Specifying ACH and air recirculation ladybug 1.9

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Hi all,

I am modelling a laboratory to understand the energy implications of different ventilation rates. The model is in the process of being calibrated against actual energy use. Generally ventilation rates in laboratories sit between 8 & 12 ACH. I have used the HB ventilation component to set the ACH rate.

if the ACH is set to 10.5, The output from my simulation is showing that the outdoor ventilation rate is 0.7 ACH. If the ACH is set to 10.8 the simulation results show an outdoor vent rate of 3.2 ACH.

With a simulated outdoor air rate of 3.2 ACH my energy use is twice the size it should be at 8ACH (not confirmed with real building but minimum recommended flow rate for an occupied laboratory is 8ACH)

Can anyone help me to understand what might be going on or if i am verifying the airflow rate correctly.

Alan macdiarmid model 16-9 2pm from uni PC .gh (536.3 KB)

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Hi @Nicholaskerr! If I am not mistaken E+/OS:

  • HVAC ACH: Supply ACH (total air changes) can be higher than OA ACH if the system recirculates, but from the little bit of experience I have with labs I’m sure recirculation isn’t on the table for workspaces etc? so that might not be helpful.

  • OA ACH: OA ACH is Strictly the outdoor portion,
    control it with DesignSpecification:OutdoorAir + Controller:MechanicalVentilation

  • DesignSpecification:OutdoorAir + Controller:MechanicalVentilation sets a minimum OA flow, often much smaller than the total supply sized for loads, so supply ACH can be >> OA ACH.
  • Recirculation through the AirLoopHVAC or terminal mixing boosts supply ACH without increasing OA ACH.
  • ZoneVentilation or Infiltration objects add to outdoor air independently of the air loop (watch for double-counting if you’re using them).

On my lunch break at the brewery so I dont have time to edit or add more or do some testing which I plan to do if the issue persists. but hopefully something in there is helpful :joy:

All the best
-trevor

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Thanks for the clarification. My issue is that i am getting much lower ACH then specified by the minimum OA rate specified in the script. I was hopping not to have to build a full ironbug model for the system and be able to utilize the honeybee VAV template but from having a dig around in the IDF it looks like there are some template assumptions that are throwing things off.

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@TrevorFedyna after snooping through the IDF i’ve found that there is only one DesignSpecification:OutdoorAir. It seems one of the zones with the lowest outdoor air design specification is being used to establish the OA flow rate. I need to model different outdoor air rates for different zones. Is there a way multiple zones can be summed for the DesignSpecification:OutdoorAir object?