[EnergyPlus_Support] Constant ventilation input results in varying ACH

["Simge" <simgeandolsun@xxxxxxxxx>]

Dear EnergyPlus users,

I have modeled a single zone residential building with an outside air system in EnergyPlus. I autosized the outside air requirement of this house by using the following objects of EnergyPlus together with the following inputs

1) DesignSpecification:OutdoorAir with 0.000204 m3/s-m2 flow/area
2) DesignSpecification:ZoneAirDistribution with 1 as the zone air distribution effectiveness for both heating and cooling
3) Sizing:Zone by referring to the DesignSpecification:OutdoorAir object defined above for the "Design Specification Outdoor Air Object Name"
4) Sizing:System by autosizing the "Design Outdoor Air Flow Rate" and selecting "ZoneSum" for the "System Outdoor Air Method".
5) Controller:MechanicalVentilation by selecting the only zone of the house and assigning the DesignSpecification:ZoneAirDistribution and the DesignSpecification:OutdoorAir defined above for this zone. Then, setting the System Outdoor Air Method to "ZoneSum".
6) Controller:OutdoorAir by autosizing the minimum and maximum outdoor air flow rates, selecting "NoEcomizer" and "FixedMinimum" options. I also assigned the Controller:MechanicalVentilation defined above as the "Mechanical Ventilation Controller Name" under this object and set the minimum outdoor air schedule to always 1.

With these inputs, I was expecting to get a constant ventilation rate in the house, but I ended up getting a varying ventilation rate throughout the year. I first thought it might be because of the varying occupancy in the house, but then I realized that the ventilation ACH showed variation between seasons although occupancy did not. Could you please let me know what might be the reason for this variation in the ventilation ACH in the house? Can it be the variation in the wind speed or the temperature difference between the outside and the inside air? Is there a way to autosize a constant outside air ACH for a building using the inputs above to model an energy code (IECC 2012) compliant house? If there is, would you please let me know how?

I look forward to hearing from you,
Thank you very much for your help,

Best Regards,
Simge Andolsun
PhD Candidate 
Texas A&M University
College of Architecture



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