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  1. #1
    Join Date
    Aug 2002
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    156

    Vid commissioning for 2 stage rtu and vav boxes

    Hello all I've been gone awhile and now I'm back....My boss bid a job for the city offices to change cv rtu's to vfd drive.the rtu's are 2 stage heat and cool.Each zone uses vav boxes with reheats.today I was installing the first of 4 drives but all that was ordered was the drive,no controls.I thought I'd use static pressure transducer with 4-20 ma signal.My problem is I'm not sure how to commission it.I will still need certain airflow requirements for each stage to work properly.....any ideas???

  2. #2
    Join Date
    Aug 2006
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    Mixing oil and fire with a big spoon.
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    4,524
    you should still have a low pressure differential cut-out switch to keep the heating and cooling from running if there isn't enough air flow. as for controls, most VFD run rtu's use a discharge air temperature sensor instead of the wall mounted thermostat for controlling the heating and cooling stages.
    "If you pull one more stunt like you just pulled with Tommy, you won't have to get on a plane because I will personally kick your ass from here to Korea!" - Best of the Best

  3. #3
    Join Date
    Aug 2002
    Posts
    156
    I understand but the thermostats go to the vav boxes and the rtu gets signal to run,I don't think temp will work.as vav boxes open and close their dampers duct static changes.just trying to figure out how to set up without starving each stage of airflow. Plus I'm adding this to an existing constant velocity system,I don't and can't add too many controls since my boss figured none!!.

  4. #4
    Join Date
    Jul 2011
    Location
    East coast USA
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    you need controls to do this. The VFD would run off static pressure. you need a balance performed. you need to know the design static of each system. You have two stage heat and reheats? are these Fan powered VAVs? you need controls, how will the system know when to go into heat. This sounds like a cluster f@@k. You cant run a DX unit off a discharge air temp sensor it wont run right. What size units are we talking about.

    if you run just the unit off of static that will take care of your VFD, but you need to control space temp. VAVs will open and close depending on demand but what changes the system over? What pulls in the reheats? you see where this is going. You need discharge air sensors on each zone, return air sensors. you need to know Min -max cfm's on VAVs. there is a Min and a max CFM for heat and cool. you just cant put VFDs on a rtu and slap on a few sensors and think this will run right. Not much you can do, stop work and go home.

  5. #5
    Join Date
    Aug 2002
    Posts
    156
    Thanks for responding,this system has been in place since 2002,all rtu's have 2 stage gas heat,I personally haven't seen sequence of operation for we did not install originally.all I'm tryin to do is install the vid on this current system.we have original balance report and will be balancing again.Im going to try to dig up the sequence today I have 4 drives to install on 4 different rtu's

  6. #6
    Join Date
    Jul 2011
    Location
    East coast USA
    Posts
    1,000
    what kind of controls are on there now?

  7. #7
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,340
    Quote Originally Posted by dlove View Post
    You cant run a DX unit off a discharge air temp sensor it wont run right
    Why? We have tons of DX AHU's controlling staging off of DA. Is it the best....no, but it works.

  8. #8
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,340
    Quote Originally Posted by hvacr9 View Post
    Thanks for responding,this system has been in place since 2002,all rtu's have 2 stage gas heat,I personally haven't seen sequence of operation for we did not install originally.all I'm tryin to do is install the vid on this current system.we have original balance report and will be balancing again.Im going to try to dig up the sequence today I have 4 drives to install on 4 different rtu's
    I would look into the existing controls and equipment first. Why are you ripping out the bypass dampers and trying to install vfd's on the drives? It doesn't seem there was much thought into this solution......odds are there wasn't much thought into if this solution will really solve the underlying real problem with the system.

    If I were you, I would either ask your boss to lay the entire job out(so it's not on you). Or talk to your boss and find out what the real problem is with the system and then start from scratch will a solution.

    Something doesn't seem right.

  9. #9
    Join Date
    Jul 2011
    Location
    East coast USA
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    1,000
    Quote Originally Posted by ascj View Post
    Why? We have tons of DX AHU's controlling staging off of DA. Is it the best....no, but it works.
    I didn't say it wouldn't work. I sure you have the correct controls installed. But, for his stuff i wouldn't try it on a RTU unless it was designed to run that way from the start. Anyway he has others issues here.

    and Yeah why put VFDs anyway? I walked into a few jobs where they paid for VFDs and was told that they would save money. But come to find out the system runs full open 90% of the time. Nobody took the time to check duct design and all that. The system was to small to start with and during most days needed all they could get from the fan... just a waste of money and time.

