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Why is the zone exhaust fan not controlled by zone temperature?

asked 2018-05-21 11:02:43 -0500

RCulham's avatar

updated 2018-05-21 11:16:09 -0500

I am modelling a simple HVAC system in OpenStudio 2.5.0. I have a zone exhaust fan that is to operate when the zone temperature exceeds the cooling setpoint temperature. I defined the outdoor ventilation design specification which is constant throughout the year in the Space Type Tab grid. I then added the zone exhaust fan in the Thermal Zones tab in the grid. I defined the availability schedule as always on and the minimum zone temperature limit schedule as the zone cooling setpoint temperature. Initially I did not have a schedule for balanced exhaust fraction. I ran the simulation and noted in the error report that the exhaust flow was not balanced. I then added a simple empty air loop in the HVAC systems tab. In this loop I added an AirLoopHVAC:OutdoorAirSystem, an uncontrolled diffuser and assigned the zone. I then went back to the Thermal Zone Tab and I set the System availability manager as coupled. Looking into DView, the yearly hourly zone temperature goes from near the outdoor ambient temperature to well over the design cooling setpoint as would be expected with an outdoor only cooling system. I then reran the simulation with the exhaust fan flow set below the outdoor ventilation design spec and the minimum zone temperature in the winter was tempered. I seems like the exhaust fan is not being controlled by the zone temperature limit schedule. Any suggestions are welcome.

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@RCulham can you post a link to your model?

Avi's avatar Avi  ( 2018-05-22 08:59:53 -0500 )edit

Good morning Avi, I apologize for the delay in figuring out how to post a link. Also, before posting the files, I had to delete unrelated zones and systems. In doing so, the model would not run in OpenStudio. I had to edit the EnergyPlus input file and remark out undeleted objects, including an AirLoopHVAC Zone Terminal List, an AirLoopHVAC zone splitter and a Setpoint Manager. I have included a link to 3 files; the osm file, its corresponding idf file (in.idf) and the edited EnergyPlus idf file. [link text](

RCulham's avatar RCulham  ( 2018-06-28 09:03:09 -0500 )edit

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answered 2018-06-02 07:44:04 -0500

RCulham's avatar

Since the majority of the building space is being cooled by unconditioned outdoor, in conjunction with zone heating equipment, solving this simulation problem for the model at hand was critical. Using this type of system is not only common in service area in multi-residential buildings, but it is quite common in large industrial factors, particularly with high internal heat gains such as a foundry. I have also used this method to cool standby by generators in modular enclosures. After many trials and tribulations, I was able to model this HVAC system, by setting the exhaust fan as a supply fan in an air loop. The amount of fan heat added to the space is negligible compared to the amount of internal heat gain in these applications.

One of the scenarios I tried was placing the fan on the relief node in the air loop. This is considered to be a zone exhaust fan by EnergyPlus and did not solve the system problem. I eventually created an air loop with an outdoor air system with the economizer set to a high lockout temperature, in my case 40C. I then put a setpoint reheat manager in the supply loop to control the fan and the zone heating system. To autosize the fan, I set the cooling temperature about 4C less than the cooling setpoint temperature. This did create a warning message.

I am not sure why, when using a zone exhaust fan, the lowest zone cutoff temperature setpoint did not work.

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Asked: 2018-05-21 11:02:43 -0500

Seen: 545 times

Last updated: Jun 02 '18