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How do I model natural ventilation for LEED in EnergyPlus?

asked 2014-09-29 10:32:20 -0600

updated 2015-07-10 06:42:58 -0600

I have come across a couple projects located in South America where they are looking to model natural ventilation as the only HVAC system for LEED. This is addressed in Appendix D of the Advanced Energy Modeling for LEED v2.1 document for those who have purchased it but I want to double check my interpretation and see if anyone has used this method.

My understanding in this situation is that the Proposed HVAC system should be the same as the Baseline HVAC system with the airflows and capacities being identical to account for when the natural ventilation system can't meet setpoint, whether the building is designed for this not. Since the natural ventilation is bringing in the required outside air, there is no reason for the Proposed HVAC system to be running if there is no heating/cooling demand during occupied hours. So in this case, can you change the fan schedule to have the system only run when heating/cooling is required, not continuously during occupied hours as long as your natural ventilation is available? I'm assuming you can also set your Proposed HVAC outdoor air minimum to 0 since your are bringing in the outside air via natural ventilation, making it a return air only unit essentially.

Making this more complicated, if your natural ventilation system has a min or max outside air when it becomes unavailable, your HVAC system needs to become available with your minimum outdoor airflow to meet setpoint. It seems that this could possibly be done through the EMS or be done post process.

Does anyone have experience with this process or be able to provide any insight on my understanding of it?

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Correction on the above, the HVAC system does need to have the set min outside air since when the space setpoint is not met, the natural ventilation should be turned off and the HVAC system turned on and supplying the min outside air. It seems to do this whole process of turning on and off the natural ventilation and HVAC system based on whether the setpoint can be met by the natural ventilation system is a huge headache. You need to find the hours that are unmet and turn off the natural ventilation and turn on the HVAC system for those specific hours, a scheduling nightmare. Thoughts?

Matt Larson gravatar image Matt Larson  ( 2014-09-30 12:52:52 -0600 )edit

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answered 2014-10-15 17:35:51 -0600

I would add that you want to be careful how you model your baseline system, you need to have the same outdoor air in both proposed design and baseline case. In the past when we have modelled natural ventilation for LEED we have asked the mechanical designer for the minimum outdoor air that will be in the space from natural ventilation, this is then continuously put into the space through the simple natural ventilation object in energyplus. The conditioning of the space is then done using either what is designed, or if nothing is designed the baseline system with no outdoor air through this system. In the baseline case, the minimum amount of outdoor air is brought through the baseline system and is conditioned to meet the space requirements. In this case it is important to show that your unmet load hours are low in both cases and that the zones are being conditioned correctly.

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answered 2014-09-29 22:20:34 -0600

hongtz gravatar image

Basically you need to model hybrid ventilation system, when natural ventilation is adequate to meet cooling loads, the mechanical cooling system is off; otherwise the mechanical cooling system (providing minimum ventilation air also) will be on and the natural ventilation is off.

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Shoul I model these hybrid ventilation you described at both the baseline and proposed models? or only at proposed model?

FelipeBabugia gravatar image FelipeBabugia  ( 2017-12-07 10:23:58 -0600 )edit

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Asked: 2014-09-29 10:32:20 -0600

Seen: 1,410 times

Last updated: Oct 15 '14