VAV Box Controls

[remp]

Can someone please explain how the BMS DDC system operates a pressure independent VAV box. I know it modulates to maintian a set point temperature, but how does it do this... is it a PID loop.... what are the parametres? I got a BMS subcontractor who cannot seem to set up his control system correct. Its a fairly weel known BMS system but I have no faith in the programmers. The VAV boxes are always either on max open (100%)if below set point or min (50%) if above set point.... they are never in between. Whats he doing wrong....PLEASE HELP!!

 

[imok2]

billyq
Each zone is controled by a room stat that resets a velocity sensing device in each terminal. this device, in turn, operates thr VAV terminal damper actuator to vary the volume of air supplied to the zone.It sounds to me as if the sensitivity is set too high and the dampers are hunting. Make sure the fan speed is constant when making adjustments.
Possible causes:
1.not enough stacic pressure at inlet
2.to much static pressure at inlet
3. static pressure sensor located incorrectly
4.static pressure set incorrectly
5.controller needs calabrating
6. fan speed incorrect
7.not enough straight duct on inlet of the box
8.wired incorrectly
9.pc board defective
10 control air must be clean and dry
11 make sure you have the proper nozzle sensor as shown on the calibration label

 

[soetjahjo]

Billyq,

Important to check first if all your inputs and outputs signal to BMS DDC are correctly commisioned and tested.
Second check if your controller is set to modulating mode and not to "on-off" control.
Third you need to check the following parameters : Propotional Band and Integration time (PID settings of the controller) in the DDC controller. It may be that the propotional band is here set too small; which results to hunting behaviour as your description.