Outdoor Air Fraction Limits
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...
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%...
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???
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
Thanks Lyle. Ya, a few tests showed me that the value in
Minimum Limit Typedoesn't matter ifMinimum Fraction of OA Schedule Nameis blank. So that step in my question was extraneous. With respect to the zone design specification, see my thoughts below.