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  1. #1
    Join Date
    Apr 2010
    Location
    Upstate New York
    Posts
    43

    Hmm Carrier VVT Open Zoning System Issues

    I have a small RTU (8.5 ton) serving a small office building with 8 VVT zones. It is a basic package RTU, 2 stages of heat and 2 stages of cooling, constant volume. The system is single duct, and there is no reheat, aux heat, or supplemental heat of any sort. The system is not set up at a standard Carrier VVT system with a VVT master and "airside linkage." The previous contractor wrote a custom program that averages zone setpoints and temperatures to control heating/cooling mode and staging. I got involved when my employer relocated some ductwork as part of a renovation.

    The current setup works fairly well, but with the colder weather, I am getting some complaints about cold air blowing from diffusers in two of the zones. With dampers at minimum position or vent mode position, I would not expect this to be a problem, and was able to confirm this. With dampers forced to minimum position, airflow is not audible or uncomfortable.

    When I watched the zones in question through a heating cycle, I noticed the following: RTU burner shuts down at end of heating cycle, supply air temperature slowly drops, as supply air drops below space setpoint, damper remains at maximum position (for as long as 10-15 minutes). It looks like the supply air needs to be at least 2 degree below space temperature before the control program starts to close the damper. The occupant claims that in some cases he has been able to make the damper close by lowering his setpoint (I only have the system set for a 3 degree offset via the SPT zone sensors).

    With the current controls program, some zones are not quite satisfied when a heating cycle ends. It seems to be these zones where the damper is slow to respond to supply air temperature changes.

    Calling all Carrier controls experts... why are these dampers responding so slowly. I don't see any adjustable deadbands for zone damper heating/cooling changeover or any other parameters that would be effecting this.

    Previously, the system had been setup to cycle the supply fan. I set the supply fan to run constantly because the RTU is supposed to be providing ventilation (there is an HRV dumping fresh air into the return). I know that cycling the supply fan will eliminate the "blowing cold air" issue, but this wouldn't be a problem if the dampers were responding quickly to SAT.

  2. #2
    Join Date
    Mar 2004
    Location
    SE Michigan
    Posts
    18,215
    How about the location of the remote sensors? Its interesting that an occupant said they could get the dampers to close by changing the setpoint. Perhaps they are located in a bad spot?

    FYI, I havent worked on VVT in many years, and the system I did consisted of several controller stats, along with remote stats, relay packs, and a monitoring computer station tied into 26 RTU's.

    One more question...are the control wires shielded cable?
    To put the world in order, we must first put the nation in order; to put the nation in order, we must put the family in order; to put the family in order, we must cultivate our personal life; and to cultivate our personal life, we must first set our hearts right.
    -- Confucius

  3. #3
    Join Date
    Jun 2011
    Location
    San Jose, CA
    Posts
    115
    It sounds to me like a problem resulting from no linkage. The DAT sensors at the zone are all the box has to go off of to know what mode the unit is in. The reason they stay at open after the unit shuts off is that the box still thinks it is in heating because of the measured supply temp. You may need to setup linkage to take care of this problem.

    Does the system have a bypass? If so you should consider setting the VVT up correctly so that it controls the bypass and reports airside linakge data to the unit and back to the boxes.

    You may also need to change the minimums on the boxes to prevent over cooling. I know that you are supposed to be able to have the minimums for air quality and circulation but the reality of it is that you will still be pushing cold air into the space and it will still have the chance of overcooling unless you choke it down to nothing or next to nothing. I've had to do this in a lot of conference rooms because even at the minimum it will icebox a room with no load.

  4. #4
    Join Date
    Apr 2014
    Location
    MN
    Posts
    133
    VVT boxes should be addressed so that the highest address is the bypass, the next highest is the linkage master, and the rest of the zones fall sequentially under the linkage master. In the linkage setup for the master, you tell it how many zones (total number of VVT boxes -including the linkage master-, if a bypass exists and the air source bus and element number.
    You need to have a DAT sensor in the bypass controller as well.
    That's the basic setup for VVT zones.

  5. #5
    I don't understand how the other contractor could get around entering master and collector controllers. I agree with bacnet>CCN, DAT sensor is critical in the bypass or the system doesn't run right. But, going by Carrier's VVT Open literature the master would be the highest:

    The linked controllers must be sequentially addressed. The VVT Master must have the highest address, the bypass controller the next lower address, followed by the slave controllers
    VVT Master must have the highest address
    VVT Bypass Controller the next lower address
    Remaining zone controllers are addresses below the bypass address
    VVT Master and all its sequentially associated scanned zones must be on the same MS/TP network
    Rogue VVT zones may on the same network or a different network
    Set up is only required for the VVT Master Controller
    All Zone Controllers have defaulted linkage settings

    rpkilgor, what is the static pressure setpoint right now? The dampers always close slowly anyway, but are your problem zones closer to the heat source or far away? When you say the occupant got the damper to close, do you mean to an individual VVT box, or the damper on the RTU? The pressure dependent control may help you out a lot with the problem zones.

