View Full Version : VAS won't go occupied
05-13-2010, 12:48 PM
Have a situation this morning that occurred completely out of the blue, a VAS and AHU went unoccupied and the VAS won't go back occupied.
The VAS has two areas defined, one stays occupied 24x7 (larger of the two and it meets the minimum CFM of the AHU & VAS) the other is unoccupied from 7pm-5am.
The VAS went unoccupied while both Areas were in the occupied mode and I cannot get it to go back occupied. I was successful in going into the UPCM and overriding the AHU into occupied mode and everything is running normally except the VAS continues to show unoccupied.
05-15-2010, 03:44 PM
Did a little more digging on this issue, at face value it appears when one of the two VAV Areas goes occupied the VAS seems to think it needs to go unoccupied possibly because some of the VAV's with 0 CFM minimum flow assigned to them but there are plenty of other VAV's that are in the Area that stays occupied 24x7 with minimum CFM flows well above the VAS minimum flow.
When we set the VAS min flow to 0 CFM it will go occupied, so it appears even though all of the VAV's of the two different areas are within the VAS there is possibly a voting issue or the VAS is not necessarily seeing the highest minimum CFM on the VAV's?
05-15-2010, 08:13 PM
The VAS should be looking at the minumum cooling flow setpoint of the occupied VAVs to compare to its minimum flow setpoint.
05-16-2010, 01:10 AM
Agreed, but yet looking at the actions it's taking that does not seem to be the case. There are multiple VAV's with a minimum heat & cool CFM of 420 and the VAS has a minimum CFM setpoint of 400 yet it's going unoccupied.
duke of earl
05-18-2010, 09:57 PM
Go to Setup Tab-then VAS Tab-Click on Override Tab and look to see what has control in the upper right hand of the open window. It will say Unoccupied by ............... Whatever program is controlling the VAS is where you need to look.
05-19-2010, 10:59 AM
There are no overrides for a VAS other than the VAS can issue overrides to the VAV's such as drive to max, drive to min, etc.
05-25-2010, 10:18 PM
I see this is a few days old, did you get it figured out yet?
05-26-2010, 11:40 AM
Nothing yet, I spent the weekend digging through TS & our UPCM's trying to find a source of the problem and I did come across one item that seems suspicious to me.
The UPCM in our AHU's control the heat/cool mode of the VAS. Basically since we have a HW & CW system with modulating valves on the AHU's the only time the AHU ever truly goes into 100% heat mode is during MWU and that is only if the air handler was stopped due to being unoccupied.
I noticed in TS we have two BI's one for AHU MWU and one for AHU H/C mode. AHU MWU references binary variable #5 in the UPCM which goes true when the AHU is in MWU. However, the AHU H/C references the AHU MWU which of course references the UPCM.
The times I've seen the VAS go unoccupied it has also shown in it's status to be in the 'heat' mode, however, the AHU showed MWU was off. My hypothesis at this point is that the H/C referencing MWU referencing the UPCM was somehow confusing the BCU. So I have changed the H/C to now reference the UPCM directly. I'm waiting to see if it reoccurs.
Is it a bad practice to have an object reference another object which in turn references another object? I don't see how referencing a 'middle' object could be a good practice when you go could direct to the originating object.
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