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RE: [EnergyPlus_Support] Zone Ventilation : Design Flow Rate

I do not know whether I should say anything on this topic.  I have done so a few time related to different questions.
As I understand it, the Eplus simulation process does not actually introduce air flow, in the non AFN model.
When an ventilation object is added, the amount of air flow in the object is to determine the amount of heat to be added or subtracted from the zone.  OA in and zone air out.  All the other HVAC air flow rate may not be affected, because these airflow rates are also only used to calculate the heat transfer amount. 
This is also the reason why so many user could not understand what is actually happening in the VAV system.  The VAV damper position is calculated from the zone temperature and the supply air temperature difference and the zond demand.  When the damper is at the limiting position, extra cooling and heating must be made available, without the set point not met warning.  Damper at the mimiting position means that no extra heating or cooling can be picked from the system coils.. 
Similarly, the OA mixer, etc also uses the air flow rate to calculated heat energy involved.  100% OA means that you have a fixed amont of  heating and cooling from OA through the AHU and all of the return air is exhausted through the relief nod of the mixer.  A new name is generated, DOAS without heat recovery..
In zone mixing, again the air flow rate is to determine the heat energy to be carraied from zone to the other.  When a volume of air is flowed from zone A to zone B, an equal amount is flowing back from zone B to zone A.
This is the reason why, the ventilatin air can be injcted in and exhaust without any openings in a zone.
It dowa not matter, whether you have doors open or not between zones.  Opening all doors to a corridor does not mean that the air fill flow through it, unless you declare the corridor as a return air plenum mixer or path for the zones.
When one install a bathroom exhaust fan and leave the door open to the other zone, it does not mean that the air is being exhausted using the air through the door from the zone.  If the bathroom is not conditioned, the inlet may be the outdoor air.  If the bathroom is conditioned, the exhause fan may cause an unbalance in the system air flow.   I do not  know the answer to this yet. 
I think, the exhaust fan has no meaning.  For heat balance, the bathroom air comes from the inlet of the OA mixer.  The airflow rate to the bath room can be higher and to be equal to the exhaust fan rate.  The temperature of the air entring the bathroom would be that of the suipply air, which can be of a higher temperature, if the thermostat in the bathroom is set to a higher value.
This is why I am using Ventilation object so simulate louvers, instead of using full AFN model.  No louvers are acuallyt entered in the IDF.  AcH is changed in the different zones to refect the wind direction.  My objective is to use natural ventilation, external shades, heat source control, wet utside soil surfaces (eco-roof on the ground.), etc. and study the natural temperature in a residential building.  The conclusiong is that no AIr-Conditioning is needed in most situations in the tropical countries.
There seems to have many topics covered.  But there is only one, there is no actual air flow in the EPlus zone model when not using the full AFN mode. 
If thay are correct, may be some of the experts can confirm that.
If I am wrong, please  point them out or ignore them.

 Dr. Li  


To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: vinay.devanathan@xxxxxxxxx
Date: Tue, 25 Sep 2012 13:55:11 -0400
Subject: Re: [EnergyPlus_Support] Zone Ventilation : Design Flow Rate

I ran a 1 zone simulation to check for the things that Richard had suggested.

The results seem to say that (based on PTHP supply airflow sizing, zone sensible load predicted) -

the situation with a 55 cfm exhaust (using ZoneVentilation:DesignFlowRate) with 0 infiltration and 0 zone mechanical exhaust is SAME as the situation with a 55 cfm infiltration with 55 cfm zone mechanical exhaust (using Fan:ZoneExhaust)

So I guess the ZoneVentilation:DesignFlowRate object does make up equal amounts of air as it exhausts

Thank you.

On Tue, Sep 25, 2012 at 10:33 AM, Richard Raustad <RRaustad@xxxxxxxxxxxx> wrote:

Test it, test it, test it.

Run a simulation with and without the ZoneVentilation:DesignFlowRate object.
You should find that the PTHP flow rate is the same for both simulations, but the loads will change.

Don't forget to report variables needed to verify operation. PTHP OA flow rate, PTHP SA flow rate, PTHP PLR, zone sensible load predicted, etc.

On 9/25/2012 10:10 AM, Vinay Devanathan wrote:
Thanks for your replies.
I would like to know how this situation would be handled by E+ -
I have a PTHP object for space conditioning with 0 cfm OA at all times. I also have a ZoneVentilation:DesignFlowRate object exhausting 55 cfm mimicking bathroom exhaust from the space. In this situation how would the PTHP operate (0 OA cfm by definition)? Would the continuous Zone Ventilation object exhaust make the PTHP operate with a 55 cfm OA to make up for the difference?
Thank you.
On Sep 24, 2012 2:36 PM, "Jim Dirkes" <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Dear Vinay,

Linda, as usual, is correct.  (When she says ?I believe?.?, that normally means ?without a doubt?, since she has forgotten more than what the rest of us know J)

The DesignFlowRate object, in my understanding, establishes minimum outdoor airflow rates for the Air Loop, but as Linda says, has no interaction with any Zone Exhaust fans.  Below are my notes from past projects and correspondence with the Help Desk; they may be helpful.


  • Zone Air Balance
    1. The zone return air is calculated as:

CFM supply - CFMexhaust = CFMreturn.

If the CFMexhaust >  CFMsupply  , CFMreturn will be 0. If the return flow rate is 0, the OA controller must provide the air needed for the supply air duct, so OA flow will be 100%.

b.      E+ sums up all the zone exhaust air flow rates for a system, denotes this as the system exhaust air.  The flow rate at the mixed air node (the supply air flow rate) is the sum of the system level exhaust flow and the return air flow. The outside air flow is whatever is set by the outside air controller, but it is constrained to be >= the system exhaust flow. All of this is based on the assumption that zone exhaust exits the system. Zone exhaust should not be used to send air from one zone to another.
c.       System air balance is affected by cycling fans or those with differing schedules.  A cycling AHU fan, if OFF while an exhaust fan is ON, will generate an ?imbalanced airflow? warning.  The same is true if one schedule has the AHU fan off while the exhaust fan is ON.
d.      Logic for the ?imbalanced airflow? warning is:  ???????????



James V Dirkes II, PE, BEMP, LEED AP
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 Linda Lawrie
Sent: Monday, September 24, 2012 2:09 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Zone Ventilation : Design Flow Rate



I believe the answer is NO.

At 12:05 PM 9/24/2012, Vinay Devanathan wrote:

Hello All,

I remember I have had this question arise sometime previously too.

Could anyone tell me how the EXHAUST type ventilation works in ZONEVENTILATION:DESIGNFLOWRATE object. Does E+ automatically make up an equal CFM into the zone as that specified to be EXHAUSTED in order to maintain ZONE AIR BALANCE?

Hope to hear from someone soon.

Thank you.


Richard A. Raustad
Senior Research Engineer
Florida Solar Energy Center
University of Central Florida
1679 Clearlake Road
Cocoa, FL  32922-5703
Phone:   (321) 638-1454
Fax:     (321) 638-1439 or 1010
Visit our web site at: http://www.fsec.ucf.edu

UCF - From Promise to Prominence: Celebrating 40 Years

Vinay Dev


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