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  1. #1
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    Excess Load in Three Zone System

    Assume my duct work will not handle the excess supply for the three zone system I plan to have installed in my 3500 sq ft, two story house. Let's also assume that it is too expensive to upgrade the ducts. What are my options. I have read about dump zones, which don't seem to have much support and bypass circuits with counterweighted dampers, which seem to be the most common. Are there any other options?

    I think I have seen comments about dampers that have some type of electronic barometric control but now I can't find them on the Internet.

    Which manufacturers should I be researching for zone systems and controls?

    Any comments or advice is welcome.

  2. #2
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    Quote Originally Posted by mhitim View Post
    Assume my duct work will not handle the excess supply for the three zone system I plan to have installed in my 3500 sq ft, two story house. Let's also assume that it is too expensive to upgrade the ducts. What are my options. I have read about dump zones, which don't seem to have much support and bypass circuits with counterweighted dampers, which seem to be the most common. Are there any other options?
    .
    If you happen to live in Calif. there outlawed or are in the process of being outlawed.
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  3. #3
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    There is no way to get around duct upgrades or replacement. The duct system needs to built with zoning in mind in order for it to perform properly. Also, the only way to get the most out of a zone system is with a 2 stage AC system. You do not want a unit running at 100% capacity when only 1/3 of the zones is calling. When it comes to zoning there are no easy ways out. Do it right, do it once.
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  4. #4
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    The technical term for what you're describing is "relief strategy". That is the method that will be used to prevent excessively high static pressures and/or insufficient airflow across the mechanical components.

    There are only 3 options for relief strategy. You've listed 2 already, those being a barometric by-pass damper, the second being a 'dump' zone. The third option is to relieve excess capacity to other zones. The best solution is to have a computer track the temps in the various zones, the trend of the room temps, the time since the last call for treatment in each zone and how that compares to historic needs for the zone, the known capacity of each zone and use modulating dampers to control the actual airflow into each zone so that minimum established airflow is allowed under all conditions. There is one other possible solution for a very small zone and that's typically referred to as a 'non-voting' zone, meaning it's thermostat is only a limiter and cannot command equipment operation. Thus the non-voting zone gets either heating or cooling only when another voting zone calls the equipment into service.

    The third option, modulating zone dampers is available with the Bryant Evolution or Carrier Infinity zone control systems and equipment. The computer is the User Interface and depending on how your duct system is laid out, can use a single damper for several supply outlets or you could have a damper on each supply outlet to the maximum allowed per zone.

    I've installed and worked on many of the Evolution/Infinity zone systems and they're all but bulletproof if installed properly. Be advised that if the duct system is under sized, no system in the world can solve the problem. But when it's just the single zone that's under sized, a relief strategy is needed.

    If your installing company resorts to a barometric by-pass damper, it should be sized appropriately (most are way too big and don't operate properly) and the length of the by-pass should be reasonably long to allow for some air temperature change between supply and return. Often they're installed with very short runs between the supply and return plenums. Longer runs are preferable.

    One final note: The entire duct system should be accurately sized using Manauls J-8 and Manual D. Guessing just doesn't cut it.
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  5. #5
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    Quote Originally Posted by jimj View Post
    If you happen to live in Calif. there outlawed or are in the process of being outlawed.
    At the very least, the energy credits for T24 compliance are going away for new construction when a bypass is used.

    By not resolving the real duct issues for single stage equipment/part load apps and moving forward with any sort of bypass, you're just giving the contractors budget to the utility over time. The efficiency/eer takes a nose dive with the supply to return bypass. It stinks.

    Oversize the ducts by 20-30% and set/restrict minimum position on non calling zones to 70-80% volume...no bypass needed and your wallet & equipment will thank you. You could shift the same 20-30% with constant volume ductwork and add some dampered dumps in key locations. Those are the west coast solutions, can't say if it works in humid or bitter cold climates.

