[Equest-users] Two separate outside air streams for a zone

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Dear All,
I searched the archives to answer this question and didn't see anything,
but may have missed something. I am trying to model (for LEED 2009) a
warehouse that trucks occasionally drive into. The space has the usual
ventilation air prescribed for a warehouse by 62.1 supplied by RTU's.
Additionally, there are fans that turn on (3.73 kW for 16,000 cfm of
outside air) when trucks are present. I created a schedule for "trucks
present" using fraction type at 1.2 hours per day each weekday. I'm not
sure how to add the outside air or get the extra fans to only run during
those hours.

Possible solutions:

1. fans as separate "equipment load" in the zone, on the schedule and
add outside air to the standard ventilation air as a fraction of
16,000 cfm. So new vent = std vent + [(1.2/40)*16,000]. This
wouldn't give the same peak load as having a full 16,000 cfm though.
2. create dummy zone with no loads, walls, area, etc. and give that
zone and system the 16,000 cfm and 3.73 kW on the truck schedule.
Tried this and only got a runtime error with no explination.
3. change the max OA to std vent + 16,000cfm and change the Min OA
schedule to reflect truck presence (run OA at 7.25% for all hours
1.2 truck hours per day). Also, changed supply flow to system
supply=9,000 + 16,000. Tried this, but the supply flow shown in
hourly output reports was always 25,000!

Any suggestions?

Jacob Goodman , LEED AP

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Joined: 2011-01-13
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Hi Jacob,

My intuition would be something like your 2nd suggestion... Split the
space geometrically in 2 to get two spaces/zones. Set the thermal
connection between them as an air wall.

Define your RTU for one zone. Define your OA vent fans for the other.
Separate scheduling for concurrent/non-concurrent runtimes as
appropriate. Consider your model now has two thermostats for two
systems, which may or may not match reality. In any case - make sure
both systems' fan/tstat schedules are defined in relation to each other.
If you go all out and also model the additional internal loads of a
bunch of tons of blazing hot metal entering in the summertime, you may
expect to encounter a certain "normal" amountNote that some unmet hours
may be normal

Your 3rd approach seems viable on the surface though... is your supply
fan "all or nothing" (constant volume fan control)? I think you may
need to swap to something like VSD or maybe explore the PTAC "two-speed"
continue reviewing the hourly results until you can find the golden
combination of tweaks that make it happen ;).

~Nick

NICK CATON, P.E.

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Joined: 2011-09-30
Reputation: 805

Jacob,

I've got a fourth possibility to consider. I think you can accomplish
something similar to what you need in "Air-Side HVAC Zone Parameters"
for the zone(s) you're working on. In the outdoor air tab, specify an
exhaust tracking control method and the exhaust schedule to match when
the trucks are in the building. Everything else you've described can be
filled in there. It doesn't sound like an exact match - I believe this
will increase the OA through the main RTUs (shouldn't change the fan
volumes, and just models the conditioning in the main unit rather than
the makeup unit), rather than through separate units. Check your hourly
reports, but it might be close enough.

Good luck!

Eric O'Neill

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