    If I was in the same scenario, I'd be looking at an airflow issue in just the problem controllers.

  6. #6
    Join Date
    Apr 2014
    Location
    MN
    Posts
    133
    Controls21 is correct for the VVT open. I was assuming it was a CCN job. CCN is addressed like I said.
    That being said, BACnet>CCN...

  7. #7
    Join Date
    Apr 2014
    Location
    MN
    Posts
    133
    Quote Originally Posted by bacnet>CCN View Post
    I was assuming it was a CCN job.
    And I didn't pay attention to the "Open" in the title.

  8. #8
    Join Date
    Apr 2010
    Location
    Upstate New York
    Posts
    43
    As far as how the previous contractor set the system up, it looks like there is a parameter to disable linkage. Then the RTU control program just reads zone setpoint and temperature and responds accordingly. As far as I can tell, the VVT boxes are just sensing DAT (locally) to determine how the damper should be operated.

    There is a bypass and bypass controller and it does have a temp sensor. I don't know what the static setpoint is currently, probably around 0.15".

    I should be relatively close on airflow. We did an air balance to set damper mins and maxes and verify adequate airflow to each zone. The trouble zones are not the closest to the heat source, but not the furthest either.

    A couple replies mentioned the importance of a temp sensor in the bypass. What purpose does this serve?

    Do you think that setting up linkage would make the dampers respond faster? What actually dictates the operational mode viA the linkage? Bypass air temp? Actual unit operational mode? Do the VVTZCs start reacting even before their local DAT sensors start to see a difference in air temperature? What I was seeing was what seems like an unreasonable delay on damper response to local DAT sensors.

    The occupant got the VVT box serving his office to close by lowering the setpoint. I think I noticed a similar effect. Once the heat demand in a zone is satisfied, the damper immediately goes to the minimum.

    I was reluctant to start making large changes like setting up linkage, but might be moving in that direction if that would help. If i were to do that, how would i go about it? Once the addresses are all correct what would I need to do? I am assuming I would need to create a control program for the RTU in equipment builder and download it to the UC-OPN that is controlling the RTU.

  9. #9
    rpkilgor>> I saw something interesting you put in at the start of your post.

    "The system is not set up at a standard Carrier VVT system with a VVT master and "airside linkage." The previous contractor wrote a custom program that averages zone setpoints and temperatures to control heating/cooling mode and staging."

    The funny thing is, that's exactly what airside linkage is designed to do. Say you have controllers 4-8 and an RTU addressed 9. 8 Would be your linkage master, 7 bypass, and 4,5 & 6 regular zones. 4-7 would feed zone 8 info, and controller 8 would then pass it along to the RTU. So, your RTU would receive a "linkage spt" that averages all the controllers' space temps, and then adjusts its heat/cool mode based on their statuses. I think the original contractor made things harder than they needed to be.

    Because most of the workers are actually happy, I'd leave it until it became a recurring nuissance. The temp sensor in your bypass tells that controller what air it is mixing or blocking with the RTU discharge. So, if it reads wrong or there is no sensor, it can't function correctly. Because they skipped linkage, they're not using the bypass correctly. It's acting like a single zone when it's designed to be part of a whole system. Imagine zones calling for heat, the RTU is heating, but the bypass damper only opens up 20% because it doesn't know the spaces need full heat right then.

    In a nutshell, the setup is against what Carrier designed. With that in mind, for them to be comfortable in the bad areas, you'll need to get the damper maximum down to a lower %. Consistent temp would give the impression you got the problem sorted out. No airside linkage is the real problem. You don't need to create any control programs custom, Carrier has already done the work. What you would need to do is go to every controller in Field Assistant, and under control properties, choose the original control program from the dropdown box and then click apply.

    However, this would mean a lot of time where the customers were unhappy while you tried fixing during normal business hours. I'd try to put out the fires and see how the winter goes.

  10. #10
    Did notice that you said a UC-OPN is on the RTU. So, they didn't go with an RTU Open?

  11. #11
    Join Date
    Apr 2010
    Location
    Upstate New York
    Posts
    43
    Yes, it is indeed a UC-OPN not and RTU-OPN. I do not know why it was done this way.

    I spoke at length with the local Carrier rep. He didn't like the fact that linkage was not being used, but agreed that even minus the linkage, the damper controls are not behaving as expected or desired. He told me that for a VVT box to be in heating mode, the discharge air should have to be at least 5 degrees over space temperature. What I observed was (if a call for heat exists) the damper only beginning to close from maximum when the discharge air dropped at least 3 degrees below space temp!


