Recommended Infiltration Rate

Dear all,

There are 3 typical infiltration rates suggested in the Set EnergyPlus Zone Loads Component of Honeybee for tight, average and leaky buildings (see image). This is something I have always looked for but never found a simple answer, which in my opinion is very useful for most cases.

I have checked ASHRAE Handbook of Fundamentals, 90.1, 62.1, and some other papers and publications with no luck.

Could anybody give me more information about these values?

Thanks,

Jorge


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Jorge,

Infiltration rates are a tricky subject because most institutions have chosen a different method for measuring this.

Have a look at this thread I started a while ago :

http://www.grasshopper3d.com/group/ladybug/forum/topics/infiltratio…

Hope that helps. Alternatively you can browse through the energyplus documentation; they have a lot of useful info.

best,

Mauricio

Jorge,

Admittedly, I wrote those recommendations in the description without being certain which ASHRAE publication that they came from. There’s a chance that it might come from the analysis of CBECS data but I am not entirely sure.

To show that those numbers are not totally unfounded, here are the results of a real study of 65 buildings that have had their infiltration measured using a blower door test:

To help convert from the crazy IP/SI hybrid numbers here:

0.75 cfm/ft2 at 75Pa corresponds to 0.0006 m3/s-m2 at 4Pa (Leaky Building)

0.5 cfm/ft2 at 75Pa corresponds to 0.0003 m3/s-m2 at 4Pa (Average Building)

0.25 cfm/ft2 at 75Pa corresponds to 0.0001 m3/s-m2 at 4Pa (Tight Building)

As you can see, the range of real results generally correspond to the three recommended classes but there are 7 buildings that are so leaky that they are in their own category and there are number of buildings by the army core of engineers (ACE) that are consistently below the tight building class (although it’s usually not too far below).

I hope this helps give a better sense of the possible range of infiltration.

-Chris

Mauricio,

I had already checked that post out. There is indeed great information there, but my concern was getting more info on the specific values that were suggested in Honeybee. I agree that infiltration is a tricky subject, but when there is no measured data available these notions of approximate rates are great to have.

Chris,

Excellent. This is exactly what I was looking for. I would love to see a greater sample, classified by climate zone, but I guess that is too much to ask. If you or somebody else knows where to find something that gets closer to it, it would be great.

Thank you to both of you!

Jorge

Chris,

I’m a bit confused by these conversion values and infiltration rates in general. 0.75cfm/sf at 75Pa = .0006 m3/s-m2. Does the /sf refer to sf of exposed facade or sf of floor area?

The honeybee components are looking for m3/s-m2 of zone floor area. The ACHm3/s-m2 calculator uses zone info to translate a m3/s-m2 rate per exposed facade into m3/s-m2 of floor area. DOE reference building E+ models appear to use a rate of .00595 CFM/sf of exposed facade. @ 75Pa, this is right around the tight building parameters. @50, its a bit closer to the average building. Based on this, I assume the .75CFM = .0006m3 refers to per sf of floor area values.

if that’s the case (please let me know, I’m confused), what should our default flow rates per area of exposed facade be? Infiltration is a function of the facade, not the floor area, so this seems to be the better input method. Long narrow buildings should have more infiltration than spherical ones (ignoring doors and all those critical details)?

The current flow rate hints don’t explain the difference between rates per floor area and rates per facade area. Can more hints be added to the ACHm3/s-m2 calculator to clarify reasonable inputs?

@LelandCurtis ,
I understand the confusion. There are a lot conversions happening between floor area and facade area and different pressures. And, an top of this, we’re adding the conversion between SI and IP in this post.

To clarify, the recommended values in the Set EP Loads input description (0.75cfm/sf at 75Pa = .0006 m3/s-m2 at 4Pa) are per square meter of facade area. However, the actual numerical input of the Set EP Loads component is meant to take flow rates in m2 per floor area. The ACHm3/s-m2 calculator is meant to help you do this conversion by taking a flow rate per facade area at a certain blower door pressure (like 75 Pa) and convert it to flow rate per floor area at normal building pressure (4 Pa).

This was originally set up this way so that you can easily compare mechanical ventilation flow rates (which have to be per unit floor area) to infiltration rates. However, I am open to revising the infiltration input to accept flow rates per unit area of exposed envelope area (and keeping the mechanical ventilation flow rates per unit floor area) if you think that this would create less confusion, @LelandCurtis . The only issue with this is that I think some people like to assign infiltration as an ACH value and so converting the infiltration input to be per unit of facade area will eliminate this ACH workflow (since the infiltration of a zone without any exposed facade can’t be anything except 0 and not the input ACH value). Perhaps this is better energy modeling practice, though, and as you point out @LelandCurtis, this is how they do it in the DoE templates.

