Honeybee_Energy_AFN_Natural Ventilation Load_ Output

[Naga]

There is this old but the golden book that discusses Airflow Network in EP. According to my understanding, if we don’t use AFN objects for ventilation, the Zone Airflow objects are used for calculation. To be exact ZoneVentilation:WindandStackOpen with or without ZoneVentilation:DesignFlowRate for simple ventilation calculations.

On other hand, the AFN provides the ability to simulate multizone airflows. Thus, the outputs for both differ and by default HB may not be requesting and integrating in SQL outputs.

[Asisnath]

Hi @Naga. Thanks for sharing the resource. Its quite good info on AFN. Yes I also think the output component of HB_energy is not yet set to receive info from AFN.

[chris]

Hey @Asisnath ,

You are right that the lack of ventilation losses and gains with the AFN is primarily because I haven’t been able to sit down and see if there is any way to get this info out of EnergyPlus yet.

When you run the simulation with simple ZoneVentilation objects instead of the AFN, it’s very easy to get the energy gain/loss from infiltration and ventilation because all of the air exchange happens on the level of the Room/Zone. So it’s easy to isolate the gain/loss on a room-by-room basis. However, the AFN not only links together the infiltration and the natural ventilation within a Room but it also links these to the air exchanges of other Rooms, making it difficult to isolate the gains and losses just from these terms.

Intuitively, I feel like there should still be a way to do it but it will likely involve digging through the gain/loss contributions of individual AFN Crack objects (that represent infiltration) and AFN Opening objects (that represent windows). If anyone is willing to dig through the .rdd file of their AFN simulation and see if there are a set of EnergyPlus outputs that give us this info and post the names of these outputs here, I can take them and use them to write a component that gives us the overall loss/gain from infiltration and natural ventilation for entire rooms.

[Asisnath]

Hey @chris. Thanks for replying back. Perfectly explained. This output with simple ventilation object is quite an important one. We can cross check the way the opening and closing of windows occurs either as per schedule or temp. difference. For now In energy balance too it forms a quite an import part. So I dont know how can we generate an energy balance graph if we go for Afn. I will also try to see the .rdd file and let you know if we have a specfic output. Thanks again.

[Naga]

@chris

Below are the list of outputs that I generally use to calculate the Load through ventilation, infiltration, exfiltration, mixing and others when I invoke AFN objects. Few of the HVAC based outputs give zero values when the zones are assigned with Natural ventilation.

I remember using volume flow rate output and heat transfer formula (Q=c×m×ΔT) in EP EMS to derive heat transfer into and from zone just from fenestrations.

HVAC,Average,AFN Zone Infiltration Sensible Heat Gain Rate [W]
HVAC,Average,AFN Zone Infiltration Sensible Heat Loss Rate [W]
HVAC,Average,AFN Zone Mixing Sensible Heat Gain Rate [W]
HVAC,Average,AFN Zone Mixing Sensible Heat Loss Rate [W]
HVAC,Average,AFN Zone Infiltration Latent Heat Gain Rate [W]
HVAC,Average,AFN Zone Infiltration Latent Heat Loss Rate [W]
HVAC,Average,AFN Zone Mixing Latent Heat Gain Rate [W]
HVAC,Average,AFN Zone Mixing Latent Heat Loss Rate [W]
HVAC,Average,AFN Zone Duct Leaked Air Sensible Heat Gain Rate [W]
HVAC,Average,AFN Zone Duct Leaked Air Sensible Heat Loss Rate [W]
HVAC,Average,AFN Zone Duct Leaked Air Latent Heat Gain Rate [W]
HVAC,Average,AFN Zone Duct Leaked Air Latent Heat Loss Rate [W]
HVAC,Average,AFN Zone Duct Conduction Gain Rate [W]
HVAC,Average,AFN Zone Duct Conduction Heat Loss Rate [W]
HVAC,Average,AFN Zone Duct Diffusion Latent Heat Gain Rate [W]
HVAC,Average,AFN Zone Duct Diffusion Latent Heat Loss Rate [W]
HVAC,Average,AFN Distribution Sensible Heat Gain Rate [W]
HVAC,Average,AFN Distribution Sensible Heat Loss Rate [W]
HVAC,Average,AFN Distribution Latent Heat Gain Rate [W]
HVAC,Average,AFN Distribution Latent Heat Loss Rate [W]
HVAC,Average,AFN Exfiltration Heat Transfer Rate [W]
HVAC,Average,AFN Zone Exfiltration Sensible Heat Transfer Rate [W]
HVAC,Average,AFN Zone Exfiltration Latent Heat Transfer Rate [W]
HVAC,Average,AFN Linkage Node 1 to Node 2 Mass Flow Rate [kg/s]
HVAC,Average,AFN Linkage Node 2 to Node 1 Mass Flow Rate [kg/s]
HVAC,Average,AFN Linkage Node 1 to Node 2 Volume Flow Rate [m3/s]
HVAC,Average,AFN Linkage Node 2 to Node 1 Volume Flow Rate [m3/s]

[Asisnath]

@Naga @chris There are lot of outputs for Afn. I think it would be tough to amelgate to get net gain/loss.

[chris]

Thanks @Naga and @Asisnath ,

It is not difficult for me to write a component that automates all of the math to turn the AFN outputs into room-level gain/loss. The tougher part is just figuring out what this math should be from the available outputs.

@Naga , am I interpreting your list correctly that there are no output with the Total gain or Total loss from infiltration (you always need to get the sensible and the latent separately)?

[Naga]

According to the input output reference documentation of EnergyPlus V9.3, there is no output for total gain or loss from infiltration.

