Dedicated OA and Split System

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Per the TRACE users:

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

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There are a few open ends here. However, I will answer the important parts.

Starting with your first question:

1) No, it is not acceptable to model as 1 set of coils 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. You need to select the recirc system under Create Systems. Once that is finished:

i) Go to the Dedicated OA tab and select the type of Configuration (ie Cool/heat) and select the control points

ii) The important part is to select ROOM DIRECT and SYSTEM under the Deck and Level Respectively. This will keep the ductwork separate for the DOA unit (it will put air directly into the room and there is 1 DOA cooling coil for the whole system). Click image below to see details

Leave the schedules at available 100% here, but make sure you edit your ventilation schedules at the room level.

iii) You really don't have to do this, but you can also go to the selection tab, and the advanced button and set the the return air deck to room direct as well (completely recirculated at the room, if that is the case).

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2) TRACE will not automatically balance the air by adding the right amount of ventilation:

 

Here's what you need to do:

 

i) Specify the bathroom exhaust as "room exhaust" under Create rooms, airflows tab (for the restroom of course)

ii) In the restroom, specify adjacent airflow from the corridor/passageway

iii) IF THE CORRIDOR has LESS INCOMING OUTDOOR AIR (Ventilation + infiltration), the restroom will only get that amount of air, but the trick is this:

iv) Go to the airflows tab of the passageway and add another adjacent room's air transfer (I'd use the next room on the system that has the most outdoor air). You can continue this cascade, but you may need to explore a few options, once you get passed two levels of this cascade.

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Welcome to Energy-Models...just to add my $0.02 on another way to look at your toilet exhaust, with DOAS utilizing exhaust air from the toilet rooms for enegy recovery, the EA cfm will match the ventilation air rate. If you want to model any "positive pressure" in the space, then take the difference between the OACFM - EACFM per space, say a dorm room might need 45cfm of ventilation air, but only 25cfm exhaust from the dorm room's bathroom. I'd enter in the difference of 20cfm into "Room Exhaust" as to "take away" that portion of energy not recovered by the energy recovery device (required for DOAS over 3000cfm). If you enter the toilet exhaust in the room exhaust, that energy goes "out the window" and no energy is recovered.


With regards to your apprehension of trusting the DOAS feature of Trace700, I can assure you that I have it working for me on 10 buildings: 2-pipe changeover either from a boiler/chiller or on another building I had heat reclaim from a geothermal loop and another with cooling tower heat reclaim. The DOAS will give you the system you are trying to model. Also to note that you can separate the coils in the Plants so that your Split system coils represent the cooling and heating portion and the DOAS called "Opt Vent" cooling coi can be on a WWHP and heating coil can have backup boiler heat.

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

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Thanks for the replies, I'm the original poster of the above question. I now have a much better understanding of how Trace handles the Dedicated OA and ventilation, and can carry on building the model. I think I'm going to be visiting this forum quite often in future!

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Glad to clarify. (as an administrator, I edited the first post so that it shows you posted it, it will earn you points for the site). Those points aren't good for much at the moment, but will be useful in the future!

I hope that you understood what I mean by 1 system. I just posted the following to the TRACE list, to prevent further confusion:

 

 
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.
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I appreciate the dialog you are having because I am working on modeling a DOAS and WSHP systems in parallel.  After setting up my WSHP system I set about to the Options tab to ad a Total energery wheel. Then over to the Dedicated OA tab to configure setpoints, schedules, controlsm and dedicated ventilation locations.  Am I going about this in the wrong manner?

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Jim, what you are doing is the correct approach. Now, of course, gotta make sure you configure the settings per reality, and also to assign the coils in the correct place once you start making plants.

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I have a similar system that I am modeling in a dormitory with a dedicated outdoor air system with valence units in each dorm room. They are modeled as passive chilled beam heating / cooling in each dorm room.  Each dorm room gets 40cfm of OA from the dedicated unit.  The air is transferred out from each room to the corridor and then from the corridor to the common restrooms (2 per floor), exhausted back to the energy recovery unit and then out of the building.

How do I model that airflow as multiple dorm rooms transfer air to the corridor.  Should I just model all of the ventilation air going in to the corridor and then transferred to the restrooms?  It doesn't seem like you can transfer air into a room from multiple other rooms in trace without doing that cascading that was mentioned by Bob.  This seems like it might get cumbersome.

Thanks

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Eb-

Yours is a predicament that definitely requires thinking "outside the box", like your idea on putting all the air into the corridor, which in my opinion would be a fair workaround - assuming no dehumidification - which could get underestimated in that case.

The cascading idea won't work on such a large scale. There are several air balance algorithms  in TRACE that will make it impossible to daisy chain a very large number of rooms, not to mention that it would also be cumbersome.

Your idea will work, provided that the ventilation is neutral. Otherwise, it will result in oversizing of the chilled beam units (since the ventilation cooling load won't be accounted for), and an undersizing of your hallway unit (since it will get an excess of cooled air).

However, if you aren't really concerned with load design and this is for energy only, your suggested method should be accurate enough

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Eb> Using the corridor as a common path of transfer air is not allowed by any code, as to maintain a safe path of egress. You may transfer to the corridor ceiling plenum but the path of egress must be maintained. You should consider the overature of CFM in the dorm room and the positivie pressure of the corridors (supplied spearately) as a positive building pressure.

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

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Thanks for the comment.  It helped me understand what is really going on.


The air is transferred from the rooms to the corridor plenum and then through a transfer duct in to the restrooms.  I guess in that case, the air transfer effectively is from the rooms directly to the restrooms, and that is how I will model it.


Thanks again,


Eric

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If I have a decicated outside air system with both a supply and exhaust fan which dumps to an energy recovery wheel in the DOAS unit, how does trace account for the exhaust fan power?  All I can see is the OPT Vent fan power accounted for, which I take to mean the supply fan only.  The main system is single zone fan coil units and the exhaust system fan tab is greyed out on the fan systems screen.  Any help is appreciated.  Thanks.

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Anonymous||||> To answer your question, you will need to account for the total HP within the Opt vent fan. This value is most accurate with using the borehorspower (bhp) values from the unit selections (sales engineer). Next, look within the fan coil unit's [Advanced]tab and Omit the return air fan, to "light up" the main exhaust fan in the fan tab. That said however, do not confuse the Main Exhaust fan with the fan within the DOAS w/ energy recovery. Remember this is a single-zone fan coil so they don't have those things in the FCU.

Something more to consider with your design with respect to DOAS and quantifying the correct EA rates for positive pressurized spaces:  For example- if a room is to have 50cfm OA from the DOAS, and you want to pressurize the space by 30%. 50cfm x 70% = 35cfm exhaust air...but this air is what you want to come back to your energy recovery device for the energy savings, what do you do? Exhaust 15cfm from the room. You cannot do negative infiltration, does not work. That said, be sure that no room exhaust fan has criteria in it back at the Systems...it's just exfiltration.

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

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Thank you Bobba Fett.  That is a great help.  I have the bhp for both the outside air supply fan and the exhaust fan from my fan schedules, so I will add those together and distribute amongst the OPT vent fan power.  I came across your previous post regarding positive pressuration via google and it was a huge help!  Keep up the good work!

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[Offtopic] You should sign up...we have so many anonymous's running loose around here that it's hard to know which is which. Oh and woorking late? or are you on the left-coast?

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

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