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  1. #1

    Distech RTU-L, AX, OSA and economizer

    I have a an RTU-L with a shared OSA sensor located on another controller. When I bound the nviOutdoorTemp in the RTU-L, I was hoping the economizer would look to that point for its OSA. I had no luck. Any suggestions?

    OSA Temp was 60 deg.
    Econ Setpoint 65
    nviSetpoint 60
    Chilled water valve 100%
    Terminal Load 100%
    Fresh Air Damper on AO 6 10%

  2. #2
    Join Date
    Sep 2002
    Location
    Hampton Roads, Virginia
    Posts
    1,629
    Have you set nviEconoEnable to ON (both value and state to a non-zero value)?

    Also note the below from a Distech manual

    Note that if the mixed air and supply temperature are not available, the FRESH_AIR_DAMPER_MOD output will always be either
    fully closed or opened to the fixed minimum position (except in case of an emergency purge) depending on the fans state and
    occupancy mode.


    Kevin
    Last edited by klrogers; 12-07-2007 at 04:55 PM.
    "There are 10 types of people in the world.. those who understand binary, and those who don't."

  3. #3
    Okay,

    So here's what I have. Dual duct (simple unit). I configured the Hot deck as Supply Air, Cold Deck as Return Air and Return as Space (so I can control the unit based on return air, but it's the Space Temp input). I didn't want to use the Return Air Temp input to control as I needed it for one of the decks. Should I switch the Supply Air Input to the the cold deck and the Return Air Input to the hot deck? Do I have to have the mixed air sensor?

  4. #4
    Join Date
    Mar 2003
    Location
    NY
    Posts
    2,163
    I'm confused! What does your first post have to do with the last post? Are you talking about the same unit here?

  5. #5
    Join Date
    Sep 2002
    Location
    Hampton Roads, Virginia
    Posts
    1,629
    Quote Originally Posted by dapper View Post
    I'm confused! What does your first post have to do with the last post? Are you talking about the same unit here?
    Dapper, I think he is trying to use a RTU controller where a programmable would be better.

    I'm no Distech expert, but I'm thinking that if you reconfigure the sensor in the cold deck to mixed air it might work

    Kevin
    "There are 10 types of people in the world.. those who understand binary, and those who don't."

  6. #6
    Join Date
    Mar 2003
    Location
    NY
    Posts
    2,163
    Maybe it's too early in the morning but I am still really confused. How do you control a Hot deck / Cold deck system with an economizer algorithm? I don't see a lot of Hot deck / Cold deck but from what I know you maintain a constant temp in each. An RTU is going to run heat OR cooling. Does the Distech RTU-L, AX have a Hot Deck / Cold deck configuration option?

  7. #7
    No, there is no constant temperature in each deck. Because of title 24, you can only have heating or cooling running - but not at the same time. So let's look at this.

    If it's 80 degrees outside (this is Southern Cal. so it happens all the time), then the return air temp will be quite warm - say 76 to start. As the return air temp senses this, the cooling will start, and the chilled water valve will open and provide cooling down the cold deck, while the hot deck has no conditioned air except the return air, so it too is 76. Follow me so far?

    Cold deck conditioned - 55
    Hot deck non conditioned - 76

    As the weather changes and the return air starts to decrease, the cooling goes off and the heat will start, and the cold deck will now use the return air (non- conditioned).

    Cold deck non conditioned - 68
    Hot deck conditioned - 80

    There are mixing dampers on each unit that control the air as it goes to the space.

    The economizer is only required if I have a requirement for cooling and the outside air cool enough to do this. It's not that big a deal. The mechanical heat and cooling have been working fine so far.

    These are units that have been retrofitted in an absolutely decrepit school. Pinpoint accuracy is not the goal. I don't believe an EC-67 would have made any difference. A lot of what Distech does is for cold weather applications with a lot of freeze protection. This is okay as they are out of Montreal and they need all of these safeties. My frustration is there so much going on in the backdrop of the code, that it takes forever to figure it out some time.

    In this situation maybe that all I need to do is configure the hot deck as supply and the cold deck as mixed. If so, then the hot deck as supply will never go below 65 and the mixed will never go below 55.

    I also read about checking the "No limit on Fresh Air Intake". If I check that, will it now ignore the mixed air temp and let the damper do it's thing?

    This is really quite simple. I just wish it would tell you that you MUST HAVE a mixed air and supply air or and either or situation. Many times we only have one, or the other.

  8. #8
    I am confused as well. This california Law and a hot deck cold deck unit by its very nature will not work properly abiding to it. Sounds like you do need a proggramable to custumize the hell out of it.Your solution as is sounds like it may work for each season, but would have to be changed for every season change. Does the hot water have a reset schedule based on OA-T? How many zones on this unit. Space temp controlling each zones corresponding hot and cold deck dampers? Cold Deck and Hot Deck reset bands? Econ lockout? Minimum damper position? How warm are these zones when the return is 80 deg. Return sensor location?

  9. #9

    Hmm

    Freddy-B

    You are way over thinking this. If memory serves me correct, you are from a cold climate. I too am originally from a cold climate, so I understand your questions about reset etc. In addtion to that, I had never seen dual duct until I moved here years ago. I too though we had to have cooling and heating at the same time. I'm not sure where you got the return at 80 degrees?

    The mixing dampers are independent of the DDC system. They are electronic and open and close the mixing dampers from an electronic room thermostat that is wired directly to a Belimo on the damper shaft.

    The temperature control at the unit is not a problem. The issue is the economizer. That's all.

    In addition to all of this, the units are a giant POS that have been extremely poorly maintained and are really in need of replacement in order to uphold today's standards. They were originally heating and ventilating (hot deck and ventilating deck) but were retrofitted with a cooling coil in the ventilating deck. Any control is better than what they had.

  10. #10
    Got to the bottom of it today. So here's the deal. This whole thread was because I was trying to show a test to a school inspector. So keep in mind this was all for testing purposes.

    If you don't have a mixed air sensor then check the box that says No limit on Fresh Air Damper in the configuration mode.

    But here' the real trick. The economizer in this controller does not lock out the cooling unless it's mechanical cooling - in this case it's a chilled water valve. How the program works is that between 0-50% terminal load, the economizer damper will modulate to give the required temperature if the outside air temp is low enough. If the terminal load goes above 50% then the chilled water valve kicks in and the economizer dampers go closed.


    The problem I was having was that I was setting the setpoint too low and the terminal load was going straight to 100%. Once I set the setpoint only 1 or 2 degrees below the actual return temperature and waited for about five minutes, the economizer started to work.

    Thanks for all of the ideas. Much appreciated.

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