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Re: [EnergyPlus_Support] Re: Using AFN for windows, but with default infiltration

I think we should always strive to model what really happens.  I agree with adding a crack object to the AFN because all buildings have infiltration.  I do not agree with trying to make that a constant value , unless you have a mechanical ventilation system like a small fan, etc.  The crack object should account for infiltration both through the walls as well as the windows when closed.  In the US, the NFRC ratings for windows give the leakage when closed in CFM/FT2 of window area.

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"

On 4/10/2013 8:55 AM, Edward G. Lyon wrote:

Actually you should step back and ask yourself what you are really trying to do.  The crack factors should reflect the real performance of the windows/walls.  If you have calm wind conditions and equal inside and outside air temperatures, there will be no airflow calculated by an air flow network because there is no driving force.  Consequently trying to adjust the crack to guarantee a minimum flow simply can’t work reliably.  If you did get it to work, it would then overestimate what happens when it is windy with a large temperature difference.


Using EMS you may be able to calculate the potential air change rate with AFN and make a decision to use it or turn off AFN for a step.  Again the question is what are you trying to do.  A good AFN model should be telling you what happens, why do you want to distort reality unless you are trying to run a secondary ventilation system when ACH is too low.  If that is true, just how do you think you can actually implement that in the actual building.


Ned Lyon, P.E. (MA, WV)
Staff Consultant

781.907.9000 main
781.907.9350 direct 
617.285.2162 mobile 
781.907.9009 fax


From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of allinson_louis
Sent: Friday, October 04, 2013 9:03 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Re: Using AFN for windows, but with default infiltration



Thanks Joe, you understood my problem correctly. I cannot use zone infiltration as i can then no longer calculate air flow through windows. It seems I will have to play around with my crack objects to get an infiltration value that I want

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Joe Huang <YJHuang@...> wrote:
> I'm afraid you're missing the point. The original question was how to model infiltration
> when the AFN is being used.
> My point was that you cannot mix the infiltration object in conjunction with the AFN. If
> you want to model infiltration with the AFN, you have to define the leakage
> characteristics of the space for the AFN (this should probably be done anyway for proper
> modeling - in fact, all these AFN programs such as COMIS and AirNet were originally
> designed not for modeling natural ventilation, but rather infiltration).
> Sure, you can use a blower door test to get the leakage fraction, but that has to be used
> as an input to the AFN, not to the infiltration object. I've always been adverse to
> modeling infiltration as a constant air change rate, because that's nonphysical and not
> what really happens. IMHO, it's always much better to define the physical characteristics
> of the space (leakage-fraction, area of cracks, etc.) and then use a physical model like
> the Sherman-Grimsrud or the AFN to then compute the hour-to-hour infiltration.
> Joe
> Joe Huang
> White Box Technologies, Inc.
> 346 Rheem Blvd., Suite 108D
> Moraga CA 94556
> yjhuang@...
> http://weather.whiteboxtechnologies.com for simulation-ready weather data
> (o) (925)388-0265
> (c) (510)928-2683
> "building energy simulations at your fingertips"
> On 3/10/2013 9:25 AM, Oscar Hernandez wrote:
> >
> > There are many methods to calculate the real infiltration value...like the blower test.
> >
> > On Oct 3, 2013 6:06 PM, <josep.sole@... <mailto:josep.sole@...>> wrote:
> >
> > In AFN models you need to assume a lot of other hypothesis it is no easier than
> > assume the zone infiltration rate.
> >
> > *Josep Sol��
> > URSA Insulation S.A**- *
> >
> > *Sustainibility & Technical Manager *
> >
> > *M��vil +34 606 42 32 14 <tel:%2B34%20606%2042%2032%2014>*
> >
> > *www.ursa.es <http://www.ursa.es/>***
> >
> > *www.ursainsulation.com <http://www.ursainsulation.com>***
> >
> > *www.uralita.com <http://www.uralita.com/>*
> >
> > ***cid:image002.gif@...
> > **P **URALITA / URSA se compromete con el ahorro de energ��a. Antes de imprimir este
> > mensaje aseg��rese de que es necesario**.***
> >
> > *De:*EnergyPlus_Support@xxxxxxxxxxxxxxx <mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>
> > [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx
> > <mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>
] *En nombre de *Oscar Hernandez
> > *Enviado el:* jueves, 03 de octubre de 2013 17:57
> > *Para:* EnergyPlus_Support@xxxxxxxxxxxxxxx <mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx>
> > *Asunto:* Re: [EnergyPlus_Support] Using AFN for windows, but with default infiltration
> >
> > If you want to model an infiltration value use the zone infiltration. The AFN its to
> > predict the infiltrattion value when you don"t know it.
> >
> > On 3 October 2013 17:42, Joe Huang <YJHuang@...
> > <mailto:YJHuang@...>> wrote:
> >
> > I don't think the infiltration objects can be used in conjunction with the AFN. That
> > would violate the mass balance of the AFN, wouldn't it?
> >
> > Sent from my IPad
> >
> > Joe Huang
> >
> >
> > On Oct 3, 2013, at 8:35 AM, Oscar Hernandez <eng.ohw@...
> > <mailto:eng.ohw@...>> wrote:
> >
> > Hi,
> >
> > Use the zone infiltration.
> >
> > Oscar
> >
> > On Oct 3, 2013 2:48 PM, "allinson_louis" <allinson_louis@...
> > <mailto:allinson_louis@...>> wrote:
> >
> > Hi All
> >
> > I am using AFN to model air flow through window openings that I am controlling
> > using EMS
> >
> > However, when the windows are closed the only infiltration is through cracks and
> > the infiltration is much less than 0.1ach. I want to model a 'default' 0.1ach
> > when the windows are closed, as this is a typical value used in simulations.
> >
> > Is this possible?
> >
> >


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