Lately, I’ve been working with the AFN component to achieve a specific value of infiltration rate in accordance with the regulatory requirements I’m following.
To calculate Qafn from the initial simulation, you can extract the variable AFN Zone Infiltration Air Change Rate or AFN Zone Infiltration Volume and convert their values to match the unit of measurement of the reference Qifrea using the geometric dimensions of the zone. In my case, where Qifrea = 0.0003 m3/(m2 s), I can either divide the air volume (in m3/hour) by exposed area* 3600 or multiply the air change rate (in 1/hour) by the zone’s volume and then divide it by exposed area*3600.
To obtain a unique reference pressure, I’ve noticed that using the annual average of the hourly Qafn values and then dividing it by Qifrea yields better results compared to averaging the hourly values of Qafn,h/Qifrea ratios.
In my case, the situation is further complicated because the model consists of multiple zones, and still, the resulting reference pressure must be unique for the entire model. Simulating with different formulas, I found that the formula which yielded the best correspondence between Qifrea and the annual average of Qafn across all zones was as follows:
Regrettably, when testing the same method on different models, I haven’t obtained comparable results, and thus, I couldn’t validate the use of one formula rather than another.
However, the employment of this comprehensive method introduces a significant problem that I noticed when analyzing the building’s heating demand: although the resulting reference pressure ensures the desired annual average infiltration, its annual trend is highly variable, characterized by pronounced peaks during the winter months. These peaks result in a heating demand that is twice as high as that of a scenario without AFN.
This makes me think that the annual average infiltration cannot be compared with the regulation or code targets, as they probably do not account for such periodic peaks. Consequently, despite the hourly infiltration trends appearing more realistic, I am hesitant to rely on the results I obtained.
In conclusion, I haven’t delved further into the approach discussed so far, as I discovered the existence of the “AirflowNetwork: MultiZone: SpecifiedFlowRate” object, implemented in EnergyPlus version 9.6.0. This idf object allows me to assign a constant airflow to each external surface, calculated simply as Qifrea for the area of the considered surface. By adopting this method, I achieve a constant infiltration rate that aligns with the regulatory requirements without renouncing the advantages of using the AFN.
Hope all of this helps you,
P.S. It would be cool to write the formulas like @SaeranVasanthakumar did above, but I haven’t quite figured out how to do it yet.