Split System with DOAS for LEED

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Hi all,

I am a new Trace 700 user and I'm busy modelling a building for LEED. I'm
struggling with two fundamental aspects of how the HVAC system modelling
works in Trace.

1. The building uses a split unit heating/cooling system, with a completely
separate dedicated outdoor air system for fresh air. The fresh air supply is
constant and does not vary with the heating and cooling load. The split unit
draws air directly from each zone (recirculating indoor air only) and is not
at all connected to the supply ducting of the fresh air. Since there are two
completely independent systems serving the same zones, how should this be
modelled for LEED? Would it be acceptable to assume that a single system is
being used, and that the heating/cooling is done directly on the supply air?

2. All zones receive fresh air, with only the bathrooms exhausting air,
creating a definite airflow network between the zones. If I set every zone's
"Adjacent air transfer from room" to the passageway connecting all the
zones, will Trace determine the correct airflow rate and direction between
each zone? Or is the adjacent air transfer directional, so that the bathroom
would need to be set to have an airflow transfer from the passage, then the
passage would need an airflow transfer from multiple zones (but trace only
allows a single zone to be set)?

I'd appreciate any feedback. Thanks in advance!


Robert Arnott's picture
Joined: 2011-10-01
Reputation: 0

Classification: UNCLASSIFIED
Caveats: NONE

Good timing Robert, I am modeling almost the exact same system.

I called Trane, but I still don't like how the system is working.

Under create system, select the dedicated outside air unit and fill in the
information. This is equivelant to modeling two separate pieces of

I modeled it the way Trane instructed, but I had my doubts.

I also modeled one system like normal but used 0 ventilation and another
system with 1sqft and nothing else but 100% ventilation (7400 cfm in my case)

The thing I didn't like about the method with 2 separate systems was that it
showed the return air of the second system to be 14800 cfm, I can't figure
out why it doubles the return air cfm.

Jeurek's picture
Joined: 2010-10-07
Reputation: 0

Hello Bob and John,

I am on my 7th Barracks and 10th overall building using DOAS systems
ranging from 2-pipe changeover Fan Coil Units to Geothermal Heat Pump
Systems with scavenger reclaim for dehumidification reheat and
Multistack geochillers for DOAS.

John is correct, select your primary System (unitary equipment) with
info for the DOAS filled out in the dedicated OA tab, just select that
it is room direct as to model your decoupled ventilation, and make sure
your Fixed-Setpoints are the same as your room setpoints so that you do
not burden the split system or cause need for insulation of the DOAS
ductwork. Remember that you can drag and drop the coils into the
different plants so that a electric back-up htg coil for the splits can
be activated if your split system needs additional heat, and your new
"Opt-Cooling" coil can be attached to your chiller/multi-stack/WWHP and
Opt Vent htg coil can be attached to the boiler/boiler backup.

It's very useful to look at your equipment energy consumption output to
see if the energy is being accounted for correctly. If something isn't
connected properly in the plants or coils assigned incorrectly, the row
of kwh or therms will "disappear" to give you a clue that something is
wrong with this picture. When building the plants always consider back
up heat whether it is boiler or electric reheat coils.

Also with DOAS in your room airflows, if you have a space that is 45cfm
of ventilation air and you only want to exhaust 30cfm from the toilet
room, that's 15cfm positive pressure, consider only entering 15cfm in
the room exhaust and the 45cfm ventilation air, then do not assign a
room exhaust fan. Your System Checksums will show the ventilation air
and what is being returned back to your DOAS for energy recovery

In the Detailed Reports Tab of you Output refer to the Airflow Balance
at coil peak. Remember that you wanted a decoupled ventilation, so the
Total Outflow will equal your Total Inflow. Back at your System
Checksums add your Main Fan to the Nom Vent minus Rm Exhaust and that
should equal your Return Air quantity. OA+SA-RmEA=RA. Trace will not
allow a building to be positively pressurized, only balanced airflows.

Doug Breese
Be Sustainable -- Never let today use up tomorrow!.

Breese Doug's picture
Joined: 2011-10-01
Reputation: 0


Sorry, but no, it is not acceptable to model as 1 system since your model
will not match your design documents, and your LEED reviewer will probably
have a system overload and die. You do not want that blood on your hands.

They are both very common points of confusion in TRACE (as John confirms!)
so I answered your questions at:

I posted an image too, and the post is pretty long, so I left it on the

I briefly addressed John's return air volume problem as well. Don't forget
to click the image to enlarge it!,


Bob F's picture
eQUEST UserLEED EA Credit 1 TrainingTRACE 700 User
Joined: 2010-06-30
Reputation: 18

Classification: UNCLASSIFIED
Caveats: NONE


Mr. Fassbender, Your answer confuses me.

You say it is not acceptable to model as 1 system, but the image you
attached shows that you are using the "dedicated OA" tab.

The second concern I have is that my outside air unit has energy recovery,
but the fan coils do not. Like Mr. Arnott's system, the heating and cooling
unit is return only with no outside air. So if I make a fan coil system,
then input the information under the 'dedicated OA' tab, if I input my
energy recovery under with 'options' tab will Trace know to credit the
energy recovery to the DOA unit, because the fan coil has no energy recovey.

Mr. Fassbender, can you explain what you meant by not modeling with 1

Also, I need to look deeper into the exhaust situation, thanks for the tip
before I even needed it.

John Eurek PE, LEED AP

Jeurek's picture
Joined: 2010-10-07
Reputation: 0

Hi Doug, John, Anne, Bob, Scott,

Thank you all for your help - all your replies have helped me to carry on
with the model, I'll probably be back here with more questions once I've
progressed further. It's good to know that there's such a helpful community!


Robert Arnott's picture
Joined: 2011-10-01
Reputation: 0

John/Other Bob-

Yes, I see where the confusion comes in. Sorry about that. I was answering
Robert's question, which I interpreted as, "should I model as 1 system -
with no DOA?" or "1 system with 1 cooling coil only".

*To clarify, it should be modeled as 1 system + 1 dedicated OA
system/unit*(which many people refer to as two systems, but TRACE does
not! Thus, the
confusion). Anyway, I hope that clarifies what I meant. The method that John
suggested (of two actual systems in TRACE, is a possibility I suppose - this
is what many eQUEST users do).

However, you really should take advantage of TRACE's ability to model DOA,
which is a feature that other programs don't have.

Regarding John's question

Yes, TRACE will know where to recover energy. If you have a fan coil system,
and specified DOA, plus energy recovery:

Model the DOA, as mentioned earlier
Then, add the energy recovery.
It is critical that you specify the deck locations (that way TRACE will know
that it affects the DOA unit, and not the FCU).
Specify the decks, as ventilation deck and exhaust side deck and VOILA!

TRACE understands that the ventilation deck goes to the DOA, and is also
pre-conditioned by the exhaust air.

Please let me know if there is other confusion.

Bob Fassbender

Bob F's picture
eQUEST UserLEED EA Credit 1 TrainingTRACE 700 User
Joined: 2010-06-30
Reputation: 18