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Hi All,
In the System Air Side HVAC when we go to Outside air tab
there it asks for Min ouside air achedule whether we have to define
Min outside air schedule or leave it as undefined.

And it shows a value of -999 for occupied hrs in min outside air
schedule what is the meaning of that, If we want to show and energy
savings form baseline to proposed case through demand controlled
ventilation then whether we have to create this schedule for both the
cases or leave it as undefined, and by putting OA control method as
sum of zone in baseline and based on DCV return sensors in proposed
case we can get savings due to demand controlled ventilation.

Also on the same tab 1st parameter it asks is Min OA ratio it should
be left autosize or user defined

Sambhav Tiwari


sambhav tiwari's picture
Joined: 2011-09-30
Reputation: 0

Hi Sambhav,

I have some answers and some homework for ya:

1. When you need clarity on the meaning of a schedule value, the
intuitive thing is to right-click that value for an explanatory help
entry, but what you generally need to do is go back to where that
schedule is referenced (under the air-side HVAC tab, either in the
system windows or spreadsheet view) and right-click those fields. In
this instance, the meaning of -999 in the min OA schedule is best
explained within that short help file entry as follows:


Takes the U-name of a SCHEDULE that contains hourly values of the ratio
of minimum outside air flow to supply air flow. Values in the
MIN-AIR-SCH vary from 0.0 (no outside air flow; economizer inactive if
specified) to 1.0 (100% outside air flow). A value of 0.001 actives the
economizer. A value of -999 returns the hourly value to MIN-OUTSIDE-AIR
or to the program's calculated minimum outside air value (found on
report SV-A for the SYSTEM or for each zone for zonal systems). During a
warmup period, this schedule is normally set to zero and can then be set
to -999 during other hours to allow the specified or calculated
ventilation minimum damper position to be used. Note that schedules are
ignored in the design routines; therefore, the minimum ventilation air
desired for design must be specified either at the zone or system level.

2. How to handle DCV in a baseline/proposed comparison depends
partly on who's reviewing your model, if anyone. Assuming your project
is LEED, then the LEED v3 EAp2 excel documentation that you have to fill
out offers the following subnote:

"...Enter ventilation flow rate (not %). Baseline & Proposed
ventilation rate must be equal unless Demand Control Ventilation is
modeled for credit, in which case Baseline should reflect ASHRAE
62.1-2007 or local code minimum volumes."

Personally, I haven't had enough projects to feel like my head is
completely wrapped around all the idiosyncrasies of eQuest DCV modeling
to give you a complete answer, but my approach is project-by-project to
work the proposed model as required to ensure DCV is being correctly
modeled with expected behavior and capacities - this normally involves
redefining various airflows for ventilation/exhaust/return that were
generated by the wizards, and checking some hourly reports. Because
this degree of modifications are necessary, I must then make a similar
concerted effort to address the OA rates in the baseline model. The
guidance above seems rational to me, even if you aren't modeling LEED.
It may seem unfair/meaningless if you have min OA requirements much more
stringent than 62.1 (yay, government work!), but that gets back to the
original question of whether you're modeling for LEED or not.

3. Whether you should or shouldn't use a min OA ratio at the
system level depends on your actual system/DCV behavior - notably
whether you're talking about the zonal or return-air DCV option. Min OA
ratio is generally a fall-back for when you do not have a zonal-level
method of defining OA required. You pretty much need to search the help
files for MIN-OA-METHOD and carefully read the article with that title
in its entirety.


Nick-Caton's picture
Joined: 2011-09-30
Reputation: 805