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# When used at low flow rates, the airflow direction became opposite to the intended direction.

Hi all,

I am trying to simulate using AirflowNetwork and AirLoopHVAC with heat recovery in a multi-zone house. In a sample file (AirflowNetwork_MultiZone_SmallOffice_HeatRecoveryHXSL.idf) "Design Supply Air Flow Rate" is 1.9 (m3/s), but I would like to simulate with a lower flow rate.

I changed the "Design Supply Air Flow Rate" and flow rate of fan in the sample to 0.05 (m3/s) and got the following error.

** Severe  ** AirflowNetwork: The airflow direction is opposite to the intended direction (from node 1 to node 2) in AirflowNetwork:Distribution:Linkage = ZONE1SUPPLY1LINK
**   ~~~   **  During Warmup, Environment=CHICAGO_IL_USA ANNUAL COOLING 1% DESIGN CONDITIONS DB/MCWB, at Simulation time=07/21 00:00 - 00:10
**   ~~~   ** The sum of the airflows entering the zone is greater than the airflows leaving the zone (e.g., wind and stack effect).
**   ~~~   ** Please check wind speed or reduce values of "Window/Door Opening Factor, or Crack Factor" defined in AirflowNetwork:MultiZone:Surface objects.


The error message says to reduce the Crack factor, but I don't want to reduce it because I want the simulation to account for the effect of cracks. If there is any other way to resolve this error, please let me know.

Thanks.

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@yoshiwara what simulation tool are you using? Please mention it in the title or body of your post, as well as add a tag so that others can provide better help. It sounds like EnergyPlus, based on your use of "AirflowNetwork" and "AirLoopHVAC", but it would be good to confirm.

( 2022-07-20 10:05:28 -0500 )edit

Thank you. You are right, I am using EnergyPlus.

( 2022-07-20 18:48:22 -0500 )edit

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I've made the change you indicated and tested with the latest release (22.1) and with my build of the in-development code. In both cases I am getting warnings, but the simulation completes. What EnergyPlus version are you using and what weather file? I wouldn't say my simulations are successful, there are 1000+ warnings about low flows and out-of-range temperatures, here's an example:

*************  ** Warning ** HeatExchanger:AirToAir:SensibleAndLatent "OA HEAT RECOVERY 1":  Average air volume flow rate is <50% or >130% warning continues. Air flow rate ratio statistics follow.
*************  **   ~~~   **   This error occurred 824 total times;
*************  **   ~~~   **   during Warmup 0 times;
*************  **   ~~~   **   during Sizing 0 times.
*************  **   ~~~   **   Max=0.115243  Min=0.115243


I would not expect such a drastic change of a single field to work very well, there's a lot that can go wrong with that, especially when you're changing one of the design inputs that's really likely to clash with other inputs. If you do go through the process of changing other inputs to silence all the warnings, you're likely to end up with a different design and that's probably not what you want. I'm guessing it would be better to limit the flow another way that doesn't change the overall design. Why are you interested in such a low flow rate?

Update

Looking at your file, I see that you have lowered the flow rate in more places than I did. That accounts for why mine completed and yours did not. It appears that the pressure solution is driving things backwards in a sense, and it may be that the pressures here are just not compatible with a simulation of the distribution system with such low flow. There are still other issues that could be playing a role. If I take the original example file and change the "AirflowNetwork Control" field in the AirflowNetwork:SimulationControl object from MultizoneWithDistribution to NoMultizoneOrDistribution, that simulation will complete and the only warnings are about unused AirflowNetwork objects being present and missing outputs. Both of those things make sense. If I take your file and do the same thing, I get a lot of warnings about low temperatures and such, so the HVAC is having a problem with the low flow rates. I also tried MultizoneWithoutDistribution with similar results.

So, what to do? I think there are two options:

1. If you can fix the warnings that are cropping up in the NoMultizoneOrDistribution simulation, then you can go back and try the AFN simulation and see what happens (with both MultizoneWithDistribution and MultizoneWithoutDistribution). Keep in mind that the distribution simulation may still not work, but the results are now at least more defensible. The handling of fans in AFN is necessarily somewhat simplified, and while it's possible there's a bug lurking in here somewhere, the distribution solution just may not be able to handle the low flows in this model.
2. If you take a look at the results of the MultizoneWithoutDistribution simulation of your current ...
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Thank you for your response and confirmation. I am using EnergyPlus 22.1. I noticed your suggestion that if I change one input, I need to change the other related inputs as well. I checked the idf file again and tried the relevant changes, but the result remained the same. Here is the idf file I tried at that time (This is the AirflowNetwork_MultiZone_SmallOffice_HeatRecoveryHXSL.idf with the parameters changed). There is no weather file because it is calculated with Sizing Period.

( 2022-07-26 19:41:24 -0500 )edit

I am considering a simulation of the thermal environment in Japanese houses. In Japanese housing, whole house ventilation systems with low flow rates of 0.5 AirChanges/Hour are often used and I would like to simulate them. For example, a house with a floor area of 150 m2 and a story height of 2.4 m would have an air volume of 360 m3 and a flow rate of 0.05 m3/s for 0.5 AirChanges/Hour. If you know of any good ideas, please let me know.

( 2022-07-26 19:43:00 -0500 )edit

@yoshiwara Thanks for adding the link to the file, it appears that you changed the flow rate in a few more places than I did and that explains the difference in outcome. I'll update my answer.

( 2022-07-30 15:07:08 -0500 )edit

@Jason DeGraw Thank you for your detailed explanation and advice. I will try to follow your suggested policy of Option 1 and solve the problems one by one while reviewing the parameters.

( 2022-07-31 20:18:44 -0500 )edit