View Full Version : Trane VAVs in pressure dependent mode
06-14-2011, 09:50 PM
We have a new building where the engineer used oversized existing vav boxes. These boxes can not see the low cfms he designed for so they do not operate correctly. It is common to see 0 cfm at 28% open. Trane says they will convert to pressure dependent mode when this happens. In some cases this seems to work but in many others it does not. I find an area at 77 when the set point is 72. I am told pressure dependent mode will open the box as the room temp climbs. Trane has been out numerous times with little success. He hooks up with Rover and makes some changes but most don't seem to work. If we set max cfm very high the boxes will see the cfm and begin to work correctly but these high cfm setpoints cause the rooms to get too cold. I would appreciate any help. Thank you.
06-15-2011, 06:00 AM
The VAV boxes are capable of reading to +/-10% of their capacity, ie a box with a catalog rating of 1000 cfm can read reliable down to 100 cfm and up to 1100 cfm. Beyond that the control revert to pressure dependent mode. The problem is not the Boxes or the local Trane office, it is the fact that the boxes are oversized. To fix the root cause of the problem you would have to install the correct size VAV boxes. I know that is something you don't want to hear, because of the money involved, but there is no other solution.
duke of earl
06-15-2011, 07:24 PM
As a former Trane Employee, I know that engineers would oversize VAV boxes for accoustical purposes which made for quieter operation. For the record, the flowring technology is not a very accurate method of airflow measurement.
06-15-2011, 10:48 PM
Thanks guys, I was afraid that is what it would come down to. These are 14" boxes rated at 544-5000. In most cases the max cfm is designed below 500. One troublesome unit is min 90 and max 290. Sometimes we can get readings in the upper 300s but other times no. This box will read 0 cfm at 19% open, then stay there no matter how hot the room gets. This is the one that I saw room at 77 with setpoint of 72. I am told it should open in pressure dependent mode but it sure doesn't look like it will. We have raised the max cfm to 1200 which gets the box to open, then it sees cfm, then it starts controling to 1200 cfm. This is very loud coming out of the two small diffusers, and it freezes the people. What a mess because the engineer said he was told to save money by reusing these boxes. I wasn't part of those discussions but I am disappointed the engineer didn't realize he was going too far. What a mess, I counted 30 boxes messing up. Thanks again.
duke of earl
06-19-2011, 09:59 PM
06-26-2011, 09:15 PM
Now they are talking about putting in round retro fit dampers. I understand they go before the box. I think their main purpose are for replacing the old Trane Air Valves they no longer make. I am not sure it makes sense to use them on oversize boxes. I think they would make sense on Fan Powered boxes but not too sure they would make sense on simple Pinch Down boxes. It seems simpler to replace the whole box. Does anyone have any experience or advice on using these retro fit dampers in front of an existing pinch down box? Thanks, John
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