5 Replies Latest reply: Apr 30, 2012 2:27 PM by ikrull1 RSS

Fan Powered VAVs with ECM Motors

cpeterm

I have a question if anyone within Johnson Controls or the Connected Community has programmed a new BACnet VMA with with an application for a parallel fan powered VAV box with a variable speed ECM fan motor?    These types of fan powered VAV boxes seem to have high energy efficiency if the speed of the motor can be slowed down during lower heating/cooling loads.   CCT has an application for a series VAV with variable speed, but not parallel.

  • Re: Fan Powered VAVs with ECM Motors
    cpotteke

    Mark,

     

    Is this a situation where a dual duct VAV application may work, or one where the fan & heating side just pulls air from the return plenum? It sounds like you might have to use a combination of canned and custom programming to achieve this.  If possible, start with a parallel box configuration and add the command & speed control of the fan as a separate control loop...

  • Re: Fan Powered VAVs with ECM Motors
    jeff_taylor

    Hi Mark

    The key reason that we have varied the speed on a VAV box fan (series or parallel) is to aid a balancing contractor to easily setup the fan speed for the first time using a software tools like CCT versus some type of hardware setting supplied on the fan motor.  We have built custom applications for developers in the past for series fans - I have not seen a request for a parallel fan.

     

    For a parallel fan powered box, the key design consideration is setting the proper fan speed is to get the most effective delta T across the heating coil and provide the right "throw" of air into the space.  At the moment, I am at a loss to think of a sequence of operation where I would want to vary the speed of the fan as part of the application logic.

    • Re: Fan Powered VAVs with ECM Motors
      cpeterm

      I'm new to these types of boxes, so I'm interested in a discussion about the best way to set them up.   The canned routine for a Series FPVAV>Variable Speed fan routine keeps the fan on at all times - as it should with a series box.   As I understand it, the parallel box only needs to run the fan when the zone temp is below the effective heating setpoint so that it will pull air from the plenum.   I wrote a custom application that runs the fan at 3 different speeds:  High Speed is the speed the balancer set and it runs on this speed when we are 3 degrees < Effective Heating Sp.   Then as we approach 2 degrees <  Effective Heating Sp we run at Medium speed; then as we approach < 1 degree from Effective Heating Sp, we run at low speed.    The routine seems to work well, the fan indexes up and down accordingly and not repeatedly on 4 different boxes programmed this way.   I'm just not sure if it's correct or if we should just fire it at high speed until the zone temp satisfies then shut it off.   One of the issues is that the high speed is rather noisy and the occupant prefers the reduced speed...