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Options for setting infiltration rates in Energy Plus

I've recently been investigating the different options for setting infiltration rates in Energy Plus and am hoping for some confirmation and/or correction of how I understand the different approaches available. The question is not so much about the inputs or equations for each model but rather how they can be practically used in Energy Plus.

ZoneInfiltration:DesignFlowRate


This is the model I use most often with a defined air-change rate and primarily for commercial buildings. In this case the air change rate is constant (ignoring the modifier schedule) and the infiltration rate varies with zone volume only. Is generally fine for code compliance - which is the bulk of the work I do.

ZoneInfiltration:FlowCoefficient


This model would appear to be more appropriate for residential applications but can't figure out how it can be practically applied to a multi-zone model. From blower-door tests I can get all the necessary parameters, however, this model calculates a volume flow rate that appears to be independent of any zone parameters (e.g. surface area or volume). If I create an infiltration object for each zone separately and use the experimental parameters then the same whole-building infiltration rate would be applied to each zone - which would be incorrect. (I ran a simulation today using this approach and the mass flow rate was similar but not the same for each zone - I can't explain the differences)

Given the level of uncertainty around infiltration rates it may be sensible to stick to the first option, however, it does seem valuable to be able to take into account site exposure, building height etc. which are available with the second option.

Any thoughts greatly appreciated.

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I think the modifier schedule should not be ignored; the result difference is non-negligible. Then, the question is how big the modifier should be? A default modifier of 0.25 is used in EnergyPlus. Any thoughts?

( 2017-05-27 17:38:44 -0500 )edit

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By way of background:

ZoneInfiltration:DesignFlowRate: This is the simplest infiltration model based on environmental conditions (wind and temperature).

ZoneInfiltration:EffectiveLeakageArea: This model is based on the Sherman and Grimsrud (1980) model, alternatively known as the “Basic” ASHRAE infiltration model, and would be considered an improvement over the previous model.

ZoneInfiltration:FlowCoefficient: : This model is based on the Walker and Wilson AIM-2 model, alternatively known as the “Enhanced” ASHRAE infiltration model. Of the three non-AirflowNetwork models, this model is widely considered the most accurate having undergone numerous validation studies by Lawrence Berkeley National Laboratory (LBL). But indeed, it takes more effort to calculate the needed coefficients from foundation type, presence of a flue, etc. This LBL publication has a lot of great information on calculating these parameters.

All three models have difficulties when trying to apply them to a multi-zone model. For example, how does one handle stack effect across multiple zones? How does one accommodate airflow between zones? These models can't really deal with many issues that are important for residential buildings. That said, in our BEopt tool (a residential modeling tool built on EnergyPlus), we currently use the ZoneInfiltration:FlowCoefficient approach. However, we actually model it via EMS in order to correctly handle interactions with duct leakage imbalance, mechanical ventilation, natural ventilation, etc. You could look at our implementation for more information.

Longer-term, we're looking to switch to the AirflowNetwork model to solve some of these issues and others. We're currently assessing the appropriateness of the model for residential buildings and are looking to make some improvements to the model next year.

Long-story short, I don't think EnergyPlus is quite there for modeling infiltration in residential buildings.

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Thanks for the feedback - the E+ documentation for ZoneInfiltration:FlowCoefficient provides a table of coefficients for different levels of site exposure and number of building stories but I haven't seen a similar table for the ZoneInfiltration:DesignFlowRate method. I'll spend some time digging into ASHRAE Fundamentals and see what I can figure out - the main effect I'd like to capture is site exposure - windy rural sites vs urban sites for example.

( 2016-07-20 18:18:29 -0500 )edit

In my opinion, ZoneInfiltration:DesignFlowRate is MORE appropriate for residential buildings than commercial.

1. The space is generally has a neutral static pressure.
2. There is minimal infiltration variation with height

Both of those characteristic will provide a more or less constant infiltration rate. Aside from exterior conditions the only thing that would significantly affect it would be interior exhaust fans. If you really want to account for differences in infiltration for spaces that have zones and such, I feel like that gets crazy complicated.

Wind speed is factored into ZoneInfiltration:DesignFlowRate, and it does have coefficients associated with the formula it uses, which should be a sufficient approximation. The ZoneInfiltration:FlowCoefficient may be a 'better' model but I'm just not sure it buys you much for residential purposes. I think it would almost be easier to do an E+ AirflowNetwork model instead of trying to determine stack/ wind coefficients for every zone.

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I might take a look at the Airflow Network sometime but not sure how I'd relate this back to measured parameters - sounds like an iterative process to adjust leakage areas etc to match ACH50 from the blower door test - sounds hairy. And as you suggest - there is no point trying to calculate answers to a higher level of accuracy than the input assumptions (real people tend to be unpredictable ...).

( 2016-07-20 18:27:14 -0500 )edit

You could use CONTAM to perform simulations to determine infiltration rates using the leakage rate obtained from the blower door test. Further, you can adjust the wind speed modifier based on terrain. NIST provides another tool to export the CONTAM infiltration results to a related EnergyPlus model to populate the ZoneInfiltration:DesignFlowRate object and schedule it to adjust infiltration based on the CONTAM results. You can verify blower door results within the CONTAM model with the Building Pressurization Test mode of the program.

I realize that this adds another tool to your simulation workflow, but if you know you will use this method before you create your E+ model, then you can generate the E+ IDF file from the CONTAM project.

The above information is detailed in documentation provided by NIST:

CONTAM to EnergyPlus infiltration - http://dx.doi.org/10.6028/NIST.TN.1873

Commercial building infiltration - http://www.nist.gov/manuscript-public...

Exporting CONTAM to EnergyPlus - http://www.nist.gov/manuscript-public...

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After a bit of thought ... maybe a better way to have asked the question would have been ...

How best to configure an infiltration object in E+ to match known performance obtained from blower-door tests?

From a standard test we can obtain characteristics such as ACH50, flow coefficient, pressure exponent, equivalent leakage area etc. These test results are for the whole building so it makes sense for the E+ model to work on a whole building basis.

The FlowCoefficient method allows flow coefficient and pressure exponent to be specified and then determines infiltration rates based on the other coefficients (to account for stack and wind pressures) but it's going to be tricky to implement well as previously discussed.

I would prefer to use the DesignFlowRate method but it's unclear to me how this method can be configured to match test results. The documentation says that the design flow rate is the maximum amount of infiltration expected at design conditions - but what value from the measurement data and what coefficients should be used (appreciating that the test does not provide information on site exposure or stack pressures)?

I hope this question is a little clearer - ultimately I guess I'm looking for some kind of verification that the test performance is being replicated in the model.

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If you have the blower door results, you could convert that to an I_design value @ 4 Pa (equation in ASHRAE fundamentals) to be used in the ZoneInfiltration:DesignFlowRate object and the coefficients in the report @Stuart Dolsmentioned: http://www.nist.gov/manuscript-public...

( 2016-07-29 12:57:59 -0500 )edit