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Hardsizing outdoor air in HVAC: When multiple input fields are given, which one takes precedence?

asked 2016-09-14 16:15:43 -0600

gokul gravatar image

updated 2017-04-16 11:17:36 -0600

I am trying to hardsize the outdoor air requirements for my models in Openstudio 1.12.0, and I'm having problems in getting the systems to use the value of the outdoor air I have given as an input
I'm running two models: - Model 1 (baseline) has VAV systems serving on OS:AirLoopHVAC loops serving the corridor zones , ZoneHVAC:PackagedTerminalHeatPump serving apartment zones and ZoneHVACPackagedTerminalAirConditioners serving office and exercise zones. - Model 2(Proposed) has ZoneHVACPackagedTerminalAirConditioners serving apartments, Make up air units serving corridor zones, and furnaces serving the eercise and office zones.

I want to specify the outdoor airflow rate for all these systems. With regard to this, for the Make up air units, as per the answer provided for [] , the outdoor air provided in "OS:DesignSpecificationOutdoorAir" for the spacetypes of the zones overrides the the value entered in the MinimumOutdoorAirFlowRate in OS:ControllerOutdoorAir and DesignOutdoorAirFlowRate in OS:Sizing:System.

However, OS:DesignSpecificationOutdoorAir does not override values of outdoor air I enter in "outdoor-air-flow-rate-when-no-cooling-or-heating-is-needed" under OS:PackagedTerminalAirConditioner or in ZoneHVAC:PackagedTerminalHeatPump. The values entered in DesignOutdoorAirFlowRate in OS:AirLoopHVAC and MinimumOutdoorAirFlowRate in OS:AirLoopHVAC:OutdoorAirSystem also override the DesignSpecificationOutdoorAir value.

Is this always the case? Does anyone know of a clear heirarchy of which entry overrides the other, specifically for each of these systems? (PTAC,PTHP, VAV and Makup air units). Why is this level of precedence different only for a make up air unit when compared to other systems? I am not able to find any documentation which says one clearly overrides the other, and the modelling suggests that this may be system specific. Can someone clarify?

Also, when I check the energyplus output files for a system, say, ZoneHVAC:PackagedTerminalHeatPump, there are two values of outside air: the User-Specified Outdoor Air Flow Rate and the Design Size Outdoor Air Flow Rate. How is this design size outdoor airflow rate value calculated? Does it use the inputs from DesignSpecificationOutdoorAir?

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answered 2016-09-15 08:55:59 -0600


I can give you a partial answer, when using AHUs, ie your VAV system in EnergyPlus/OpenStudio, the main controller for OA is: OS:ControllerOutdoorAir, in here you can control the amount and schedule for your outdoor air. As far as I know this overrides everything. So in the minimum OA entry field, put your minimum outdoor air value. Then, in the minimum limit type field put "fixed minimum", finally in the minimum outdoor air schedule field put in a fractional schedule to describe how you want your outdoor air to come into your unit. Ie, if you have 24hour fresh air it would look like this "outdoor air schedule, Fraction, through: 12/31. For: AllDays, Until: 24:00, 1". That will control your OA through your AHUs.

I believe the next level of hierarchy for OA control is in the zone equipment, the numbers you put in here will control how much OA goes into your zones. The design specification outdoor air does not override any of these values. I think this is more for sizing your system than the running of your system.

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While valid for constant volume systems the outdoor air controller can not set the loop flow rate for variable volume systems as the volume flow is set by the terminal units. In other words during a time of minimal heating/cooling the total loop flow has the potential to drop below the minimum outdoor air as specified in the outdoor air controller. This also applies to outdoor air fraction. If you specify 0.5 OA but the loop flow drops below drops to a point where 0.5 OA is below the specified minimum OA flow, the actual fraction will drift up to 1.

Adam Hilton gravatar imageAdam Hilton ( 2016-09-15 10:26:36 -0600 )edit

A solution is to apply DesignSpecification:OutdoorAir to the vav terminals and this will ensure that the minimum OA is always met. In OS this is just a matter of changing the 'Control for Outdoor Air' prompt on the terminals from 'No' to 'Yes'.

Adam Hilton gravatar imageAdam Hilton ( 2016-09-15 10:27:53 -0600 )edit

@annie-marston, @adhilton thank you for the answers. I did not know about the Control for Outdoor Air prompt! The other question is the make up air units. I am modelling it as a packaged single zone unit serving multiple zones. Regardless of whether the Control for Outdoor Air is 'yes' or 'no', the controller takes in the value of DesignSpecification:OutdoorAir. Is there a way for me to make the controller take the value in the OS:ControllerOutdoorAir?

gokul gravatar imagegokul ( 2016-09-15 11:22:05 -0600 )edit

And the other terminal units, PTAC and PTHP, is there a way to apply DesignSpecification:OutdoorAir to them as well, when both fields are populated? I guess what I am looking for, is a single way of controlling OA in all these units. If there's a way to make only the DesignSpecification:OutdoorAir, or the respective tabs for OA in OS:ControllerOutdoorAir/ZoneHVAC:PackagedTerminalHeatPump control the OA uniformly, then I can just change that value, and not play with different entry tabs for different systems.

gokul gravatar imagegokul ( 2016-09-15 11:28:53 -0600 )edit

Again the way i control the OA in zone package objects is to hard size the value in the object, so for instance in the PTHP object the is cooling outdoor air flow rate, heating outdoor air flow rate and no load outdoor air flow rate would have hard sized values. At the bottom of this object is "design specification ZoneHVAC sizing object name" this might be helpful for you. check how to use it in the I/O reference guide.

Annie Marston gravatar imageAnnie Marston ( 2016-09-15 12:24:57 -0600 )edit

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Asked: 2016-09-14 16:15:02 -0600

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Last updated: Sep 15 '16