[EnergyPlus_Support] Re: The Airflow Network, Infiltration, and Mechanical Systems

["Jean marais" <jeannieboef@xxxxxxxxx>]

Hi Sam,
You'll have to check the eng. ref., but from what I remember, it does take the mechanical ventilation into account, i.e. when the mechanical ventilation is on there is a slight overpressure resulting in less infiltration.

"More advanced infiltration calculations are possible using the EnergyPlus AirflowNetwork
model for natural infiltration driven by wind when the HVAC system does not operate and/or
driven by wind and forced air for times when the HVAC system operates. Exfiltration (the
leakage of zone air to the outside) is generally handled better as zone exhaust air in the zone
equipment description.
The question of typical values for these coefficients is subject to debate. Ideally, one should
do a detailed analysis of the infiltration situation and then determine a custom set of
coefficients using methods such as those laid out in Chapter 26 of the ASHRAE Handbook of
Fundamentals. The EnergyPlus defaults are 1,0,0,0 which gives a constant volume flow of
infiltration under all conditions."
---implies that it does----

"AirflowNetwork:Distribution:Linkage objects are reported during the air distribution system on cycle, since no airflow is assumed during the system off cycle. The airflows for the AirflowNetwork:Multizone:Surface objects are weighted by the air distribution system part-load ratio. The zone loads are the sum of energy losses during both the air distribution system on and off cycle at each system time step. The energy losses when the air distribution system is on are calculated using the system â??onâ?? air flow rate multiplied by the air distribution system run time fraction. The energy losses when the air distribution system is off are obtained from the multizone airflow calculations (without forced air through the air distribution system) and these losses are multiplied by (1.0 - system run time fraction), assuming no airflows through the air distribution system when the fan is off."...

Airflow
Airflow = Airflow during ADS on cycle * ADS Part-load ratio + Airflow during ADS off cycle *
(1.0 â?" ADS Part-load ratio) where ADS = Air Distribution System
---implies that only the energy calcs are adjusted, not the flowrates themselves---------

Reading the io ref. I would say you can only expect this behaviour if you model MultizoneWithDistribution, i.e. with a AFN distrubution network. However, I'm pretty sure you can schedule all infiltration to OFF whenever your mechanical systems is on. Or checkout the hybridventilation options...there are a lot of features that can be used.

Let us know what you learn.

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "samuel.brunswick" <samuel.brunswick@...> wrote:
>
> I'm attempting to use the Airflow Network to model dynamic infiltration in my building. I'm using the "MultizoneWithoutDistribution" setting. The model has a VAV system and I'm wondering, will the Airflow Network take into account pressurization from the mechanical system when it calculates infiltration rates?
> 
> My hunch is that the AFN doesn't account for the mechanical system pressures, because I'm seeing increased infiltration rates during periods when the fan is on and I would think that the fan would give the building a positive pressure and thus the infiltration rates would decrease.
> 
> Anybody know for sure?
>




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