    Skip is right about the load calcs and duct design, without it, you're just guessing and hoping for the right outcome. A poorly executed strategy may not live up to its real potential or expectations.

  6. #6
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    Quote Originally Posted by btuhack View Post
    At the very least, the energy credits for T24 compliance are going away for new construction when a bypass is used.

    By not resolving the real duct issues for single stage equipment/part load apps and moving forward with any sort of bypass, you're just giving the contractors budget to the utility over time. The efficiency/eer takes a nose dive with the supply to return bypass. It stinks.

    Oversize the ducts by 20-30% and set/restrict minimum position on non calling zones to 70-80% volume...no bypass needed and your wallet & equipment will thank you. You could shift the same 20-30% with constant volume ductwork and add some dampered dumps in key locations. Those are the west coast solutions, can't say if it works in humid or bitter cold climates.

    Skip is right about the load calcs and duct design, without it, you're just guessing and hoping for the right outcome. A poorly executed strategy may not live up to its real potential or expectations.

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  7. #7
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    Red face ZONING EXPERIENCE = RELIABILITY ! !!

    Quote Originally Posted by mhitim View Post
    Assume my duct work will not handle the excess supply for the three zone system I plan to have installed in my 3500 sq ft, two story house. Let's also assume that it is too expensive to upgrade the ducts.
    Are there any other options?

    Which manufacturers should I be researching for zone systems and controls?

    Any comments or advice is welcome.
    If it's too expensive to address ductwork, it's too expensive to have a reliable system. So, PLEASE DO NOT ask for ANY Performance Guarantees beyond when the installer pulls out of your driveway.


    Let's assume DYI ... Done Yourself Insanely


    WWW ZONEX COM

    PAY ME NOW - OR PAY ME 4 X LATER !
    THANK YOU VERY MUCH.

    VERY FEW contractors will get zoning installed and operating right.

    When the budgets right, your comfort and lack of worries will be right.
    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

  8. #8
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    I SURE AM GLAD YOU ENDED your statement with - ANY comment is welcome. RE READing comments # 4 & 5 six times will be well worth your effort.
    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

  9. #9
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    Smile

    First of all to dan_sw_fl. This is absolutely not a DIY. What, you think I haven't been paying attention to you guys? Plus I have no problem letting my contractor tear into drywall if it is neccessary to do the job right!

    Dan, It makes total sense to use a two stage compressor for a zoned system and if that is true wouldn't one of the new inverter/fully variable speed compressors be ideal? The problem is I have only seen HP in variable speed and no AC units, have I missed something. HP is not an option for me at $0.17/KwH. If there are any inv/var speed AC units being offered please let me know and are they an expensive premium? A lot if these decisions don't pass the cost/benefit equation even though they would be cool to have.

    Skip - great info. I will save your comments and discuss them with my potential contractors. I really like the idea of computer controlled, modulating dampers. When you say "...accurately size your duct system" is this based on the manual J and D-8 for a non-zoned system (which are then slightly oversized for the relief strategy for the zoned system) or do the calcs take into consideration the details of each zone and then give a duct size? Sorry about my lack of understanding on the calcs.

    If I end up needing a bypass (hope not esp. if it is illegal in Kali) then the long circuit makes a lot of sense even if it is not the most economical way. Would you under insulate this circuit to enhance the heat or cooling loss for this application?

    btuhack - you seem to be describing a static version of what Skip is talking about. If I understand correctly, no zone is ever fully shut off, which bleeds off just enough pressurre to prevent icing and the other horrible things I don't yet understand. Plus this bled off, conditioned air is still contibuting to the entire space since no house is a sealed, compartmentalized system. Am I right. Plus you are saying add 20 - 30% to the duct sizes above what the calcs call for, right?

    Thanks again for all of your comments and knowledge now if one of you would just move to the Sacramento area I would be in good shape.

  10. #10
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  11. #11
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    Mhitim, yes and yes.

