3.7.4.2 Air Side Economizers - OA Schedule Method

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In Title 24 analysis, we have OASchMthd, which can be set to Constant, 
FollowHVACAvailability and Scheduled.  For T24 compliance, this is prescribed 
to be FollowHVACAvailability, with an offset of 1 hr, which is based on the 
prescribed schedules that provide a 1 hr lag between HVAC availability and 
occupancy.  

For 90.1, are we allowing user schedules for everything?  If so, we can't 
prescribe this input.  I assume we just want to allow user selection of this 
input and use the same input for proposed and baseline?  

Original issue reported on code.google.com by rhedr...@archenergy.com on 29 Sep 2014 at 4:35

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I assume scheduled means a user enters the hours OA is available? What if those 
hours don't correspond to the HVAC system operating. What does constant mean?

Original comment by bikerose...@gmail.com on 29 Sep 2014 at 4:47

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Is there anything in the PRM or ACM that describes this?

Original comment by bikerose...@gmail.com on 29 Sep 2014 at 4:51

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This is a schedule that determines when minimum OA must be provided.  Constant 
means that ventilation is always provided.  

Improper user schedules is a significant concern.  I'm not sure whether this 
schedule will override the HVAC availability schedule and force the system on, 
but I don't think so.  If the user sets the HVAC availability schedule to match 
occupancy, and puts in an offset from HVAC availability that is non-zero, then 
what?  

Original comment by rhedr...@archenergy.com on 29 Sep 2014 at 4:53

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Nothing that I see in either.  

Original comment by rhedr...@archenergy.com on 29 Sep 2014 at 4:55

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Isn't HVAC availability typically 24 hours a day to allow night cycling, with 
fan on/off representing time when fans are forced to run? If the OA follows 
HVAC availability does that mean there is always OA even when cycling on during 
unoccupied hours?

Original comment by bikerose...@gmail.com on 29 Sep 2014 at 5:08

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No, HVAC availability is set to 1 during the normal workday and to 0 at night 
and weekends.  Night cycle controls override the availability schedule and 
force the system on as needed by the night cycle controller.  

So, in Title 24, on a weekday for an office building, HVAC availability is 1 
("On") from 6 a.m. to 1 a.m..  Occupancy is non-zero from 7 a.m. to 1 a.m..  
Min OA is provided 7 a.m. to 1 a.m..  Night Cycle controls can turn on the 
system between 1 and 6, but OA will be zero.    

Original comment by rhedr...@archenergy.com on 29 Sep 2014 at 5:16

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Roger's descriptions are correct, except I thought I'd clarify that the NACM 
prescribed 'Office' schedule is on from 5am to 12am, and occupancy is first 
assumed to occur at 6am.  Here is some additional input I have on this issue:

The different OA scheduling methods are not defined in the NACM. The various 
methods were developed around what E+ readily supports, with the exception of 
'FollowAvailability'.  This method was intended to mitigate the user having to 
create a separate schedule for the OACtrl object, which in general, would track 
the system availability schedule.  Instead of the user defining the schedule, 
the OpenStudio translator creates it based on the schedule assigned to 
System:AvailabilityScheduleReference and the OutsideAirControl:StartUpDelay 
property. It is expected this method saves the user time, but also mitigates 
user-error that may stem from not coordinating these schedules properly.

I can say from experience this is a critical assumption, and therefore should 
be addressed in the NACM/PRM. Previous to eQUEST v3.63, the Wizard did not 
create schedules to coordinate the system availability and min OA schedules.  
This resulted in OA being introduced by the system during night-cycling if 
modelers did not pay attention to this property. Since the building envelope of 
the baseline design is often worse than the proposed, in heating-dominated 
climates the baseline will typically night-cycle more hours than the proposed. 
Therefore, not defining a min OA schedule artificially improves the performance 
of the proposed design. As a result, the eQ wizard was modified to facilitate 
properly creating min OA schedules that are coordinated with the night-cycle 
control option.

Original comment by da...@360-analytics.com on 29 Sep 2014 at 5:45

• Changed state: Discussion

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Right, but I can see two occasions where the design may not follow that default 
schedule (there are probably more). In CZs 1-3, automatic OA dampers are not 
required. This is likely only for systems without economizers (small systems 
and those in CZ 1A and 1B). So we can't force dampers to be shut during night 
cycle and morning warmup. Also, some designs will use open OA dampers for 
economizing or night flush during unoccupied periods. So I think for 90.1 we 
have to let the user create the OA schedule. 

Original comment by bikerose...@gmail.com on 29 Sep 2014 at 6:52

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I have checked in modifications to the economizer rules that allow user 
selection for the proposed and baseline.  The default is 
"FollowHVACAvailability" with a delay of 0. 

Original comment by rhedr...@archenergy.com on 1 Oct 2014 at 9:53