Unbalanced Airflow

asked 2018-04-05 10:54:05 -0600

Tim Johnson
617 ●1 ●9

updated 2023-04-22 08:06:35 -0600

14250 ●86 ●24 http://bigladdersoftware.com/

I have an issue with a dedicated outdoor air system supply and exhaust flows being unbalanced in OpenStudio 2.4. I am getting a severe error in my heat recovery wheel as a result. The DOAS is set up as shown below and delivers air to dozens of zones.

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I am losing most of my air between the supply and return sides. The airflow is consistent from OA intake, through the coils and to the terminal units. The flow from the terminal units through the return from the zone stays consistent, but somewhere between the zone return node and the airloop return node, I am losing 80% of my flow. as shown below. I am showing the standard density volume flow rate in CFM of the supply node where the setpoint manager is located and the return air node immediately before the economizer.

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I do not have any exhaust fans at the zone level and can't think of any reason I would be losing air in the loop. I am hard sizing all of the terminal units with flows greater than the required ventilation rate specified in the DesignSpecification:Outdoor air object, but I don't think that would result in unbalanced flows. The larger of the two flows correlates to expected supply air flows specified for terminal units. I have tried switching loop sizing from Ventilation to Total, toggling the DCV option, and autosizing loop flows vs. hard sizing them with no luck. My zone level system is a four-pipe fan coil with outdoor air set to zero. My assumption is that isolates the fan coil from the DOAS loop. What am I missing?

My OSM is here.

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Mixing hardsize and autosize across zone/system/plant is always a bad idea. In this case you have autosized the zone's airflow rates, but hardsized the vav terminals serving them. This will certainly lead to undesirable behaviour.

TomB ( 2018-04-05 16:03:56 -0600 )edit

Interesting. I assume you are referring to the Sizing:Zone object being set to DesignDay. I set my ventilation flows in this object and it did not change the outputs. Should I be making a new DesignSpecification:OutdoorAir object for every space, then setting zone flows as my fan coil flow? That does not seem practical. My understanding is that the system (zone or airside) inputs would override and zone or system sizing objects. Is that not correct?

Tim Johnson ( 2018-04-05 17:37:44 -0600 )edit

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answered 2018-04-06 17:38:02 -0600

Tim Johnson
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I chose the nuclear option to fix this problem. I deleted the whole air loop and rebuilt it. I stuck with autosizing the zone flows and hard sizing the single duct no reheat VAV boxes and zone level fan coils and it seems to work as expected. I am not sure what sort of gremlins I managed to work into my model, but this seemed to get rid of them. @TomB, thanks for all the suggestions, I appreciate you helping me out.

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answered 2018-04-05 21:16:26 -0600

TomB

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NB;This is not a complete answer (yet), but the image below will help.

If you want to hardsize the zone airflows, change the Cooling/Heating Sesign Airflow method to Flow/Zone

image description

The Terminals should follow the zone sizes, so best to leave terminals to autosize. Check the zone airflow and terminal airflows match in the .eio file. I don't see any good reason to hardsize the minimum and maxium terminal airflows to the same value. Perhaps you are trying to get 100% outdoor air via this approach?

EnergyPlus sizing routines start at the zone. Zone sizes lead to system sizes and then on to plant sizes.

Your model looks like it is set up to try to 100% outdoor air. Control for that at the OutsideAirSystem - set it to a Proportional Minimum with a schedule that is always 1.