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  1. #1
    Join Date
    May 2003
    Location
    Indiana
    Posts
    10
    We are working on a hot deck cold deck system with pnumatic zone dampers. The cold deck damper is closed with no air preasure and the hot deck is open. The rooms have Johnson T-4002 201 single purpose high velocity stats. The problem we are having is that there are two actuators on one stat and the calibration is not going too well. We can't get the dampers to
    modulate properly. I have tried to find info on the stats and all I found is how to calibrate them. Does anyone know if they can work a mixing box properly with two actuators? And would changing the spring preasure in one actuator help in any way?


    Thanx in advance for any advice

    [Edited by joedeck1 on 08-15-2005 at 11:49 PM]

  2. #2
    Join Date
    Feb 2005
    Posts
    50
    Just guessing, sounds like these two zones have been grouped together perhaps due to a TI. If that stat is a single pipe feeding both actuators, you may have a fitting,tube,or diaphram leak.If single pipe, it is fed with a five thou[.005]restrictor then that branch signal is teed to the actuators.Any leak wont let the pressure build to stroke the dampers. If the stat is two pipe,test with a gauge and a full call for cooling,you will soon find your leak.Dont forget to re-calibrate,to mid point if hot and cold water, or re adjust strke if steam or dx.

  3. #3
    Join Date
    Nov 2001
    Location
    Seattle, WA
    Posts
    7,744
    Are you sure that the thermostat controls the hot/cold deck dampers actutators directly? Some have a hot/ cold deck discharge controller. Some have a variation.

    The hot deck is normally open, called "fail safe",should a simple thing like the pneumatic compressor stop then all the building would go into heating to prevent freezine.

    The cold deck goes to a normally closed position to help prevent the chiller(s) into going into full cooling.

    If the T4002 is calibrated correctly it's quite possible that you have pneumatic pilot relays which stage the dampers to sequence so as one opens the other closes. The relays should be on the side of the damper actuators and they need to be set properly or you never will get any control.

    More info if you please.
    "The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
    - Alexis de Toqueville, 1835

  4. #4
    Join Date
    May 2003
    Location
    Indiana
    Posts
    10
    I guess I didnt explain very well. I have air preasure and no leaks. My failsafe position is heat. The problem is getting the stat to modulate so the air can be mixed in the mixing box. The dead band is so close you get full heat or full cool with no mixing. I was wondering if a T-4002 was capable of mixing. It says in the specs it is a single purpose stat. Have you guys used this stat in a mixing box situation? I think a T-4054 is made for a mixing box but I'm still trying to find a solution as of now.

    Oh and the stat is the only thing operating the dampers.

    The crazy thing about this deal is the hot deck cold deck and actuators are all new. Four penthouses full of it. And they did nothing down in the building, didn't replace a single stat.

    Like having a new car with no steering wheel

    [Edited by joedeck1 on 08-16-2005 at 09:04 PM]

  5. #5
    Join Date
    Aug 2005
    Posts
    145
    What you need to know is the hot and cold deck actuator spring pressures. You can add a positive positioner to shift the spring range on one actuator if the deadband is too small.

  6. #6
    Join Date
    Feb 2005
    Posts
    50
    I guess I misunderstood, I assumed this was a Multizone air handler.Sounds like what you have is a hot and cold mixing box. First check your actuators,the spring ranges cant overlap.If this is a typical JC job,you might have a 4-8 N.O. htg actuator and a 9-13 N.C. clg actuator.Out of the box stat sensitivity is 2-3 #s per degree F.,so you can see that a 3 degree temp change will send you from full heat to full cool.Check to see if: someone changed[increased] your sensitivity,your air balance is correct,and the stat is in a good location,near the "mixing" portion of the space, not in the path of the supply air. You can always change the sensitivity,even down to 1# per degree.

  7. #7
    Join Date
    Nov 2001
    Location
    Seattle, WA
    Posts
    7,744
    What old fitter said!

    There is a big different to us old guys that know what a real hot and cold deck application is compared to mixing boxes.

    The mixing boxes come out of the factory either set up with one damper actuator to modulate between the hot/cold supply. Or they can have two which are factory installed with the proper spring ranges.

    If the thermostat is acting as if it is a two position thermostat then most likely someone has messed with the sensitivity as stated above. Check the sensitivity then recheck the calibration and you should be OK if there are no other problems such as linkage hangup or some other odd control sequence.

    Also, the easiest way to troubleshoot the dampers is with a squeeze bulb or better yet, make your own variable supply pressure using a minimum position switch which you probably have laying around.

