SV-A: Discrepancy between system OA rate and sum of zone OA rates

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Good day everyone,

When I review the SV-A report for a PVAVS, there is a discrepancy between
the OA rate at the system level and the sum of the zone OA rates and I'm
not sure why. Hoping someone can help me out.

Here's the SV-A report:

[image: Inline image 1]

The OUTSIDE-AIR-FLOW was entered for each zone. No other OA inputs were
entered at the zone level.

At the system level, below shows some of the OA inputs:

[image: Inline image 3]

And here's the MinOA Sch:

[image: Inline image 4]

Any help would be appreciated!


Darryl Kasun's picture
Joined: 2017-12-02
Reputation: 0

Hi Darryl,
From your SV-A report, if you sum the zone outside air flows, then add the zone exhaust flows, and divide by the supply cfm, you get the OA ratio of 0.223.
Hope that helps.
Also, I think I need bifocals.

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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Bill Bishop's picture
Joined: 2012-02-25
Reputation: 7

This has been fixed in DOE-2.3 in the 7174 eQUEST build. That is, zone exhaust fans in a multiple-zone system do not create their own hidden OA flow demands, at least not until they are larger than the sum of the zone OA flows.

In DOE-2.2, I worked around this by never inputting these zone exhaust flows. If I was trying to be scrupulously fair, I would input the fan power and a schedule as a line item at the meter. And set any zone minimum supply air flows as needed. Usually some or all of the air to RRs and janitor?s closets is transfer air, so one doesn?t want the zone exhaust fan airflow to set a minimum on the zone supply airflow anyway, which is what happens when these exhausts are input at the zone level. One can also distribute the exhaust fans and flows to the other zones in the system such that the ?virtual? zone exhaust airflow at any zone is never greater than the OA flow; and at least then one is assured that the system OA flow at any hour will not drop below the connected exhaust flow if DCV is enabled.

Another item deserves a comment. In DOE-2.2 versions, when a return fan is sized with a flow less than the supply fan flow (as shown in this SV-A), then this differential sets a minimum OA flow (in this case 1580 cfm) which OVERRIDES all the ZEROS in that MIN-OA-SCHEDULE. That is, if the AHU cycles on at night, it does not do it with OA dampers closed as is the intent shown by the schedule below, and is hopefully the intent of the design. The workaround was to always set the SA and RA with the same cfm. I?m not sure how/if this is fixed in DOE-2.3

Happy Modeling, or Random Number Generating,

Porter, Fred's picture
Joined: 2015-02-03
Reputation: 0