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Modulating Control Damper

There are basically 2 kinds of modulating dampers. Depending on your application, you will see either 2 position, or full modulating dampers. Since I think you Q: hinges on the full modulating damper,...here it is in a nut shell. Full modulation occurs as a DIRECT result of input of external controls (which are usually DC vac) that control the position of the damper according to what voltage, DC they receive. The nice about DC is that it is temperature and pressure reactive. o to 12 vdc is usually the range that these dampers operate in, since through thermistors a certain temp gives off a certain voltage value, thus moving the damper either open or closed or anywhere in between. There is more to it, but just a basic run down.

I hope this helped....

Mike T.

A good resource to learn can be found in the Maxitrol. com web site.


  • Economizer
    Economizer Member Posts: 14
    Modulating Control Damper Control

    Could someone please explain to me how a 24Vac modulating control damper is powered and controlled? In this specific situation there are (3) dampers used for airside an airside economizer.

  • Brad White_200
    Brad White_200 Member Posts: 148
    Economizer Principles

    If you are looking for which color wire goes to which terminal, that is not me :)

    That said:

    Most air-side economizers use enthalpy based sensors rather than just temperature alone. Specifically, the controller sensors measure and compare the total heat content (temperature + humidity) of both the inside -"return"- air and the outside air.

    In cooling mode, the controller will select the lower of the two. For example, it may well be warmer indoors and cooler outdoors, but if it is more humid outside, the return air, even if a degree or two warmer, gets the nod.

    The outside air damper then modulates toward open and the return air damper modulates toward closed, more or less together*.

    The relief damper modulates toward open more or less in parallel* with the outside air damper, to keep the building from becoming too pressurized.

    The dampers are often driven by 24 VAC damper motors on signal from the controller, drive open and drive closed. The OA dampers often fail closed (Spring-Return) as a safety/freeze protection measure though.

    *I say "more or less together or in parallel" because I prefer to index the dampers individually, not to a common signal nor mechanically linked. Air dampers do not follow very linear profiles of position to actual airflow. A 25% open damper may well pass 60% airflow. So individual indexing is key for effective control.

    Many packaged enthalpy economizer controllers are pre-wired with harnesses and work off a "canned" EEPROM program which simplifies this sequence across a lot of packaged systems.
  • JJ_4
    JJ_4 Member Posts: 146
    Non linked...

    I once saw the aftermath of an air handler where someone had put actuators on that were not spring return (close osa, open ra) on failure and both ended up closed...sort of like the imploded tanks in pictures last week.
  • Brad White_200
    Brad White_200 Member Posts: 148
    Damned good dampers, those!

    That is why we specify static pressure safety switches on systems, even low pressure, where a damper may dead-end close on you, such as a fire or smoke damper.

    Thanks for bringing that up, a sobering illustration.
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