Two OA systems supplying one space?

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I'm currently running loads on a small school. For the classrooms and offices, we're going with ductless minisplits with ventilation supplied by a DOAS. The DOAS will condition the OA before putting it in the classrooms, so the load should be negligible.

In the cafeteria, we're using a ducted split system and a separate OA system from the one supplying the classrooms. For the time being, we're not using an ERV to precondition the cafeteria OA.

Since there are so many people in the cafeteria, the ventilation requirement is around 2/3 of the total airflow in the space. Naturally, this adds a substantial load to the area.

One possibility we've considered is using the DOAS to supply some of the air to the cafeteria to reduce part of the load. My question is, is there a way to model this scenario in Trace? Say we were using the DOAS to supply 25% of the required ventilation--we'd need to have 25% of the OA being introduced directly into the space @ 72 deg F (or whatever) and 75% of the OA being introduced into the RA duct, unconditioned.

Thank you!

jasonlecker's picture
Joined: 2013-07-26
Reputation: 0

I think the answer is no but I may have a workaround.  I'm running on the assumption you're only concerned about load - not modeling?

So - if I understand - you are bringing 75% of the OA into the return of the split system that serves the cafeteria and 25% of the OA directly into the kitchen space.  All this air is coming from a single DOAS.

I think what you want to do is input the 75% of ventilation into the cafeteria in your rooms tab and the remaining 25% in the kitchen.  I think then what you would have to do is create a separate system for each room - one for the kitchen and one for the cafeteria.  In the kitchen - when you put in your DOAS - pick room direct.  When you put in your cafeteria DOAS - pick return/outdoor deck.  It won't be entirely accurate from a model perspective because it will create 2 fans, coils, etc. - but it should get you your load.

Pearlyp's picture
Joined: 2012-08-09
Reputation: 2

Well the downside for this one is there might not be enough energy to supply the whole two spaces. - YOR Health Products

Anonymous's picture