[chris]

Thanks @Naga . It looks like we can just add them together ourselves, then. I think the only open question at this point is whether the infiltration loss/gain includes both the air flow from the AFN Cracks and from the AFN Openings (the latter of which represent the open windows). The Input/Output reference doesn’t seem clear in this respect. If someone could run a Honeybee AFN simulation with operable windows and see if the window-opening pattern is visible in the infiltration loss/gain, then I’ll have all that I need to add a component that fills in the missing energy balance term.

[Naga]

@chris I will try it for single zone and let you know

[SaeranVasanthakumar]

@chris, @Naga

FYI, I actually had this same question back in 2017, and posted it on unmethours: https://unmethours.com/question/26907/how-to-output-heating-gainslosses-from-zone-ventilation-in-the-airflow-network.

As you can see, I eventually figured out that the AFN Zone Infiltration output is the sum of ventilation and crack flows into the zone. But, not a bad idea to double-check!

S

[Naga]

@SaeranVasanthakumar, you are absolutely right.
To segregate infiltration only through cracks and ventilation & crack combined, I generally use AFN Surface Opening Status output.
I used to calculate the average infiltration through cracks and then minus it from ventilation & crack combined infiltration to avail heat gain/loss only through ventilation.

[RafaelA]

I have done some workarounds to include AFN outputs in HB Energy Balance, but only during winter months when all the AFN Zone infiltration output could be assumed to be infiltration. I just added Sensible+Latent heat loss/gain outputs at zone level and trick the Energy balance component with the right header.

The only way I see to dissociate AFN infiltration from ventilation heat gains/losses in a more consistent way is by using the outputs at surface level:

• Opaque surface cracks can be accounted directly, since they always function as infiltration.
• Window cracks for infiltration are only used when the opening is closed. When windows are open, then ventilation settings apply. As @Naga mentions AFN Surface Opening Status would tell when each window airflow goes into Infiltration or Ventilation.

The bad news is that only Air volume, mass or pressure differences are reported for each window, so there is an extra step needed there to get the heat gain/loss. If this can be addressed, then I think we could be heading towards a solution.
AFN Linkage Node 1 to Node 2 Mass Flow Rate [kg/s]
AFN Linkage Node 2 to Node 1 Mass Flow Rate [kg/s]
AFN Linkage Node 1 to 2 Average Mass Flow Rate [kg/s]
AFN Linkage Node 2 to 1 Average Mass Flow Rate [kg/s]
AFN Linkage Node 1 to Node 2 Volume Flow Rate [m3/s]
AFN Linkage Node 2 to Node 1 Volume Flow Rate [m3/s]
AFN Linkage Node 1 to 2 Average Volume Flow Rate [m3/s]
AFN Linkage Node 2 to 1 Average Volume Flow Rate [m3/s]
AFN Linkage Node 1 to Node 2 Pressure Difference [Pa]

Another limitation I found was regarding heat transfer between zones.
The set of AFN Zone Mixing outputs gives the total Sensible/Latent heat gain/loss from air mixing between zones. Unfortunately, it doesn´t allow to differentiate where these come from if a zone is connected to more than one another zone. Reporting at surface level could also solve this, filtering internal windows.

For medium sized models, all these are already a large amount of data and I often end up with result files of several hundred MB. Hopefully this won´t be a limitation.

[chris]

Thank you all. This is very helpful and it will at least allow me to write something that results in balanced loads for the Honeybee energy balance. For now, I think that I will just use the Zone Infiltration Sensible/Latent Heat Gain/Loss values to ensure that infiltration gain/loss data collections are output from the simulation results whenever gains_and_losses_ are requested and the model is using the AFN. This will mean that the natural ventilation contribution to the load balance will be mixed in with the infiltration but at least we will be able to see how this combined airflow term fits within the whole balance.

At a later point, we can implement something that takes the model, the room air temperature, and the detailed Surface AFN results to give the contribution of window openings vs. infiltration cracks using the heat transfer formula that @RafaelA suggests . But we can get to this later.

I’ll try to get the first goal implemented by the end of today.

[chris]

The AFN outputs have been integrated into the result requesting and parsing components:

fix(output): Add results for AFN gains and losses

So the AFN energy balance is at least balanced now:

AFN_GainLoss_output.gh (108.4 KB)

[Asisnath]

Thanks @chris. A much needed update

[Asisnath]

From above discussions to be clear, for time being the infiltration will denote both ventilation and infiltration gain / loss in energy balance provided working with AFN. Am I correct?

[chris]

Yes, that is correct. At a later point, we can try to write something more sophisticated. But that’s how it will work for now.

[Asisnath]

Hello all and @chris. Sorry to restart this conversation again. Did you find a way to segregate ventilation loss/gain and infiltration loss/gain in AFN.
In a lot of analysis of design, it becomes confusing to infer from the balance chart using AFN and comparing with simple ventilation object. Even if we compare infiltration values of both cases there is a huge difference. I assume that in AFN, the infiltration value is not the sum of ventilation and infiltration heat gain/loss.

[chris]

Hey @Asisnath ,

I do not have an easy means of separating the infiltration from the ventilation loads when using the AFN but I am pretty sure that the “AFN Infiltration” is the sum of both infiltration and ventilation. So your charts should still be balanced when using the AFN.

Until E+ supports better AFN outputs, it looks like the only way to separate infiltration from ventilation is by requesting surface-level AFN flow rates and checking whether the flow is from a window. But even that workflow might mix some of the infiltration of closed windows with the ventilation of open windows and you still have to do some heat balance math yourself with the indoor vs. outdoor air temperatures.