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Getting infiltration rates right?

asked 2014-09-23 13:50:19 -0500

mbrusic's avatar

updated 2015-07-10 20:47:24 -0500

I know this forum is mostly for energy modeling but some of us have to model existing buildings, and when you model existing buildings sometimes you have to measure them. So - how do you determine what the ACH from natural infiltration is in a leaky old building? It could be 0.6; it could be 1.6.

What methods do you use? Utility bill calibration seems to be about the best the industry can do, but I'd like to know if anyone has better ideas.

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Mike, should this question be a bit larger? Maybe also talk about what infiltration method we should use to best represent a building? (eg: Sherman-Grimsrud in eQuest, EffectiveLeakageArea, FlowCoefficient, etc in E+)

Julien Marrec's avatar Julien Marrec  ( 2014-09-23 14:43:43 -0500 )edit

You could, but ultimately all of those models will end up with some parameter representing the flow resistance/discharge coefficient/ELA/whatever of the building that's not easily derivable or measureable. In most infiltration models it's a multiplicative constant. I think the fact that all those models have a parameter that's unmeasureable within plus or minus 50% is more important than which model you select.

mbrusic's avatar mbrusic  ( 2014-09-23 14:55:39 -0500 )edit

You're absolutely right. Plus, it slipped my mind but infiltration modeling methods were already covered in this post

Julien Marrec's avatar Julien Marrec  ( 2014-09-24 04:26:10 -0500 )edit

@mbrusic. Please tag this question with something like "infiltration".

__AmirRoth__'s avatar __AmirRoth__  ( 2014-09-24 09:55:42 -0500 )edit

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answered 2014-10-10 10:18:55 -0500

lisang's avatar

I agree with a comment above, that no matter which model you choose in an energy model, there will be coefficients/constants that are not attainable without further testing such as blower door and tracer gas tests. And if you enter a constant infiltration rate, you don't capture the effects of weather and system operation on infiltration. In real life, infiltration is not constant.

I published an article in the July issue of ASHRAE Journal which outlines equations to calculate coefficients in one of the EnergyPlus infiltration models. You use building height, surface-to-volume ratio, and net system flow (supply-return-exhaust) normalized by surface area, to calculate those coefficients. You do have to assume a baseline leakage value (like m3/s/m2 @ 4Pa) but having these coefficients allows infiltration to vary with weather and system operation (building pressurization, essentially). In the buildings I simulated, sometimes doing this saved energy, sometimes it increased energy. But it was still better than using a constant infiltration.

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Lisa did a presentation on her work for the IBPSA NCC chapter a little while ago, you can find her presentation here: if you would like a little more information.

Annie Marston's avatar Annie Marston  ( 2014-10-24 08:58:11 -0500 )edit

The EnergyPlus IO manual lists for Idesign lists two scenarios: 1. 7.5mph value and using Blast coefficients and 2. 10mph value and using DOE2 coefficients. We are actually using this for a mid rise multifamily building, and we are trying to quantify energy cost savings with air sealing using a whole building modeling approach. I tried to use the DOE2 coefficients 0.224 on wind speed difference, and use Idesign at ACH at 4pa values, my results are 4 times more savings on a per unit level than using BEopt or Remrate results savings level. Is there any recommendations? @lisang?

xfang's avatar xfang  ( 2015-03-13 15:34:13 -0500 )edit

@xfang Here at NIST, we've done work in correlating infiltration, modeled with CONTAM (a multizone model airflow simulation program), with weather, HVAC operation, and building characteristics. We developed a set of equations to calculate coefficients in the EnergyPlus ZoneInfiltration:DesignFlowRate object. You need the building height, exterior surface area, volume, and system pressurization (supply, return, exhaust fans). You can find the equations in this article.

lisang's avatar lisang  ( 2015-09-23 09:41:26 -0500 )edit

Just recently, we developed an Open Studio Measure that incorporate those infiltration relationships into your model. You do need to input system flow rates (total building supply, return, and any exhaust fans). You can find the Measure on BCL hereor look for the Measure titled "Adding infiltration correlated for weather and building characteristics" within the Open Studio program. Subscribe to updates on this Measure by sending an email to (Subject: Subscribe).

lisang's avatar lisang  ( 2015-09-23 09:41:46 -0500 )edit

Thanks @lisang! I just downloaded the BCL script, but it didn't get applied successfully, mainly the coefficients on A, B, D didn't get applied, and defaulted to OpenStudio default.......maybe the building height etc. didn't get registered? It's a bit odd....

xfang's avatar xfang  ( 2015-12-22 12:35:59 -0500 )edit

answered 2014-10-06 15:56:49 -0500

Particularly for existing buildings there needs to be a qualitative examination. You'll be able to gauge the envelope's construction type -- is it made of materials that would tend to yield higher or lower infiltration rates? Are there many penetrations and joints? Are thermal images of the windows available when the HVAC system is on and when it is off?

Then also assess the construction or maintenance quality which may be a factor that you can observe...was the envelope well-sealed originally, and has it been maintained?

In some cases you may be able to acquire trend data or deploy data loggers to show how temperature changes when the HVAC system is off. In past experience this will mainly be useful as backup for your assumptions in cases where the infiltration rate is very those instances the client probably has some idea that the building is leaky.

For most cases it will be difficult to confirm or deny the constants in the more detailed formulas, but by comparing the utility bills and maybe the space temperature trends you can partially calibrate.

This works in conjunction with knowing the other energy using systems in the building -- the more of those that are well-known, the less energy use that is eventually left over to vary as infiltration or process loads.

Lastly, run a sensitivity analysis to determine how important infiltration is to the results. (And your other major unknowns that were assumed to have a certain value.)

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Asked: 2014-09-23 13:50:19 -0500

Seen: 1,843 times

Last updated: Oct 10 '14