OA schedule

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Hi All,
It is necessary to specify the min OA schedule as -999 for
occupied and 0 for unoccupied hrs at either system or zone level or it
can be skipped.

Thanks
Sambhav

sambhav tiwari's picture
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Joined: 2011-09-30
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-999 means use input values rather than scheduled values. If you want the program to use input values for a period of time, then override the input with 0 for another period of time, you have to use -999 for the times you want the program to use input values. Blank may not mean the same thing as -999.
Jeremy

___________________________________________
Jeremy McClanathan, P.E., BEMP, HFDP, LEED? AP BD+C

Jeremy McClanathan's picture
Joined: 2011-09-30
Reputation: 200

depends on the modeling requirements. 90.1? owner requirement? leed?
something else?

for 90.1-2007, appendix g, table 3.1, no. 4 schedules, specifically
states the "hvac fans that provide outdoor air for ventilation shall run
continuously whenever spaces are occupied and shall be cycled on and off
to meet heating and cooling loads during unoccupied hours." 90.1-2004
doesn't specify fans specifically for outdoor air for ventilation, it
just says hvac fans - which is the same as the system supply fan in a
non-doas/economizer capable of completely closing the outdoor air damper
system so the outdoor air can't be shut off.

so for a 90.1-2007 simulation the minimum oa should not be zero during
unoccupied hours.

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
Reputation: 0

For LEED or other rating systems referencing ASHRAE PRM, it does seem
that the 90.1-2007, appendix g, table 3.1 statement is misleading.

However, the mandatory shutoff damper controls in section 6.4.3.4.3, I
believe, clarifies that the outdoor air should be 0 if cycling system
during unoccupied hours, except if the use of OA reduces energy costs
(night purge, economizer).

If you specify 0.001 in your MIN-AIR-SCH (enables use of economizer), I
would review your system's hourly OA levels to make sure it is behaving
as expected, especially for multizone systems. I have found that this
can lead to a significant amount of additional heating, as the
economizer will be used to achieve the SAT control setpoint, even if
this is not appropriate in the middle of the winter... As such, I
typically set the unoccupied MIN-AIR-SCH value to 0.

- David

David Reddy3's picture
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good point david, considering a baseline (i.e. minimally compliant
building) would have to meet the requirement i think the intent of the
appendix g requirement is to simulate a worst-case scenario concerning
outside air scheduling and energy consumption. not unlike the
simulation requirement for a proposed building that a thermal block not
specified with a cooling or heating system must simulated with one that
is the same type used in the baseline building simulation (app g, table
3.1, no 10, d).

your reference to 6.4.3.4.3 (2007) would explain why a usgbc/gbci
reviewer could comment on the oa schedule, even though app g is pretty
specific on how the oa should be scheduled for simulation purposes.

Sambhav, if you're working on a leed project can you ask the review team
(thru the project on leed online) to clarify which oa requirement method
should be followed in the simulation - app g or 6.4.3.4.3?

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
Reputation: 0

I don't see any conflict between App. G and 6.4.3.4.3. I interpret the "cycling" language as requiring that the fans cycle to meet heating and cooling loads but without providing ventilation. Providing ventilation when cycling conflicts with the G3.1.2.5 requirement that ventilation rates be the same - you won't get the same ventilation during cycling since the proposed and baseline cases will differ in the frequency and duration that they cycle.

Regards,
Bill

Bill Bishop's picture
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Yes, thanks Bill, these are exactly the same points that I was going to
follow up with...
-DR

David Reddy3's picture
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the app g, 3.1, no 4 text (2007) states:

"HVAC Fan Schedules. Schedules for HVAC fans that provide outdoor air
for ventilation shall run continuously whenever spaces are occupied and
shall be cycled on and off to meet heating and cooling loads during
unoccupied hours."

so separating outdoor ventilation from fan cycling does not appear (to
me) to be the intent of the app g requirement. why would a fan that
provides outdoor air for ventilation be required to operate if it is not
providing outdoor air? e.g. doas system or outdoor air supply fan
separate from fan in the ahu, or even if the outdoor air ventilation fan
is the ahu supply fan (per app g, 3.1, 4).

how does the requirement (6.4.3.4.3) apply to an evaporatively cooled
system with zero return air? i.e. no return air path. or an evaporative
system with no return air path in cooling mode but 80% return in heating
mode when mated with a furnace section? app g, 3.1, 4, sounds more
logical (to me) without even getting into the debate about how to
properly simulate a baseline building system of system type 3 or 4 when
the proposed building system is an evaporative cooler with 100% outside air.

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
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