipment for the obvious reasons but in your case the contractor most likely has added additional btu for heating cycle and or extreme cooling cycle. With variable speed technology over sizing is generally a minimal point as long as it has not affected to overall cost of the job. One thing i would suggest is ensure that the contractor pressure checks all copper connections with at least 700 psi for 24 hours prior to starting system. I have seen to many leaks in the flared fittings over the years due to in adequate joint connections and or pressure testing to prevent any possible refrigerant leaks in the future of your system. You will no doubt experience and completely different environment with the variable speed technology than the old through the wall units you currently have. It is not just added heat or removing heat that is important in our indoor systems. The manner in which the energy (heat) is introduced or removed drastically affects the condition of the air in our homes. Technology is assisting the hvac&r field as never before. Variable speed technology is ready and available in ductless and even central systems for homes today. It is the future of our business and will be standard within the next 10 to 20 years. Enjoy your new system and definitely keep your self informed and apart of the process of your new heating and cooling system.

  4. #4
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    Mar 2010
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    I agree with the other comments. The capacity plan you’ve laid out seems entirely reasonable to me if these units are all part of a comprehensive inverter heat-pump system. I assume this to be the case but I am unfamiliar with Daikin equipment.

    As pointed out already, high efficiency inverters are designed to operate 24/7 and simply ramp up or down according to current load. They are terrific. Over capacity isn’t really a concern with these systems.

    From a financial point of view, downsizing may be possible, but in my experience you’ll like the zoning and having the cross-room air movement in the open area the two wall units bring you. IMO.

    Kudos for at least trying to establish your actual heatload numbers too. As long as those are good enough to keep you from being undersized you seem to be in a good place.

    K

  5. #5
    Join Date
    Aug 2002
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    From here even for heating that sounds huge! 7K is the smallest so maybe! 2) 12K for 750Sq Ft? Possibly 2) 9K is closer.

    Member "EPACertified" is in NJ

    Reach out to him and get a second opinion!

  6. #6
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    Thread Starter
    I re-ran the heatload to calculate just the space to be conditioned with mini splits (I counted the attic as unfinished, insulated space and fixed a data entry error). It recommends a 2.5 ton system now.

    Daikin doesn't make a 2.5-ton condenser, just 2 and 3 ton models. Based on that, along with some of the comments above, it looks like the system shown in my first post is OK after all. I know other brands like Mitsubishi make 2.5 ton systems but they're 3-port condensers so we'd have to lose a head. So I guess I've sort of answered my own question but I appreciate the feedback. I'm really looking forward to the upgrade.

  7. #7
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    Quote Originally Posted by DanielNJ View Post
    I re-ran the heatload to calculate just the space to be conditioned with mini splits (I counted the attic as unfinished, insulated space and fixed a data entry error). It recommends a 2.5 ton system now.

    Daikin doesn't make a 2.5-ton condenser, just 2 and 3 ton models. Based on that, along with some of the comments above, it looks like the system shown in my first post is OK after all. I know other brands like Mitsubishi make 2.5 ton systems but they're 3-port condensers so we'd have to lose a head. So I guess I've sort of answered my own question but I appreciate the feedback. I'm really looking forward to the upgrade.
    3-Tons is huge for 1000 Sq Ft in NJ. Even with a inverter system sizing is still critical.

    Please post the calculation used to get to this #

  8. #8
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    Thread Starter
    Quote Originally Posted by pecmsg View Post
    Please post the calculation used to get to this #
    Design Indoor Cooling Temp. :75°F
    Design Outdoor Cooling Temp. :89° F
    Indoor Humidity: 50
    Outside Walls: Siding w/ R19 insulation
    Windows: Low E or triple pane - blinds
    Floors: Concrete slab no edge insulation
    Ceiling: Ceiling under attic space R-11
    Doors: Metal (actually fiberglass)
    Basement Walls: block brick or concrete - no insulation (basement is partly finished/heated)
    Basement Floor: no insulation underneath
    Window avg ht.: 5' 0" Overhang: 1.5' Top to overhang: 1.5'
    Front door orientation: Southeast
    Outside Wall NE: 777 sf
    Outside Wall NW: 540 sf
    Outside Wall SE: 777 sf
    Outside Wall SW: 540 sf
    Windows NE: 46 sf
    Windows NW: 61 sf
    Windows SE: 76 sf
    Windows SW: 51 sf
    Doors NE: 17 sf
    Doors NW: 20 sf
    Doors SE: 20 sf
    Doors SW: -- sf
    Floor: 774 sf
    Ceiling: 758 sf
    Basement walls above grade: 434 sf
    Basement walls above grade: 496 sf
    Basement floor: 774 sf
    Width: 23 ft or less
    Feet below grade: 4
    Appliances: 2
    Fireplace: 0
    People: 4
    Fresh air: 28cfm (recommended)
    Conditioned: 1333 sf; 13,791 cu ft
    Construction: Average
    Duct system: ---
    ---
    CALCULATION
    Total BTU's cooling: 26,974
    Sensible Load: 23,974
    Latent Load: 3,000
    Total BTU's Heating: 113,529
    ***
    MANUAL S
    System Type: Heat Pump air to air / 2 stage or variable compressor
    Climate: Heating & Cooling
    Elevation: 280 ft

