Pressure Dependent Systems and Econo
Got a couple Trane pressure dependent zoning systems with RTU's that have economizers and just weighed relief dampers. Static duct pressure is controlled by a bypass duct from supply plenum to return plenum.
Problem is.......sometimes I get a slight majority for cooling, during econo enabled mode. Because most boxes are towards the closed position, the bypass is pretty open. Since the economizer is calling and most of the return duct is closed......I end up with mass amounts of supply air going into the return duct and out of the return grills.
This is the first time I have run into this.......anyone else dealt with this before?
The RTU's should have a spill relief damper to prevent this on the return.
Or the building may be under a negative pressure from exhaust fans.
the return dampers should 100% opposite the outside air dampers emove the drive link off the mod motor and check the swing back and forth 100% return open OA should be closed
Usually when I see a bypass from the supply to help control static it just dumps up in the ceiling,not connected to return,doesnt seem very efficient though.
I see an issue with that and I don't ever see it going away without some design changes. If you had power exhaust it might do better than baro-relief dampers but .... do the dampers. Work?
Are you pressurizing your bldg ?
hmmmmm ... I bet this has happened since day one . Has it? Doesn't sound very efficient ,if your already close to vent mode I can see overcooling and un-needed reheat.
I think you should look hard at the layout and show it to a trusted design expert and come up with a solution . I see sheet metal and control work needing to be done . I think it would pay for itself eventually.
Or maybe your weighted relief dampers need a proper setup. Your weighted relief dampers are before the ra dampers and feed to the outside correct?
I keep seeing the air and pressure taking the path of least resistance .
Easiest solution I can see would be to get the power exhaust option for the unit. Maybe wire it up with a pressure switch to turn on when there is any positive pressure in the return ducts, or wire it to turn on whenever the OA damper is 20-30% or more.
Just like the poster above said, air takes the path of least resistance.. Your solution involves making the path of least resistance the unit under all conditions.
Sorry been abit busy......
Yes the dampers work properly. The unit's are equipped with 100% economizers with a barometric relief. The static bypass damper is located from the supply to the return drop about 4 foot under the 2nd floor unit and about 14 foot under the 1st floor unit. It's a return plenum ceiling. And yes, they have always had this issue.
I was thinking of either adding power exhaust to the units or getting rid of the bypass damper and adding vfd's. I would assume adding the power exhaust option would be easy, but I'm not sure about adding the VFD's. I got to give Trane a call.
BTW- The units are 50ton Trane Voyagers.....don't have the vintage with me, but about 10 years old.
How are the units controlled now?
There is a Trane Varitac CCP controlling the RTU's and bypass.
Originally Posted by y7turbo