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Thread: RTU open

  1. #1
    Join Date
    Jan 2013
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    Dyersburg, TN
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    RTU open

    I have several Factory mounted rtu opens mounted in RTU. I am getting regular complaints of cold air blowing before the heat comes on. After monitoring and visiting the site many times i saw what was going on. There is no fan status on these units so it is not a delay in the fan status switch. I called tech support and they said that the fan will operate for a certain amount of time. They told me the fan is like on a linear setpiont and until it hits a certain point the PID loop for the heating is activated. They said i would have to speed up the PID loop for the heating algorithm. Not good on PID loop tuning. Which one do i need to adjust to do this. thanks

  2. #2
    Join Date
    Apr 2014
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    MN
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    If you're using FA, you can go to the properties page and click on BACnet points to get to the "BACnet Reverse Acting Heating PID" loop. Click on htg_pid_ra and it will open up a pop up window where you can adjust the PID and the update interval.
    I think the default update interval is 30 seconds. I'd probably start there, drop it to 15 seconds. Maybe bump up the proportional and integral depending on where they're set.
    I'd probably look at adding a fan status point, should be too expensive.

  3. #3
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    I attached a screen shot of the heating pid for you to take a glance at. What should i adjust them to
    thanks
    Attached Images Attached Images  

  4. #4
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    If it was me, the first thing I would do is put the derivative down to .1 or 0. I don't think I've ever had to use the derivative in any of the hundreds of loops I've looked at for HVAC. Maybe someone else has some input on this.

    Maybe drop the update interval to 15 seconds and put the proportional around 12. Don't make too many changes at once.

    There's also a delay in the heating sequence that says the indoor fan has to be on for at least 30 seconds before the heating loop starts. So you've got that to deal with if you're running the fan on auto. Are you sure that's not the issue? I couldn't find any info on the specific heat sequence when a fan status point isn't installed. What tech support told you that?

  5. #5
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    Yes tech support told my this. I have timed it it is around 5-6 min before the heat come on. This is happening on all the units. They said they see this in some applications where the units might be oversized our when there is not a big load on the system. so do think changing the loop parameters as you mentioned would help?

  6. #6
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    Carrier tech support?

    If what they're telling you is accurate, I think speeding up the loop would help.

    Why not just add a CT to the motor and be done with it? (again assuming that tech support is correct with what's causing the delay). Get a few RIBXGTA's and a few feet of wire, can't imaging it would take more than 15 minutes a unit.

  7. #7
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    Are you saying that it would fix the problem. There would be no delay between the fan on and heat on

  8. #8
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    MN
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    There's always going to be a 30 second delay after the fan starts. That's built into the heat sequence.
    I'm assuming the fans are running in auto, correct?
    I can't say with 100% certainty since I haven't seen one without a supply fan status point, but based on the answer tech support gave you I would think it would.
    Next time you're there try jumping out the fan status point and see if the heat starts up faster. You might have to configure input 3 to be fan status first.

  9. #9
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    Yes fans are running in auto. So u think a fan status would speed that up. If not, would you change the pid loop as you mentioned above. One problem I would have is a few of these units have ECM motors. What would you do there?

  10. #10
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    I'd probably try changing the pid loop info first and see if it helps.
    On the ECM, I'm not sure. I put a CT on my furnace's ecm motor and just adjusted it so it makes on the lowest speed setting, I'm not sure if that would work in your application, but I don't know why it wouldn't.

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