Results 1 to 10 of 10
  1. #1
    Join Date
    Jul 2005
    Location
    south
    Posts
    602

    pneumatic face/bypass damper sequence

    I have several 1980's vintage bufflo forge 100% OSA air handlers with pneumatic face and bypass dampers and hot water valve that have seen it days. Customers wants to repair/replace pneumatics so units are functioning properly.

    Pneumatic schematics are long gone and i am no pneumatic whiz so i am struggling with the proper sequence
    This is my logic, feel free to chime in and correct me, i know i'm missing something

    Components---- OSA dampers, face dampers,bypass dampers, hot water valve actuator
    reciever controller, discharge air transmitter, freze stat


    call for heat (based on suppy air setpoint)----- hot water valve and face dampers modulate open, bypass dampers modulate closed, osa dampers open

    call for cool ( based on supply air setpoint)----- hot water valve and face dampers modulate closed, bypass dampers modulate open, osa dampers open
    Freeze stat activated--- osa dampers closed, hot water and face dampers open, supply fan off

  2. #2
    Join Date
    Apr 2014
    Location
    MN
    Posts
    134
    Typically there's a switch over point somewhere around 35F OAT where the hot water valve is opened 100% and the bypass damper modulates to maintain a discharge temperature. This helps prevent freezing the coil.
    Above 35F the bypass damper is closed and the face damper is opened and the hot water valve is modulated to maintain the discharge air temperature.
    On start up if it's cold, I'd flood the coil first and delay the fan a bit to prevent tripping the freezestat.
    Also on a freeze, I typically have the bypass damper fully open (face damper closed) to help protect the coil as well, the rest of your freeze sequence sound right.

  3. #3
    Join Date
    Apr 2014
    Location
    MN
    Posts
    134
    And if you're asking how to do this with pneumatics, I have no idea!

  4. #4
    Join Date
    Sep 2013
    Location
    Green Bay, WI
    Posts
    37
    There would be one receiver controller for the damper and the valve. Above 35-38 set point it controls the valve and due to the spring range the damper is open. Below the set point a solenoid valve between the the receiver controller and the valve would bleed off and the valve would go wide open. Then the damper would modulate to maintain set point.

  5. #5
    Join Date
    Sep 2006
    Location
    Alaska
    Posts
    475
    Watched several of these units freeze the heating coil. Then everyone would say that can't happen because it doesn't get cold. The main thing is to have is an interlock which doesn't allow the face damper to open until the temperature of the heating coil is 120 degrees. Then at least you won't freeze the coil.
    Law Of The Thermostat: He who has the thermostat wins!!!!!

  6. #6
    Join Date
    Aug 2002
    Location
    Southold.calm
    Posts
    6,364
    Don’t forget minimum outside air

  7. #7
    Join Date
    Aug 2002
    Location
    Southold.calm
    Posts
    6,364
    Or this:
    Attached Images Attached Images

  8. #8
    Join Date
    May 2002
    Location
    Baton Rouge, Louisiana
    Posts
    3,961
    Unfortunately can't write it out that good but if I was there and had some tubing and fittings.....................
    Quote Originally Posted by MatrixTransform View Post
    very soon it is you that will be pwned

  9. #9
    Join Date
    Jul 2002
    Posts
    936
    Think of it as above 35 OA the stat is sent to the valve, below 35 the stat is sent to the bypass.

  10. #10
    Join Date
    Jan 2012
    Location
    California
    Posts
    92
    I'm confused by the description... 100% OSA and then bypass damper. So is the bypass damper around the coil or is there a third deck? Hot Deck, Cold Deck, center deck for 100% osa no conditioning? If it is 100% osa the damper on the inlet of the unit is for shutdown only 2-pos control.

    Face and bypass have the dampers linked in opposite positions for a two deck setup. Apparently it is a constant volume design. You should have separate controls on the cold deck and hot deck. So with only one receiver controller it might have DX for cooling and hot water... need more input.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event