    LOAD CALCULATION DATA
    (Used calculation numbers shown above)
    Sensible Heat Ratio 0.89

    SIZING RESULTS
    *Cooling - Minimum: 26,974 BTU's
    Cooling - Maximum: 32,369 BTU's
    Emergency Heating - Minimum: 91,675 BTU's
    Emergency Heating - Maximum: 168,874 BTU's
    Aux Heat Minimum: 21.64 kw
    Aux Heat Maximum: 39.86 kw
    Last edited by DanielNJ; 03-16-2019 at 01:29 PM. Reason: Fixed data entry for ceiling

  9. #9
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    Quote Originally Posted by DanielNJ View Post
    Design Indoor Cooling Temp. :75°F
    Design Outdoor Cooling Temp. :89° F
    Indoor Humidity: 50
    Outside Walls: Siding w/ R19 insulation
    Windows: Low E or triple pane - blinds
    Floors: Concrete slab no edge insulation
    Ceiling: Ceiling under attic space R-11
    Doors: Metal (actually fiberglass)
    Basement Walls: block brick or concrete - no insulation (basement is partly finished/heated)
    Basement Floor: no insulation underneath
    Window avg ht.: 5' 0" Overhang: 1.5' Top to overhang: 1.5'
    Front door orientation: Southeast
    Outside Wall NE: 777 sf
    Outside Wall NW: 540 sf
    Outside Wall SE: 777 sf
    Outside Wall SW: 540 sf
    Windows NE: 46 sf
    Windows NW: 61 sf
    Windows SE: 76 sf
    Windows SW: 51 sf
    Doors NE: 17 sf
    Doors NW: 20 sf
    Doors SE: 20 sf
    Doors SW: -- sf
    Floor: 774 sf
    Ceiling (under attic): 576 sf
    Basement walls above grade: 434 sf
    Basement walls above grade: 496 sf
    Basement floor: 774 sf
    Width: 23 ft or less
    Feet below grade: 4
    Appliances: 2
    Fireplace: 0
    People: 4
    Fresh air: 28cfm (recommended)
    Conditioned: 1333 sf; 13,791 cu ft
    Construction: Average
    Duct system: ---
    ---
    CALCULATION
    Total BTU's cooling: 26,237
    Sensible Load: 23,237
    Latent Load: 3,000
    Total BTU's Heating: 112,703
    ***
    MANUAL S
    System Type: Heat Pump air to air / 2 stage or variable compressor
    Climate: Heating & Cooling
    Elevation: 280 ft

    LOAD CALCULATION DATA
    (Used calculation numbers shown above)
    Sensible Heat Ratio 0.89

    SIZING RESULTS
    *Cooling - Minimum: 26,237 BTU's
    Cooling - Maximum: 31,484 BTU's
    Emergency Heating - Minimum: 91,008 BTU's
    Emergency Heating - Maximum: 167,646 BTU's
    Aux Heat Minimum: 21.41 kw
    Aux Heat Maximum: 39.44 kw
    What program was used?

    Link to site!

  10. #10
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    Thread Starter
    Quote Originally Posted by pecmsg View Post
    What program was used?

    Link to site!
    loadcalc.net

    Has been posted to this forum & recommended in other threads.

  11. #11
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    Quote Originally Posted by DanielNJ View Post
    loadcalc.net

    Has been posted to this forum & recommended in other threads.
    OK that works. Did you use the basement in your calcs? That a completely different load calculation.

    I'll let others run the #'s but im very confident a 1 1/2 -Ton Inverter system is much closer match to 1000Sq Ft.

    Why a 4 head model?

  12. #12
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    Thread Starter
    Quote Originally Posted by pecmsg View Post
    OK that works. Did you use the basement in your calcs? That a completely different load calculation.
    The instructions say to include basement walls & floor in the construction details if it's a finished/heated basement, so I did that but I didn't include it in the space to be conditioned.

    Quote Originally Posted by pecmsg View Post
    I'll let others run the #'s but im very confident a 1 1/2 -Ton Inverter system is much closer match to 1000Sq Ft.
    Nearly every thread on this forum advises people to do a load calculation and definitely go by that. I got wide variety of differing opinions from estimators "eyeballing" the place.

    Quote Originally Posted by pecmsg View Post
    Why a 4 head model?
    Two in the upstairs bedrooms so occupants can close the doors and still be comfortable.

    Two on the main floor is because there's not a desirable place for a single wall unit in the middle of the area, so we got recommendations to use two smaller heads -- one in the front in the living room, and one at the back in the dining room. As previous replies noted, this might help cross-room air movement since putting just one wall unit at the back of the house might not have had enough throw. And if I do find out two heads on the main floor turns out to be overkill, then I think I'd relocate one of them to the finished attic in the future.

  13. #13
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    Your a little South of me so anything that's over 750 Sq Ft per ton raises Red Flags!
    My 1st floor is 1000 sq ft and a 12K window shaker does it perfectly so? 2 8K on the second floor because it hasn't been renovated and still no insulation in the walls.

    Basements are below ground so the Load / Loss is completely different. if anything they get a dehumidifier and mini split heat pump. NJ or Eastern LI same environment

    Yes a load loss calculation is required but the information provided must be accurate. 2 thumbs up for doing it and questioning the results. As far as the # in Load Calc I have to let someone else run them, my program went with the old computer.

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