Natural ventilation according to ASHRAE 90.1-Appendix G

Help and discussion on using the Detailed HVAC option

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Posts: 1

Natural ventilation according to ASHRAE 90.1-Appendix G

Postby Miguel031 » Thu Jul 06, 2017 5:51 pm


I would like to know how to set up a correct simulation for natural ventilation according to the modelling method of ASHRAE 90.1 appendix G (working with 2007), I understand that I have to stablish a "hybrid" HVAC system in which the mechanical ventilation (equal to the one modeled on the baseline building) has to be turned on when the natural ventilation is insufficient for thermal confort,however, as I always use detailed HVAC systems, the "mixed mode" option is present in the HVAC tab for every space and in the VAV Fan options feeding the zone in the HVAC modelling mode, I don't know where should I program this mixed mode and how to determine it's behaviour. Additional to this, what exactly are the air changes per hour that I have to set up either in the sheduled or calculated mode? are this quantities determined by the mechanical specialist in his design? or I have to enter the ACH corresponding to the minimum CFM required according to ASHRAE 62.1? Do porcentage of apertures in interior windows play a significant role in the natural ventilation?

Following are the steps that I think I must follow, please correct me if I'm wrong:

Set the natural ventilatión to scheduled
I didn't considered infiltration, the building is brand new and well constructed and I don't have a value for this.
Maintain the default ACH for each space
Model the detailed HVAC system equal to the one on the baseline
Set the VAV fans to mixed mode, select one of the zones being feeded as the control zone
check the "mixed mode" in the HVAC tab for each zone to be naturally ventilated
The cooling and natural ventilation setpoint are left as default

I'll be waiting for some comments, thanks
Posts: 9
Location: Denmark

Natural ventilation according to ASHRAE 90 1 Appendix G

Postby Timothybap » Tue Feb 26, 2019 11:38 pm

Good evening people
Ive recently started working with DB. The building I am working on graduation project should work like this:
- Natural ventilation when the inside temperature is > 20 В°C winter or 26В°C summer.
- Mechanical ventilation when the inside temperature is > 20 В°C winter or 26В°C summer and natural v. does not provide enough ACH.
- HVAC Cooling above 26В°C mechanical ventilation to provide ACH.

I found online: "The Minimum fresh air per person activity tab defines the fresh air requirements of each person in the zone and is used to generate appropriate mechanical outside air distribution rates when the mechanical ventilation Outside air definition methods on the HVAC tab is set to any of ... This data is also used to calculate natural ventilation outside air flow rates into the zone when the natural ventilation Flow type on the HVAC tab is set to 2-Min fresh air per person."

I set option #2 for both of them. So I was wondering: will my building use natural air flow, and only resort to mechanical ventilation when it cant provide that ACH value?
And how will the mechanical ventilation cooling setpoint temperature influence on that behaviour?

Thank you so much

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