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# ASHRAE 62.1 - Ventilation Rate Procedure - Reported Average Outdoor Air Fraction

I have a model with several VAV systems and have been working on setting them up so the ventilation (outdoor air) is done via the ASHRAE 62.1 ventilation rate procedure. I am comparing the results from the Standard 62.1 Summary report created by EnergyPlus and comparing them to a manual calculation.

My manual calculation is identical to system ventilation requirement for heating, I have however noticed some differences for the cooling.

My manual calculation gives the following average outdoor air fractions (Xs) for cooling:

• VAV_1: 0.107
• VAV_2: 0.090
• VAV_3: 0.091
• VAV_5: 0.490

Here's a snippet of the report generated by EnergyPlus:

ASHRAE 62.1, definees Xs as $Xs =\frac{Vou}{Vps}$

For the cooling report, Xs doesn't seem to be calculated according to the previous formula for VAV_1, VAV_3 and VAV_5 but is for VAV_2. How can that be?

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Can you provide a test file, either to the help desk or my email address?

( 2017-01-25 15:41:06 -0600 )edit

@rraustad Thank you but it was actually an input error on my part. I had inadvertently set the Minimum Zone Ventilation Efficiency of the DesignSpecification:ZoneAirDistribution to 1 early on while doing some tests. I now get the right result for Xs, and the results of my manual calculation align with the EnergyPlus report so it's all good! I'm however not sure why this error resulted in those values for Xs since it doesn't depend on Ev (or Evz) but on Vps and Vou which if I am not mistaken don't depend on Ev (or Evz) either.

( 2017-01-25 17:11:45 -0600 )edit

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I had inadvertently set the Minimum Zone Ventilation Efficiency of the DesignSpecification:ZoneAirDistribution to 1. I now get the same results as my manual calculations.

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