HB AFN model results not consistent with the default simple simulation

Hello everyone,

I’m currently working on a project that focuses on the impact of natural ventilation (NV) as a passive cooling strategy. In my initial simulation, I used a simple HB model with no operable windows to establish a baseline. In the subsequent step, I simulated the same building but introduced operable windows during the summer and spring, utilizing AFN for more accurate NV results. I must mention that none of these cases were conditioned.

While I expected the operative temperature and relative humidity (RH) results during the winter to remain consistent between the two scenarios (since the windows were kept closed), I observed noticeable differences between the two cases.

I suspect that the primary reason for these differences may be the infiltration rate, which is significantly higher in the first case (annual average nearly 5 times larger). I attempted to address this by adjusting the reference delta pressure according to this discussion https://discourse.ladybug.tools/t/afn-infiltration-and-ventilation-questions/13471/11?u=shadab_mdr, but the impact was limited.

I also ran the first case (with consistently closed windows) through the AFN model, further complicating my assessment.

I’m currently at a crossroads, not sure which model to rely on for the basis of my results. Any insights, guidance, or suggestions on addressing the infiltration rate discrepancy and determining the more accurate model would be greatly appreciated.
I also attached the operative temperature vs. RH graphs for a zone with consistently closed windows once with AFN and once with a default model to show the difference better.

Hi @Shadab_mdr ,

The AFN is really a whole different way of modeling airflow compared to fixed flow rate assumptions so I’m not that surprised to see a bigger difference in flow rate and indoor RH. The AFN is going to respond to things like outdoor wind in a way that a fixed flow rate is not. So you might try lowing the wind in your EPW if you’re trying to match the results. Or you can increase the fixed flow rate of infiltration to account for the fact that you might be in a windy climate.

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Hi @chris Thank you for your response. To clarify, the higher zone infiltration air change rate was observed in the default model, not the AFN model, so I’m not entirely certain if it’s due to the wind effect. The infiltration input was 0.000569 m3/s-m2 for both models but the AFN model results in zone infiltration flow rates much lower than expected. But do you think it would be a valid approach to rely on the AFN model as it is and define the building with closed windows modeled by AFN as my baseline? The reason for this is that I need to use the AFN model for the subsequent steps where the windows are open for specific periods during the year. It seems the only way to reach the consistency I’m looking for is by using the same energy model for all steps (even when there is no NV).

Thanks for clarifying, @Shadab_mdr . In that case, perhaps your EPW is less windy than might otherwise be anticipated. It could also be that any buoyancy in the AFN is much less than what the constant infiltration flow rates are based on, which often assume infiltration resulting from multi-story buoyant flow.

Whatever the reason for this is, I would not do a comparative study where one case assumes static flow rates and the other uses the AFN. As I said earlier, they are just two very different ways of accounting for air flow and infiltration. You can either use the AFN for both cases or you can use the default constant infiltration rates and simplified window natural ventilation. But comparing the simpler assumptions to the AFN is much like comparing apples and oranges.

The AFN will model the air flow in a way that is truer to real building physics but this also means that there are many more properties to be set on the AFN if you want it to be a good representation of a real building. If you’re not able to fill in these AFN properties with values that you know match the real building, sometimes the simpler flow rates are the more accurate ones to use.

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