pneumatic face/bypass damper sequence

[licensed to chill]

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

[bacnet>CCN]

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.

[bacnet>CCN]

And if you're asking how to do this with pneumatics, I have no idea!

[bigguy158]

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.

[simux]

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.

[pecmsg]

Don’t forget minimum outside air

[pecmsg]

Or this:

[joey791]

Unfortunately can't write it out that good but if I was there and had some tubing and fittings.....................

[incontrol]

Think of it as above 35 OA the stat is sent to the valve, below 35 the stat is sent to the bypass.

[catalina_mike]

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.