  10. #10
    Join Date
    Mar 2003
    Posts
    7,326
    use the static pressure transmitter to control the drive speed. use the old fan contactor for your enable. when the existing controls call for fan on, the contactor will pull in. Take your power direct to the drive, and out of it, grab the line and load termianl of the fan contactor and run your enable contacts across there. when fan is called for, contact will close, enabling vfd. set the vfd to ramp to minimum of 30 htz at 4 ma, and 60 htz at 20 ma. put some relatively slow ramp timing in there and wave goodbye. boss wants half ass, he called the right guy, lol.

  11. #11
    Join Date
    Aug 2002
    Posts
    156
    The city received an efficiency grant,to put motion sensors for lighting,change out fixtures,add vfds to rtus.so he just figured getting vfds without consulting anyone.We also figured balancing.each system uses one master stat for each rtu,then vav boxes with reheats to all zones attached to that rtu.I have al engineering info on programming proportional dampers on vfds for each zone required airflow.My intention was to add discharge sensors to control staging,differential sensors,or static sensors to send ma signal to vfd.The problem I have is with the vav boxes,with the reheat enabled the damper rarely shuts,open for cooling,shuts down to min pos,if space temp drops even another degree dampers open again and reheat is enabled.ducting is perfect sized properly units have no problem supplying airflow.My issue now is that I need to get these vfds to operate.I considered eliminatin the reheats and close dampers 80 percent once cooling or heating is satisfied for each zone,then I can control my vfd with static sensor and staging with discharge temp.i spent today finding out that every vav box was powered off for last 5 years dampers wide open.Whatever I come up with will be better than it was before.....

  12. #12
    Join Date
    Aug 2002
    Posts
    156
    Oh ya the vfds are controlled by zone sensors for the vfd I can and have adjusted all min max settings at stats to control airflow,also a 60 to 80 degree adjust wheel insind sensor

  13. #13
    Join Date
    Jul 2011
    Location
    East coast USA
    Posts
    1,000
    Quote Originally Posted by hvacr9 View Post
    The city received an efficiency grant,to put motion sensors for lighting,change out fixtures,add vfds to rtus.so he just figured getting vfds without consulting anyone.We also figured balancing.each system uses one master stat for each rtu,then vav boxes with reheats to all zones attached to that rtu.I have al engineering info on programming proportional dampers on vfds for each zone required airflow.My intention was to add discharge sensors to control staging,differential sensors,or static sensors to send ma signal to vfd.The problem I have is with the vav boxes,with the reheat enabled the damper rarely shuts,open for cooling,shuts down to min pos,if space temp drops even another degree dampers open again and reheat is enabled.ducting is perfect sized properly units have no problem supplying airflow.My issue now is that I need to get these vfds to operate.I considered eliminatin the reheats and close dampers 80 percent once cooling or heating is satisfied for each zone,then I can control my vfd with static sensor and staging with discharge temp.i spent today finding out that every vav box was powered off for last 5 years dampers wide open.Whatever I come up with will be better than it was before.....

    The city received an efficiency grant,to put motion sensors for lighting,change out fixtures. Add vfds to rtus.
    They will be sorry they did that, just did a big build out using light sensors, I already disconnected 20 of those and changed several to regular switches.
    so he just figured getting vfds without consulting anyone.
    1st Major problem
    We also figured balancing each system using one master stat (how will using a master stat help balance a system?) for each rtu, then vav boxes with reheats to all zones attached to that rtu. I have all engineering info on programming proportional dampers on vfds for each zone required airflow.
    My intention was to add discharge sensors to control staging.
    Staging what? The compressors? Heat?
    differential sensors, or static sensors to send ma signal to vfd.
    The problem I have is with the vav boxes,with the reheat enabled the damper rarely shuts.
    You need the air so not to burn out heat strips. The money they will spend reheating 55 or 60 degree air to warm up a room will blow their minds over time.
    ,open for cooling,shuts down to min pos,if space temp drops even another degree dampers open again and reheat is enabled. Is this system in a heat mode or cool mode during this time? What program is telling it what to do????
    ducting is perfect sized properly units have no problem supplying airflow. Whew, that takes a load off
    My issue now is that I need to get these vfds to operate. Good luck if that’s your only problem
    I considered eliminating the reheats and close dampers 80 percent once cooling or heating is satisfied for each zone. Not a good idea, people will complain of temp swings.
    What will tell the system to switch from cool to heat, or vice versa?. You are butchering a true VAV system. You really need a control guy to walk you through this.

    Then I can control my vfd with static sensor and staging with discharge temp. I really see no reason to have a discharge air temp to control staging.
    i spent today finding out that every vav box was powered off for last 5 years dampers wide open. Whatever I come up with will be better than it was before.....or worse

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