Hi All, 

I would like to consult the derivation of the System Outdoor Air Intake (Vot) for baseline system 7 VAV under ASHRAE 90.1 - 2010.

We received a GBCI reviewer comment that there is disproportion between the baseline systems Vot and the Vot for the proposed case (100% OA DOAS), as shown in the IEQp Minimum Air Performance Calculator. 

In relation to baseline system 7 VAV, it is our understanding that the System Outdoor Air Intake (Vot) is not equal just the sum of zones Outdoor Airflows (Voz), but involves adjustment for the system ventilation efficiency. The Vot is then calculated as follows:

      Vot = Vou/Ev

where:

- Vou = System uncorrected OA requirement (sum of zones OA Vbz, i.e. OA req. before application of the zone Air distribution efficiency [Ez])

- Ev = System ventilation efficiency. For the purpose of this calculation Ev = worst case Evz (ventilation efficiency for the critical/worst case zone of the system). 

Now, in our case (energy model in IES-VE, below example values for a baseline system 7 VAV for a typical 3rd flor), we have the following: 

 - Vou = 564l/s

 - Ev(Evz worst case) = 0.37

Vot is then:

      Vot = 564/0.37

      Vot = 1524l/s

To compare the Vot with the sum of Voz (zone OA airflows after the application of assumed Ez=0.8), the Voz sum is calculated as:

      Voz sum = Vot/0.8

      Voz sum = 705l/s

The ratio of this particular System Outdoor Air Intake (Vot) to the sum of Zones Outdoor Airflow (Voz) is then:

      Vot/Voz sum = 1524/705 = 2.16

The System Outdoor Air Intake (Vot) is 2.16 times higher than the sum of Zones Outdoor Airflow (Voz). For the whole building (office building with retails and restaurant), the ratio between the sum of systems Vots and the sum of Zones Outdoor Airflow (Voz) is then about 1.8.

       ***

The disproportion/increase in the baseline systems Vot on the sum of Voz is clearly due to the low worst case Evz. 

When investigating the derivation of the Evz (worst case Ev) value, it was found that the Evz 0.37 is for a little storage room in the core of the building. Heating and cooling loads for the space are zero, resulting into the Design Primary Airflow (Vpz) being equal to just the Heating Outdoor Airflow (Voz). Verification of the Evz calculation for the zone was performed following the ASHRAE 62.1 - 2010 equation A-2:

       Evz = 1 + Xs - Zpz

where:

-   Xs = System Outdoor Air Ventilation Fraction (Vou/Vps) = 0.37 (From IES-VE)

-   Zpz = Primary Outdoor Air fraction (Zpz = Voz/Vpz) = 1

       Evz = 1 + 0.37 - 1

       Evz = 0.37

The above confirms that the Evz for the zone is low due to the zero heating and cooling loads in the zone. 

       ***

I would like to ask the following:

1/ is this realistic - does the above sound reasonable, or are we overlooking an elephant in the room?

2/ Assuming that the above calculation of Evz is correct, we find it surprising that the whole System Ventilation Efficiency (Ev) can be pulled down due to such a small/un-representative space (while the area of the storage room with Evz 0.37 is 2.75m2, the whole floor plate - formed mainly by offices with ventilation Evz in the region 0.80-0.90 - is approx 1500m2). Is this really so?

3/ What is your experience with the system 7 VAV Vot - have you come across Vot based on such a low Evz? If yes, how was it received during review?

With many thanks,

Lenka