Revit MEP: Load Calculations - Ventilation Rates

[Anonymous]

Hi all,

I don't mena to pile on here (I have read many posts/articals explaining Revit's load calculation short-comings), but I have a question about how Revit caluclates a zone's ventilation rate.

In a zone's instant parameters I have set the outdoor air info. to be 5 CFM per person and 0.06 CFM per sq.ft. The given zone contains 50 people and is 1,260 sq.ft., therefore I would expect a ventilation rate of roughly 325 CFM. However in the load calculation report I get a peak ventilation rate of 246 CFM.

I have changed the occupancy schedule to be 100% from 7am to 10pm but this have not effected the peak ventilation rate. Can anyone give some insight into how Revit calculates the ventilation rates?

Thanks in advance,

SMS

[Anonymous]

A more accurate way to ensure that correct ventilation rates is modeled is to calculate the required outdoor air per appropriate ventilaton rates. You can go through the book called Mastering autodesk revit MEP 2015. http://www.ventis.com.au/accessories/

[jfriggstad]

I realize this is an old post, but I'm running into the same issue.  The standard method as per ASHRAE 62.1 is to add the "per person" rate to the "per area" rate, but Revit seems to pick the highest value of the two.

A workaround is to equate the ASHRAE method to the "Revit method":

ASHRAE Office Space:  5 cfm/person, 0.06 cfm/sqft, 5 people/1000 sqft

Do the math and use either 0.085cfm/sqft or 17cfm/person.

Let me know if I'm missing something.  I'm hoping to switch over to using Revit's energy analysis tools (from Trace 700) but am not too confident in it's methods. 

[JustinStirling26]

Any good explanation for this methodology would be greatly appreciated!

[iainsavage]

follow link, scroll down to Outdoor Air Method and see options.

https://knowledge.autodesk.com/support/revit-products/learn-explore/caas/CloudHelp/cloudhelp/2018/EN...

[JustinStirling26]

Are the "Air Changes per Hour" for air exchanges of outdoor air only or for total air changes of the space?

[iainsavage]

As far as I’m aware that value is only for outdoor air. 
Other air would be included in Specified Supply Airflow.

You could test this though by putting in some values and working backwards with pen & paper to verify your results.

PS: I may have given the wrong link earlier, I think that was for zones. This link is for spaces (but there is very little difference)

https://knowledge.autodesk.com/support/revit-products/learn-explore/caas/CloudHelp/cloudhelp/2018/EN...