    The scheme I described was explained and demonstrated to me by a guy named John Proctor. He's a mech eng based in Marin and contributes his time to the California energy commission and PG&E training. His group has done the studies and has the numbers to back the claims of efficiency expectations. I think the commissions view of bypass damper apps and future code is based largely on his findings.

    I had little belief in the opportunities that zoning offered to most residential apps despite what the home improvement shows would have you believe and don't consider it a panacea for all, but the method described is the best option for those who don't invest in the infinity method($) which addresses /accomodates for part load conditions. Your load diversity will define if zoning is right for you, so get an accurate room by room load calc before any decisions. Example: take 2 tract homes, same block, same design but rotate one 180 degrees and add some tree shade and you could have 2 very different situations to deal with.

    Proctor does training classes for PG&E, it might be worth your while to hold off on any projects untill you can attend.

  12. #12
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    http://www.engineeringtoolbox.com/he...gs-d_1117.html

    With HSPF of 10, COP = ~3, NET electric cost = $0.17 /3 = $0.057 /kW heat output.

    One needs to determine the economic cross-over point based on the actual COP close to 35'F. I guess the $0.17/kW rate would put your economic balance point in comparison to CHEAP natural gas as a little disadvantage close to low-mid 40'F.
    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

  13. #13
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    Quote Originally Posted by mhitim View Post
    Dan, It makes total sense to use a two stage compressor for a zoned system and if that is true wouldn't one of the new inverter/fully variable speed compressors be ideal? The problem is I have only seen HP in variable speed and no AC units, have I missed something. HP is not an option for me at $0.17/KwH. If there are any inv/var speed AC units being offered please let me know and are they an expensive premium? A lot if these decisions don't pass the cost/benefit equation even though they would be cool to have.

    To the best of my knowledge, HP's are the only available modulating cooling units at this time. It doesn't make sense for the manufacturers to have multiple lines when there's just a defrost board and 4-way valve to add to make the system into a HP. That said, yes, they are at a significantly higher premium to purchase. But the benefits to be reaped include variable capacity to as low as 30% of full volume. In fact, the caution with those units to be sure to NOT oversize the ducts too much or one could find insufficient service to some rooms when operating under certain conditions. The designer simply needs to use the room/zone info to properly size each zone in reference to the output of the unit.

    I will also add that $.17/kW compares closely with out electric rate locally and I can tell you I've done the numbers and a modulating HP with an HSPF of 13.00 can compare very favorably with a high efficiency gas furnace! IMO, these are a real game changer!


    Skip - great info. I will save your comments and discuss them with my potential contractors. I really like the idea of computer controlled, modulating dampers. When you say "...accurately size your duct system" is this based on the manual J and D-8 for a non-zoned system (which are then slightly oversized for the relief strategy for the zoned system) or do the calcs take into consideration the details of each zone and then give a duct size? Sorry about my lack of understanding on the calcs.

    [COLOR="rgb(75, 0, 130)"]The total load should first be calculated using Manuals J-8 and S. Whether both heating and cooling are being considered should also play into the calculations and equipment selection. With a V/S outdoor unit, it's entirely possible to oversize the unit by as much as 1-ton to achieve higher capacity for heating purposes and still provide proper cooling and dehumidification in summer. Once the equipment is selected using the appropriate science, then it's time to design the duct system. Manual 'D' should be used and the loads should be set according to the output of the HP and the size of the zones. The Infinity/Evolution zone systems variable zone damper capability should factored in for best results. This is definitely not a job for the uneducated or for those who guess. The system designer should able to both explain the design methods as well as produce ACCA documentation supporting the design. As such, the designer/installing company should be able to guarantee the results.[/COLOR]

    If I end up needing a bypass (hope not esp. if it is illegal in Kali) then the long circuit makes a lot of sense even if it is not the most economical way. Would you under insulate this circuit to enhance the heat or cooling loss for this application?
    Shop carefully for a fully competent company and you should get great results.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

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