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  1. #1
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    Single large 4-zone unit vs. two smaller 2-zone units?

    We are building new frame-construction on monolithic slab in North Florida. Upgraded insulation all the way around (low-E glass + R13 Spider blown-in insulation in walls + Icynene spray foam on roof deck so all ductwork will be within insulation envelope.)

    Working with highly reputable HVAC contractor. Will be doing Carrier heat pump, and leaning toward variable-speed 18VS. Units will be located above garage. Per my request, I asked for an HVAC plan with 4 zones:

    ~3300 sf downstairs (3 zones)
    ~600 sf bonus room over garage (1 zone)

    His load calculations estimate 4.6 ton for whole house. Or if we use two units, 2.7 ton + 2.1 ton (some extra for additional fresh air intake).

    We're considered two options:
    Option 1: Single 5 ton Carrier 18VS unit (4 zones)
    Option 2: Two smaller Carrier 18VS units (2 zones each)

    Questions for Pros:

    1. Pros and cons of one large unit vs. two smaller units? (duct work, energy efficiency, maintenance issues, air filtration differences, comfort)

    2. Complexity/reliability of a 4-zone unit, as opposed to two 2-zone units.

    3. Any other things we're missing or should be considering?

    This forum is great! I've learned a lot, and look forward to hearing from you.

  2. #2
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    1. Bonus ROOM : multi or mini-split
    2. 3,300 Sq Feet : ... 2 Three-Zone systems.

    Duct work issues are somewhat less complex .!
    < 1% of mechanical contractors would be able to address issues with YOUR HOME.
    .. I DON'T CARE IF they are Know as Highly Reputable or Not.

    .. Have they done 30 residences like yours in the last 2 years. .?
    or are you volunteering to be the Initial guinea pig or Intermediate guinea pig.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  3. #3
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    Thread Starter
    Quote Originally Posted by dan sw fl View Post
    1. Bonus ROOM : multi or mini-split
    2. 3,300 Sq Feet : ... 2 Three-Zone systems.

    Duct work issues are somewhat less complex .!
    < 1% of mechanical contractors would be able to address issues with YOUR HOME.
    .. I DON'T CARE IF they are Know as Highly Reputable or Not.
    Thanks Dan for responding. I understand your words of warning, and that's why I'm here trying to get additional input from pros like you before we commit. The contractor does dozens of large, new home installations per year with multiple custom home builders in our area, so I'm as confident as I can be with the capability of the HVAC contractor. It just comes down to this question of 2 units vs. 1 unit, and the contractor really just says it's primarily a tradeoff of lower installation cost (1 unit) vs. redundancy (2 units).

    To answer your questions..

    Bonus room would be another full zone (not a mini-split). If we went with two units, the plan would be one unit split between master and main living/kitchen, second unit split between kids bedrooms and bonus room.

    For your 2nd point, are you suggesting two, 3-zone systems, for a total of 6 zones?

  4. #4
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    5 ton system on 4 zones. would point out that hi seer systems have variable speed, and are controlled by manufacturers zone controls. the systems and zone controls are communicating, they "talk" to each other.

    The problem w/this or any large unit w/4 zones, is that the smallest zone calling by itself may be too small for the system to run on. ie, a 1 ton zone on a four ton unit won't work. unless the unit is 2 stage, the controls will decide not to run and start to put out fault codes.
    Even some two stage units are 3/5 ton stages, so 3 ton on a 1 ton zone is still too much.

    So each individual zone would have to all be above the minimun airflow requirement for the main system, or else....



    2 separate systems would give redundancy, and the output of system would tend to better match the smallest size zone
    Col 3:23


    questions asked, answers received, ignorance abated

  5. #5
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    Quote Originally Posted by billygoat22 View Post
    The problem w/this or any large unit w/4 zones, is that the smallest zone calling by itself may be too small for the system to run on. ie, a 1 ton zone on a four ton unit won't work. unless the unit is 2 stage, the controls will decide not to run and start to put out fault codes.
    Even some two stage units are 3/5 ton stages, so 3 ton on a 1 ton zone is still too much.
    So each individual zone would have to all be above the minimun airflow requirement for the main system, or else....
    Thanks billygoat! The unit we are looking at is the 5-speed Carrier 18VS heat pump. My understanding of this product is that the lowest setting (speed/power?) is 25% of the maximum. If this means 25% of the maximum heating/cooling power, then the lowest setting on a 5 ton system, would be 1.25 ton. Is this correct? If our smallest zone is at least ~1.25 ton, we should avoid the minimum load problem you describe above, right?

    Another general question I have is how much more complex is a 4-zone system compared to a 2-zone system. Is a 4-zone twice as complex as a 2-zone or is it like 10 times more complicated?

    And I'll pose this last question to contractors/installers... How many 4-zone systems have you ever installed?

  6. #6
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    I've worked on a 3 zone, which has the option of going to 4 zone. Not sure what the minumun airflow for zones/output is on particular Carrier you have, likely varies w/manufacturer.

    Not everyone that comes in will be able to troubleshoot/install such a system, since zoning controls and design limitations have caveats w/ them, like Dan said.

    4 zone would likely just have another 2 dampers, thermostats, and maybe second zone controller (extension) to allow the 4th zone.

    2 zone w/2 system would somply double what is needed for a 2 zone by itself.

    if its a communicating system, everything would be propietary (and would have to be to work w/high seer eq) so future replacement of equipment may require all new zone controls, or stay w/same manuf, assuming they still support the old controls.
    Col 3:23


    questions asked, answers received, ignorance abated

  7. #7
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    also, that's a lot of duct attached to one unit w/ 4 zone- would require a good deal of space to fit all ducts to unit, since would be at least 4 supply ducts and 4 return ducts. even if they extended the main ducts to get 4 smaller ducts for zones, will still need access to each zone damper on each duct.

    2 zone would need have two systems but w/fewer duct connections to deal with.
    Col 3:23


    questions asked, answers received, ignorance abated

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