    [Edited by DeltaT on 08-16-2005 at 10:27 PM]
    "The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
    - Alexis de Toqueville, 1835

  8. #8
    Join Date
    May 2003
    Location
    Indiana
    Posts
    10
    This is an old style hot deck cold deck setup. There are four penthouses each has a hot deck cold deck that operates close to a quarter of the buildings space. Each hot deck cold deck has around 25 zones. Each zone has what I call a mixing box, where your hot deck comes off of the top and the cold deck on the bottom and then the air mixes,goes into one duct and out in the room. Each zone represents a room. The specs called for one actuator with a single shaft to control both dampers. For some reason they could not use a single shaft and actuator to work both dampers so they used two. One on top for hot deck and one on bottom for cold deck. Hot deck N.O. cold N.C. So I have two actuators and two dampers with one stat for each room. I can't get the dampers to modulate. For example I had a room temp of 74 I set stat for 65 and nothing happens. I turn stat to 55 and get full cooling. Turn to 85 and get full heating. Can't get anything in between. I have adjusted everything I know to adjust without anything close to a solution. I just don't think the stat is capable of modulating two actuators.

    Hot decks are supplied by steam boiler in another building. I have only worked on a few hot deck cold deck systems. They are not very efficient IMHO. always heating and cooling at the same time and relying on dampers to mix the air to a good discharge temp does not even sound logical to me. Running a steam boiler in the summer to pull humidity. What were they thinking?





    [Edited by joedeck1 on 08-17-2005 at 12:40 AM]

  9. #9
    Join Date
    May 2003
    Location
    Indiana
    Posts
    10
    Originally posted by okemechtech
    What you need to know is the hot and cold deck actuator spring pressures. You can add a positive positioner to shift the spring range on one actuator if the deadband is too small.
    I have looked at the spring tension. with two actuators on one stat, let me know if I'm wrong here . Both actuators are 8-13 psi range, 8 closed 13 open. If I change the tension on one say like the cold side, since the hot is N.O. It will still close the hot side because it is set at a lower tension.

    Now if I had a different stat that was capable of operating in two different preasure ranges it could be possible. Keep in mind if I go that route I would need to change tension on close to 100 actuators. Man that would be a bad week....

  10. #10
    Join Date
    Aug 2005
    Posts
    145
    So you at least see how the dampers are operating. There is an overlap, where both dampers are open at the same time. This condition blends the air so that there is always some airflow. Think of it this way; The normally closed 8-13# cooling damper starts to open @ 8# and is fully open @ 13#, whereas the normally open 8-13# heating damper starts to close @ 8# and is not fully closed until it reaches 13#. So between an 8-13# stat signal there is some blending going on. The crossover point is at midspan or 10.5#. Of course this is theoretical operation. If there is any mechanical resistance, binding, or position deviation the spring ranges are thrown off. Make sure that you don't have any severe binding problems.

    Which is why systems like this so energy inefficient that they are no longer being installed.

    You may want to pay a little more attention to the hot deck and cold deck settings at each main coil, Resetting the deck temperatures would help reduce energy consumption.
    It is most likely already being done, but look into recalbration. Some systems were reset based on outdoor temperature or zone high/low signal selection.

  11. #11
    Join Date
    Nov 2001
    Location
    Seattle, WA
    Posts
    7,744
    Originally posted by DeltaT
    Also, the easiest way to troubleshoot the dampers is with a squeeze bulb or better yet, make your own variable supply pressure using a minimum position switch which you probably have laying around.
    [Edited by DeltaT on 08-16-2005 at 10:27 PM]
    Once again, if you can modulate the dampers with a squeeze bulb or minimum position switch which will gradually increase/decrease the branch line pressure to the dampers, then you problem is the thermostat and its sensitivity.
    "The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
    - Alexis de Toqueville, 1835

  12. #12
    Join Date
    Feb 2005
    Posts
    50
    If the whole building has had these actuators replaced incorrectly,why not just buy a new actuator for the hot deck,4-8 spring. What a hero youll be. I just ask that you act like you know exactly what your doing; anything can be fixed with pneumatics,you could even see if they sell just a new 4-8 spring,and your in business.

  13. #13
    Join Date
    Aug 2005
    Posts
    145
    My recommendation is to stick as close to the original concept of the hot deck cold deck mixing box system. You want to be careful in selecting a modification, especially when you have that many zones.
    The idea of changing one spring to a 4-8# spring range may create a ventilation problem at an 8# stat signal. The original mixing box theme would never be able to have both dampers closed at the same time if you were using one actuator.

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