Correctly Modeling Infiltration

[SakuraOno]

Hello,

I'm a student doing some analyses on non-engineered, owner-built (and oftentimes low quality) buildings in developing countries.

I am modeling a free-running (with no mechanical system whatsoever) building. Therefore my main modes of ventilation is natural ventilation and infiltration. And I'm doing simulation analysis via ApacheSim.

I read in the User Guide that if I coupled MacroFlo with ApacheSIM, natural ventilation should ONLY be represented EITHER in MacroFlo OR in Room Data but not BOTH, since this will cause them to be superimposed. My questions are:

1) what does "natural ventilation being represented in MacroFlo" mean? Does this just mean when we specify the opening types, the opening areas, when the opening is open, etc etc? So if we do this in MacroFlo, we should NOT add the air exchange type "natural ventilation" under the "thermal conditions" tab in Building Management Template (Or Room data)? Is this correct?

2) I am also wondering if the same thing applies to infiltration? Because in MacroFlo, we can define the "Crack Length" and "Crack Flow Coefficient" to represent the infiltration/air leakage for our openings when they are closed (i.e. natural ventilation when the openings are open, but infiltration when they are closed). If we do so, can we sill add in the air exchange type "infiltration" in Room Data? Or will this cause the infiltration to be counted twice?

I would really appreciate some pointers! thank you very much for your help!

Sakura

[PCully]

Hi,

Sounds like you have understood this guidance correctly.

If you are using MacroFlo (Opening types set up with Crack Flow Length&Coefficient and openable Area) then you shouldn't add Infiltration or Natural Ventilation Air Exchanges to your thermal Templates/Room Data as this would result in the air flow being counted twice.

Phil

[SakuraOno]

Hi,

Sounds like you have understood this guidance correctly.

If you are using MacroFlo (Opening types set up with Crack Flow Length&Coefficient and openable Area) then you shouldn't add Infiltration or Natural Ventilation Air Exchanges to your thermal Templates/Room Data as this would result in the air flow being counted twice.

Phil

Thanks so much!
This is now clear to me!
cheers!
Sakura

[SakuraOno]

Hi,

Sounds like you have understood this guidance correctly.

If you are using MacroFlo (Opening types set up with Crack Flow Length&Coefficient and openable Area) then you shouldn't add Infiltration or Natural Ventilation Air Exchanges to your thermal Templates/Room Data as this would result in the air flow being counted twice.

Phil

Actually, I spoke too soon, I actually still have a question regarding this.
If I understood correctly, MacroFlo only deals with the "normal" openings, such as windows and doors. what I'm modeling a porous wall (such as a wall made out of woven twigs, with lots of gaps in between), obviously there would be lots of infiltration through this wall. Would I then need to model this type of infiltration via adding an infiltration in the Air Exchanges Tab in the thermal template/Room Data?

Additionally, in the MacroFlo User Guide, it notes that "Holes have fixed air flow characteristics and do not need to be assigned air flow characteristics in the MacroFlo view." Does this mean as long as I model an opening using the "hole" function in Model IT, I don't have to specify anything else to it, and ApacheSim will simulate the the air through this hole automatically? In the case of natural ventilation (as described in my original post), do I then need to add a natural ventilation to the Air Exchange Tab in the Thermal Template/Room Data to account for the holes (since they are not assigned anything in MacroFlo)?

apologies for so many nuance questions! Thanks very much for your help!
Sakura

[PCully]

ok the porous wall is probably better represented via an air exchange rather than trying to define this as part of an opening. You'd just have to predict this and add it as an Infiltration AE.

Holes do not have opening types assigned and MacroFlo models the bulk air flow through these unopposed, note however that Apache can't simulate external holes only holes on internal surfaces so be careful how you plan to use these. To model an external hole you could just add a door and have it open 100% all of the time.

Hope this clarifies

[SakuraOno]

ok the porous wall is probably better represented via an air exchange rather than trying to define this as part of an opening. You'd just have to predict this and add it as an Infiltration AE.

Holes do not have opening types assigned and MacroFlo models the bulk air flow through these unopposed, note however that Apache can't simulate external holes only holes on internal surfaces so be careful how you plan to use these. To model an external hole you could just add a door and have it open 100% all of the time.

Hope this clarifies

Thanks for the clarification. Very good thing you told me about the external holes.
One last question, when adding the infiltration AE, for the "max flow", if using ACH, what is the reference wind pressure that Apache uses for the max flow? because the ACH for infiltration can vary according to the wind pressure (ACH at 50 Pa will be greater than say ACH at 4Pa). Obviously when doing simulation the wind pressure will change, but what does Apache use as the max (50Pa, or 75Pa, etc)?

and does it differ based on exposure (terrain type: country, suburbs, or city), for example does it use 50Pa for suburbs, but 25Pa for city?

Thanks very much for your help!

[PCully]

when you use thermal template/room data Air Exchanges I don't think they take into account wind pressure, it will be a constant air flow only depending on the variation profile you have assigned and th emax flow specified.

Phil

[SakuraOno]

when you use thermal template/room data Air Exchanges I don't think they take into account wind pressure, it will be a constant air flow only depending on the variation profile you have assigned and th emax flow specified.

Phil

Thanks for getting back to me.

So if I did not set the variation profile (meaning I have it "on continuously"), and if I set my max flow to be 0.5 ACH (for example) for infiltration air exchange, then there will be continuously 0.5 ACH infiltration regardless of the wind speed outside? Is there anyway to model this infiltration so it actually varies with the wind speed?

I understand that in contemporary houses, there's very little infiltration through the wall itself, most infiltrations are through openings (which is modeled by MacroFlo opening type), in such case, the infiltration air exchange can be assumed independently of the wind.

However, because I'm modeling something which the infiltration through the wall itself is very major and varies quite greatly with wind pressure (a wall made of twigs with lots of gaps), therefore it is actually quite important for my analysis that the wind pressure is accounted for. Is there a way to do that in IES? (without resorting to use CFD?)

Thanks very much for your help!

[PCully]

Yes that's right any Air Exchange will use a fixed flow only depending on the max value you have entered and variation profile.

I'm not sure how you could use VE To model this infiltration through these walls in any greater detail, it's not an area I have any experience with really. sorry

Phil

[SakuraOno]

Yes that's right any Air Exchange will use a fixed flow only depending on the max value you have entered and variation profile.

I'm not sure how you could use VE To model this infiltration through these walls in any greater detail, it's not an area I have any experience with really. sorry

Phil

Hi Phil,

thanks very much! it's good to know the limitation of what I can model.

For MacroFlo, however, do the infiltrations/leakage through the openings vary with the wind pressure? such that the amount of leakage from window and door is not only dependent on the "crack coefficient" or the "crack length" that I set for each opening, but also on the wind speed as it varies over time?

thanks again very much for your help!