Schedule relief fan operation during airside economizer mode

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I have a VAV AHU with a economizer which is schedule with an "On" point of 20 Btu/lb.  This AHU also has a relief fan that is activated once the AHU goes into economizer mode.  How do I properly add and schedule this relief fan operation?  Thanks for your help.

jmowrey's picture
Joined: 2011-03-23
Reputation: 0

Just pick a fan type that you are not currently using in the applicable system. I think the auxiliary fan would be best - but you might need to test a few fan types (the ventilation fan MIGHT work).

Then, set that fan up (as you normally would - with the proper static and power - etc).

  1. The trick here is to create a custom schedule - Create a schedule that is Off 24/7.
  2. Then - set the reset and lockout so that the schedule is 100 percent, anytime the outdoor air enthalpy is less than 20 btu/lb (note that reset and lockout does not work for all fan types - thus, my earlier comment on needing to test it)
  3. AND - add a rule so that the Room dry bulb temperature is set to ~1 degree less than your cooling setpoint (so that the economizer fan does not run in heating mode) I assumed the cooling setpoint was 75 here.
  4. Don't enter anything for the offset

See the attached Image

Reset and Lockout Schedule in Trane TRACE 700
Bob's picture
Joined: 2010-06-30
Reputation: 475