I will make the change to be per unit area of facade if you agree, @LelandCurtis , and @AbrahamYezioro agrees (since I know he had used the ACH workflow and originally coded the ACHm3/s-m2 calculator).

Hi @chris and @LelandCurtis,
I’m for any change that can make thinks be clearer and work better and right.
Though, I still use the ACH definition, since it is easier to explain and comprehend, at least for some kind of audience that i interact with.
-A.

@AbrahamYezioro ,
Thanks for the quick response. If you are still going to use the ACH workflow for infiltration, I can add another input to the ACHm3/s-m2 calculator to have the output to be either per unit floor area or unit area of exposed envelope. This way, you can still use an ACH workflow for infiltration but it will only assign the infiltration ACH to zones with an exterior envelope (for the reason I cited earlier).

Assuming that I add this input, the default for the ACHm3/s-m2 calculator will be to have the ACH translated to be per unit floor area since I imagine most people will use the ACH workflow to assign mechanical ventilation and not infiltration.

Let me know if this sounds good and, if so, I will implement it.

@chris and @AbrahamYezioro,

I perfer inputting infiltration flow rates per m2 of floor area, as this is how ventilation rates are input and it allows the ACH method. The ACHm3/s-m2 calculator is perfect for inputting per façade area infiltration. It could benefit from more clear annotation, or perhaps by requiring the blower pressure to be input.

That being said, I’m still confused. If I input .75 CFM/sf of façade, convert it to M3/s-m2, input 75PA, my results are .001, not .0006, with the exception of the central zone. See attached script. I’ve tried various combinations of zone heights, form factors, and perimeter zone depths. All exterior zones appear 2x what i’d expect. The interior zone = .0006 @.75, but =.0004 @.5, not .0003.

See attached gh file.

[details=“Summary”]
This text will be hidden
infiltration test.gh (507.3 KB)

Hi @chris,
I use the infiltration as ACH, and so are the local standards here.
Even E+ allows to define infiltration as ACH in one of their options. The original need of this component for me was to translate ACH to m3/s-m2 since this is why HB was writing this parameter in the IDF.
As long as i can keep having this option, i’m for any change you consider users will find more useful … which, if i understood what you wrote, is exactly what is going to be.
Thanks,
-A.

I’m glad that I put the question out here as it seems like the best route is to leave the infiltration assigned based on floor area so that we can keep the entirety of the existing ACH workflow. Instead, to help avoid the confusion that happened here, I am adding a few extra outputs to the ACH2m3/s-m2 Calculator so that you can do a sanity check and be sure that everything they set up is working correctly.

Specifically, the readMe! output now prints the assumptions that are being used for each zone. @LelandCurtis , you can use this in your file to see that the infiltration per unit of exposed facade area is, indeed 0.0006 m3/sec-m2 as the Set EP Loads component suggests. So the 0.75 cfm/ft2 values at 75 Pa that you are inputting here are a good representation of a leaky building as expected:

I also now output the surfaces that are being used to calculate the exposed facade area in a new allExposed output so that you can check and be sure that interior surfaces aren’t being used in the calculation.

These changes have been pushed to the github:


… and you can find an updated version of your file here @LelandCurtis :
infiltration test_CWM.gh (503.8 KB)

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Chris,

This is great, thank you. These new outputs will calrify all of this conversion craziness

Hi friends!

Nowadays in version 0.0.66 the infiltration rate is per façade area. I am curious, how can I input infiltration per floor area?


In HB Set Energy Plus Zone Loads, it says that 0.4 cfm/sf @75PA = 0.000285 m3/s.
If I convert 0.4 cfm to m3/s I get 0.000019.
How does the value of 0.000285m3/s calculated? Am I not considering the difference in pressures?
I could not find the answer in ASHRAE 90.1-2013…

Also, does anyone have any reference to check the infiltration rates for different facade types?
For instance here I found infiltration rates for curtain walls in cfm/sqft.

@mrbro

Yeah, I was able to hit 0.000285 by accounting for the assumed 4 Pa pressure difference.

You also missed a step in the first conversion, the infiltration rates are for per unit area, so we need to include the square foot to square meter conversion by multiplying 0.000019 m3/s by 10.7639 (sf in 1 m2) = 0.00205 m3/m2/s1

Now you can solve it using the following crack equation that relates pressure to flow rates through cracks:

\dot{Q} = \frac{1}{\rho} C \Delta{P}^n

C and n are unknown air flow coefficients, \rho is air density, \Delta{P} is the envelope pressure difference, and \dot{Q} is the resulting volume of air.

I approximated n to be from 0.6 to 0.7, and substitute 0.00205 m3 into the equation to solve for the 4 Pa pressure difference. \rho cancels out when you do the substitution and the resulting equation is:

\dot{Q} = 0.00205 \cdot \frac{4}{75}^n

You can see here for an n of around 0.67 we get the target 0.000285 m3/s/m2:

image

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