Mechanical Ventilation Zones with Only Extraction

asked 2019-11-18 05:14:19 -0600

updated 2019-11-18 05:15:07 -0600

In the model I have, some of the rooms only have mechanical extraction of air. (Supply is via the doors/opening between zones). I used a Zone:Mixing object for the interzone air exchange in combination with a zone exhaust fan. When running the simulation a warning about unbalanced exhaust airflow is generated:

* Warning * In zone LEVEL1:1.03ARCHIEF there is unbalanced exhaust air flow. * ~~~ * During Warmup, Environment=BRUSSELS ANN HTG 99% CONDNS DB, at Simulation time=01/21 00:00 - 00:10 * ~~~ * Unless there is balancing infiltration / ventilation air flow, this will result in * ~~~ * load due to induced outdoor air being neglected in the simulation.

Is there a better way to model this?

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answered 2019-11-18 07:02:48 -0600

Julien Marrec

29726 ●94 ●76 http://www.effibem.com/

Your "problem"* is very likely in the Fan:ZoneExhaust, especially at the Balanced Exhaust Fraction Schedule Name. Read carefully the documentation for this field.

I say "problem" as in the cause of the warning. Whether it's actually a problem or not depends on whether you also have an AirLoopHVAC serving the zone.

Let's assume you have two zones:

Zone A: the zone where air is being transferred from (the source)
Zone B: the zone with the exhaust fan, that receives transfer air from Zone A.

If you are setting a Zone:Mixing object to transfer the entire airflow from zone A to B that is being extracted in the zone B with the exhaust fan, then you can set this field to a schedule of always 1.

Note that having a Zone:Mixing object will not affect the heat balance of the source zone (Zone A). You probably want to add another object in that zone, for eg a virtual Fan:ZoneExhaust that would consume no power but limit the return air to the AirLoopHVAC serving zone A by the same amount that is being transfered to zone B. Or, if it's not served by an AirLoopHVAC, you probably need a ZoneInfiltration:DesignFlowRate.

Bottom line: You are responsible for balancing the airflows yourself, E+ won't do it for you (at least outside of an AirflowNetwork implementation)

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Actually it seems your warning is thrown in the ZoneHVAC:EnergyRecoveryVentilator to warn you that exhaust > supply (cf HVACStandAloneERV.cc#L1764). My comments above still stand though. Just make sure you are properly balancing your airflows, and you can then safely ignore warnings.

Julien Marrec ( 2019-11-18 07:05:15 -0600 )edit

Thanks for the answer, I appreciate it. Just one more question about the virtual Fan:ZoneExhaust, it is just an object you create but don't link to the system?

DelphineRamon ( 2019-11-20 08:43:01 -0600 )edit

I say virtual because you zero out the power since it doesn't exist physically. If the answer solved your problem, don't forgot to mark it as accepted so the thread is flagged as resolved.

Julien Marrec ( 2019-11-20 09:09:46 -0600 )edit

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