    Can someone explain the logic of how the VVT box determines it's operational mode? I wish I could see the logic page for these. I can only guess at how things are working based on the parameters that are exposed on the property page. And then there's the pressure dependent damper control, which is really a black box.

    I also noticed that once a zone's heat demand is satisfied, the damper behaves normally, and goes to minimum position.

    For now, I altered the control program for the RTU so that first stage heating doesn't end until buildings average setpoint is exceeded by 0.4 degrees. This way, all zones are usually satisfied by the time the RTU goes out of heating mode. I haven't heard any complaints since I made the changes, but I would like to confirm that there's adequate airflow for 1 stage of heat with all but a few zones at minimum position, since this is going to happen at the end of just about every heat call.

    It sounds like I would need to install an RTU-OPN to actually get linkage working.

  12. #12
    Join Date
    Jul 2006
    Location
    Southern California
    Posts
    256

    VVT System = Kiss you client and reputation good bye!!!

    Just some items to consider:
    1. Sounds like the SNAP programmer may have known (or found out the hard way) that the earlier versions of "SNAP" "Open linkage" just flat out did not
    work (earlier versions were quite "Alpha and Beta" software) so he may have decided to go custom SNAP logic.

    2. VVT can be a completely defective and likely illegal mechanical system design. In order to overcome the complaints of cold air blowing during the winter, or hot air blowing during the summer, 99% of the time I have seen HVAC techs adjust the zone minimum positions to zero, which violates the law regarding minimum fresh air exchanges. (this is how it is a defective and usually illegal design) (usually in the US there is no where are you allowed to have NO Minimum Airflow during occupancy based upon building codes requirements, especially with todays IAQ issues)

    3. VVT zoning is usually temperature controlled damper positioning, not air flow or CFM, so one could expect the zone damper position to be potentially partially open even after the zone temperature goes below the set point. This allows delivery of hot or cold air, when it is not needed by the zone (this is how the complaints start)

    4. If you upgrade to a RTU Open, be aware of the new 2.0 version, check it's ability to work with your VVT controllers, as you may have to do a module driver upgrade to the new 6.0. (6.0 is way cool but you will need to upgrade your tools also)

    5. If this project is in a cold area, what are you doing to "pre heat" the minimum fresh air intake , so your DAT doesn't nosedive when unit is done with heat staging? ( I have found in cold climates that many HVAC system designers fail to address this issue, and the DAT can go from 110 degrees while heating, then satisfy the zones, turn off the heat, then the DAT drops to 40 degrees, re-cooling all the zones and pissing off the tenants, due to the minimum damper positions not being at zero.

    Anyway, sounds like changing just the controller will likely NOT solve you design flaw issues. Yet I hope it does work for you.

    PS: Is the bypass duct and damper sized correctly, as I have seen most "undersized", which just adds to the defective control sequence.

    These VVT design flaws are not just a Carrier issue, the entire VVT Sequence and design concept is a failure of correct mechanical engineering
    multi zone engineering; The VVT sequence design was originally conceived to be the lowest "first cost" on HVAC multi zone construction projects
    by eliminating the need for reheat coils, boilers, pumps, or multiple "single zone" HVAC units.

    At best, VVT only works "OK" when all the zones have extremely similar exterior heat gain/loss "aspects", and internal zone heat gain/loss load profiles.
    I have worked on thousands of these, and the clients are always dissatisfied. When allowed there are fixes, but they cost$$$.
    Last edited by dracula; 12-08-2014 at 10:37 AM.
    "We are what we repeatedly do. Excellence, then, is not an act, but a habit" Aristotle

    Remember to "Pay it Forward"; help out the newer generation of techs, remember someone during our career helped us! ("Pay it Forward" was by someone smarter than me!!)

  13. #13
    Join Date
    Jul 2006
    Location
    Southern California
    Posts
    256
    Sorry, forgot to mention a bit about the VVT PAT sensor;
    (I noticed you stated that the VVT zones were sensing DAT "locally")

    Where are your VVT zone PAT (DAT) sensors located, before or after the VVT zone damper?

    Upstream on the AHU side of the VVT zone box is where they need to be if they
    are going to be doing VVT stand alone zone "Mode Available" sequence, without "linkage"
    otherwise they may have very sluggish (slow) mode selection because they are not sensing
    what primary air temperature is available to the zone from the RTU supply air duct.
    "We are what we repeatedly do. Excellence, then, is not an act, but a habit" Aristotle

    Remember to "Pay it Forward"; help out the newer generation of techs, remember someone during our career helped us! ("Pay it Forward" was by someone smarter than me!!)

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