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VAV Air Terminal w/ Reheat - Zero flow when HVAC available

asked 2020-01-14 07:28:14 -0600

Going2theGame gravatar image

updated 2020-01-15 11:24:44 -0600

Hi all.

I have an E+ model of an office building with VAV AHUs with reheat at the zone level VAV boxes. AirTerminal:SingleDuct:VAV:Reheat with Zone Minimum Air Flow Input set at 0.30. HVAC Availability Schedule kicks on at 6am. Heating setpoint goes from 65 (before 7am) to 70F (at 7am).

However, at this point (6 to 7am) during each day in the heating season (most of my unmet hours are in heating mode), this damper goes to ZERO airflow and therefore zero heating energy to the space. Instead of the zone temperature rising to meet the now-increased setpoint, it falls for just one hour at 7am, and then increases up to setpoint. The space with the largest amount of unmet hours is "143 DOCK" so I have used this as my test space.

For troubleshooting, I have adjusted just about every setting and schedule that applies to this system - gradually raise setpoint rather than stepwise, shift hvac availability to 4am to ensure the system is "ready" for the onset of people/ventilation, gradually raise outdoor air fraction schedule to ensure the issue isn't because of an inrush of cold outdoor air, and even double checked to make sure the diffuser's Minimum Air Flow Fraction Schedule NEVER goes to zero.

Any explanation for this behavior? Thank you!

Files and annotated image showing the issue located here.

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answered 2020-01-14 16:26:08 -0600

I can't open your model because of numerous geometry errors. I recommend cleaning those up and updating the model to the most recent version of OpenStudio.

The issue your are facing is likely do to settings in the SingleDuct:VAV:Reheat object. In your model, "Fixed Minimum Air Flow Rate" is hardsized to zero. Reading the documentation for Fixed Minimum Air Flow Rate:

"If the Zone Minimum Air Flow Method field is set to Scheduled (and the field Maximum Hot Water or Steam Flow Rate is set to autosize), then this field is optional and can be used to separately control the air flow rate used for sizing normal-action reheat coils."

I suggest autosizing the fixed minimum air flow rate and determining whether your building uses a single max ("normal") or dual max ('reverse") control logic for the Damper Heating Action field

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Asked: 2020-01-14 07:25:40 -0600

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Last updated: Jan 14