Re: [EnergyPlus_Support] e+ calculation of AHU minimum damper position in multizone system with VAV termi

[jeannieboef <jeannieboef@xxxxxxxxx>]

Thanks for the feedback, James. I know the reviewers are picky about having the ODA to the zone identicle for baseline and design case models.

This is a tricky subject and I'm kind of supprised that I've not seen this before. 

I'm simulating a project using trnsys which is a much more barebones toolkit and I've had to really sit down and think about how these systems are controlled (the user must build his own controls).

I've also been able to gain more understanding as to the why's of Appx G. And I must say that it is in general pretty well conceived.

I was considering setting the ODAmin amount at the system AHU level to be equal. The problem is that different zones require their ODAmin at different times and so the system ODAmin is constantly changing.

Sent from my iPhone

On 23.11.2012, at 16:07, Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

 

Dear Jean,

I always think of Baseline systems as fictional in many regards, so I donâ??t worry too much about small inconsistencies.  If the Baseline system is substantially different, pay close attention to the G3.1.1 Exceptions.  These describe situations in which you make a separate Baseline system for â??non-predominantâ?? zones.  There is an aspect of personal judgment involved here, but I would probably consider a zone with dramatically different outdoor air needs to require a separate Baseline system.

 

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

 

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Jean Marais
Sent: Friday, November 23, 2012 9:50 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] e+ calculation of AHU minimum damper position in multizone system with VAV termi

 

 

Your understanding is perfect. In germany we always design DOAS. this means problems for the baseline system which must have the same min ODA quantities. As the zones in the design case are unscrupiously linked to an AHU, using the same linkages for the baseline AHU may inadvertantly push the airflows high as described. As I have so little experience with real life non-DOAS systems, I thought I would ask.

I don't really know how to deal with this in the baseline...as the zones are not assigned ODA by %, but by absolute, meaning I'll never be able to group zones perfectly anyway.

On 23 November 2012 15:37, Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

 

Dear Jean,

I am not quite certain that I understand your question ? butt Iâ??ll give an answer and you can tell me if Iâ??ve helpedâ?¦

·         As you know the amount of outdoor air is calculated by the E+ sizing objects and can also be manually entered.

·         As in real life, if you provide a specific amount of outdoor air to a single air system which serves several zones, the fraction of outdoor air which is part of the air loop (e.g., 10%) at the fan is the same fraction delivered to every zone.  In this example, 10% of the airflow to every zone is outdoor air.

·         If you truly need to have very different outdoor air fractions in each zone, you must define a separate air loop for these zones with the correct amount of outdoor air in each zone.

·         Alternatively, a DOAS is commonly used to provide the minimum outdoor air flow to each zone (plus a zone heating / cooling component such as radiator, fan coil, etc.)

 

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
1631 Acacia Drive, Grand Rapids, MI 49504 USA
616 450 8653

 

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Jean marais
Sent: Friday, November 23, 2012 3:54 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] e+ calculation of AHU minimum damper position in multizone system with VAV termi

 

 

Dear Group,
How does e+ calculate the minimum ODA damper position for a multizone VAV system during autosizing?

I have the following scenario. Big zone requires lots of air (say 30,000 m3/h) for heating, but zero ODA. Small zone requires no air for heating because it has a radiator, but requires 80 m3/h min ODA. They are served by a common AHU.

If the AHU sets damper position as 80/(80+30000)=0.0026, then the small zone will require more air to get the minODA, i.e. 80/0.0026=30080 m3/h.
This would set the total demand for the AHU = 30080+30000. This obviously is unacceptable.

Does it then open the ODA damper such as to minimize the total m3/h? It may end up with the damper at 50% so that the small zone now receives 160m3/h and the total AHU is 30160. But this would mean the big zone is eating energy as it gets 30000m3/h with 50% ODA fraction...lots of ODA.

Any ideas?

Jean

PS. this example has been exagerated to show the principle

 

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