How to deal with apparently inconsistent AirLoop SystemNode Pressures ?

air-loop
openstudio
energyplus
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Consider the following portion of an airloop: image description

When I checked the air pressures at AirLoopExhausNode comparing them with barometric pressures of outdoor air, I got the following result:

image description

To me this is surprising, because at AirLoopExhaustNode I would have expected air pressures at least the amount of outdoor air pressure, because here air is leaving the airloop. If air pressure at AirLoopExhaustNode is less than outdoors, outdoor air must be penetrating the airloop at this node, which is in contradiction with constant mass flow rates, as also displayed in output.

Am I mistaken with this interpretation, or is there maybe an ENERGYPLUS BUG ? (I could send you the osm-file).

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OS-user-AT

asked 2016-03-09 16:01:06 -0500, updated 2016-03-10 05:27:33 -0500

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Node pressure is irrelevant unless you're incorporating the AirflowNetwork input objects in EnergyPlus. I don't believe these objects are available in OpenStudio yet.

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Lincoln

answered 2016-03-09 16:09:14 -0500

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For my application the exhaust air node pressure is important, because I am working on an EMS program to mix exhaust air with outdoor air, which is to serve as supply air for an air-water heatpump. (see motivation for this in this question on utilizing exhaust air with air-source heatpump)

OS-user-AT (2016-03-09 16:19:10 -0500) edit

I don't think you can do this in Open Studio for the reason Lincoln stated. You need the Airflow Network group in order to run node pressure simulations. You only have access to the Air Distribution group in OS, I think.

Adam Hilton (2016-03-09 16:44:00 -0500) edit

The OpenStudio model is of course only the initial model, which then is extended in the idf version for EnergyPlus. This is really about correctness of calculated physical properties, not whether you can do this in OpenStudio.

OS-user-AT (2016-03-10 00:27:51 -0500) edit

I don't see why it would be incorrect. You just need to build a loop using the airflow network objects to simulate the node pressures, which you can't do in Open Studio.

Adam Hilton (2016-03-10 09:42:37 -0500) edit

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The standard (non-airflownetwork) EnergyPlus HVAC simulation does not use node pressures to determine flow. Pressure is only used for pyschrometric calculations. Air node pressures are initialized to to the outdoor barometric pressure at the beginning of each new environment. Looking at timestep output reveals that some nodes remain fixed at this intial value (e.g. Zone Air Nodes and the return path nodes), the outdoor air nodes track the current barometric pressure from the weather data, and the air system nodes are a mix of these two pressures. A new issue has been posted to review this, but the impact on property calculations should be minor. https://github.com/NREL/EnergyPlus/is...