If you have a DDC control system and are constantly measuring duct static pressure, air flow through the VAVs, and valve position, you can add some control logic to automaitcally reset your duct static pressure to match the conditions in the duct at that moment. ESP to be measured 3/4 of the way down the main duct run. The problem is most engineers pick an arbitrary duct static pressure set point (or worse don't pick one and leave it to the control contractor/installer). By minimizing static pressure you minimize fan HP and hence energy costs.
#Anemostat variable volume reset control full
In practice what you want to do is minimize the static pressure in the duct but still maintain full air flow. SMACNA says ducts upstream of VAV boxes should be installed to withstand a minimum of 2" w.c. RE: VAV terminal mode of operation Teto (Mechanical) It sounds like you have a good handle on the operation of your system.
#Anemostat variable volume reset control code
If this were to occur, you are providing zero ventilation air to the occupied spaces which is a code violation. There should be a mechanical stop on your box to keep the box from going to the full closed position. I never though of it that way but it makes sense. If I understand your last question correctly and your system is using this "typical" operating sequence, I guess you could assume that any valve position on the box above the minimum position would mean that the box is in cooling mode. The heating control valve may also operate during cooling mode if your box reaches its minimum position and you continue to overcool the space. The min setting on the box should also be your ventilation rate for the space or the quantity of air required for heat, whichever is greater. position and modulate the control valve on the heating coil to maintain space temp at setpoint. In heat mode with heat at the box, the box would actuate to it's min. I would assume that the VAV box CFM's are max and min. It sounds like most of your assumptions are correct. RE: VAV terminal mode of operation haneyrm (Mechanical) 11 Jan 06 15:40
I would really try to find that Sequence - that is the key. It is still possible that the VAV closes completely in both modes, though. Using 1.08 X CFM X deltaT(deg.F.) should give you an idea if the airflows match with your suspicions. If you have an Equipment Schedule on the drawing, perhaps you can check the temperatures with the heat capacities of the heating coils. Chances are, you are correct - but what determines the heating vs. The smaller CFM you cite may be the minimum closed airflow, it may be the heating airflow, or it may be an unoccupied setback airflow. Unfortunately, that meant no ventilation air, too. One of the historical issues with VAV was that they were allowed to completely close if no cooling/heating demand was present. The system has to be operating - right now - from program logic according to a Sequence Of Operations. If you can't find the original plans, or talk to the original design Engineer, then refer to the DDC vendor. The sequence should exist in two places - on the original plans, and in the DDC controls documentation. If you can't find that, you may have trouble determining the logic. You need to review the Sequence Of Operations, first.
I would be happy to read any thoughts/comments on VAV terminal box setup and design. Is it safe to assume that the terminal will have a minimum air flow at 340 cfm? For heating, the control valve will start to open with the terminal damper staying at minimum? For cooling, the control valve will be in the closed position and the damper begin to open for and a maximum of 2040 CFM? Should the damper stop at the minimum position by mechanical means (it's a pneumatic actuator)? Should anything above the low CFM flow be considered cooling mode?. I have the prints with terminal box schedules showing 2 CFM designs. I have no information on how the terminals are supposed to function as far as air flow is concerned. In the building, the duct distribution system has terminal units on DDC with a reheat coil, pneumatic n.o. The AHU units are strictly cooling units with economizers. The AHU units have variable air volume controls on the fan (inlet vanes controlled by duct static pressure controller to 2" wc) and supplies air at 55 degrees to the building. First some background:I operate an 8 year old building with a VAV system. I have a question pertaining to VAV system operation/setup and would value your comments.
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