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Outdoor Air Fraction Limits

asked 2016-05-18 10:22:15 -0500

Adam Hilton's avatar

updated 2016-05-18 13:24:03 -0500

Does anyone know how to use the Fractional Outdoor Air Schedules???

Reading this doesn't help as I don't get results consistent to what it says.

If I set the Minimum Limit Type to ProportionalMinimum and then assign a 10 percent fractional schedule to Minimum AND Maximum Fraction of Outdoor Air Schedules I would expect to end up with the minimum set as 10 percent of the design air flow and scale down with the mixed air mass flow rate. Instead, I get this...

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Doesn't seem very proportional to me...It's a VAV system so I understand the change in fraction. On top of not being proportional, it's not even close to a max/min of 10% and at 100% of design flow OA is at 37%.

If I convert the system to constant volume by setting the minimum flow fraction at the terminals to 1 and the minimum fan power fraction to 1 it's constant but still not 10%...

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The saga continues...

Still trying to find a way to fractionally reduce the outdoor air I've moved to the DesignSpecification:OutdoorAir object. I attempted to apply a 10% fractional schedule to the Outdoor Air Flow Rate Fraction Schedule Name field. This had no effect on the results. When I physically altered the values in Outdoor Air Flow per Person and Outdoor Air Flow per Floor Area to 10% of their initial value I finally got the results I wanted/expected. I still need to be able to schedule the OA fraction though...Has no one else ever had these problems???

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A few thoughts to consider... - Proportional minimum is % of the actual airflow at that timestep, not a % of the design airflow - Make sure you are using the 'Minimum Fraction of OA Schedule Name' field if the 'Minimum Limit Type' is set to 'ProportionalMinimum' - The system OA% may stay above the minimum 10% if the sum of your zone design specification objects requires a greater OA volume. - When looking at system OA, I would also plot economizer operation to see if/when that is impacting system OA

Lyle K's avatar Lyle K  ( 2016-05-18 14:37:10 -0500 )edit

Thanks Lyle. Ya, a few tests showed me that the value in Minimum Limit Type doesn't matter if Minimum Fraction of OA Schedule Name is blank. So that step in my question was extraneous. With respect to the zone design specification, see my thoughts below.

Adam Hilton's avatar Adam Hilton  ( 2016-05-18 15:10:26 -0500 )edit

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answered 2016-05-18 15:07:13 -0500

Adam Hilton's avatar

I finally got the results I wanted, but I'd still be interested to hear an explanation as to why it has to be done this way.

The trick was to zero out the DesignSpecifcation:OutdoorAir objects. I applied the Minimum Fraction of Outdoor Air Schedule Name to the Outdoor Air Controller as I had done before. The graph ended up more beautiful than I could ever imagine....

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It seems like the values in DesignSpecification:OutdoorAir just take priority regardless of the bounds entered in the max/min outdoor air flow rates. I'm not sure if this is intended functionality or not. This is best exemplified by entering a minimum outdoor air flow rate above and below the zone sum of the design outdoor air.

Here I entered a minimum of 100 cfm and the outdoor air flow stays constant at 656 cfm (the zone sum) as expected.

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Now the only thing I changed here was the minimum to 700 cfm which is above the specification of outdoor air. There is still a clear floor at 656 cfm but it also occasionally seems like it's trying to rise to the specified minimum.

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This seems to be a flawed process here. In the first case I'm not able to find an error telling me that the minimum was below the calculated zone sum (I feel like this is important) and in the second case why would the minimum not be followed if it's above the calculated zone sum?

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Asked: 2016-05-18 10:22:15 -0500

Seen: 760 times

Last updated: May 18 '16