Minimum outdoor ventilation rate during unoccupied time

[Adnan86]

Ashrae 62.1 describes the minimum ventilation requirements for occupied zones/breathing zones considering the maximum expected occupancy in that zone and zone area. What would be the ventilation rate in unoccupied mode. Will it be calculated same way as for occupied mode considering occupancy to be zero or will it be zero considering system (fan) to be off in unoccupied mode. Any suggestions please...

 

[willard3]

That's your decision, you're the Engineer.

 

[Adnan86]

Thanx for the response Mr.willard3. Being an engineer my interpretition on Ashrae 62.1 is that ventilation is not required in unoccupied mode and the ventilation system must turn off. I want to know if there is any other standard/code requirement for providing ventilation during unoccupied mode.

 

[EmmanuelTop]

What's inside the room/building?

If the potential exists for contaminants to accumulate in a space when it is unoccupied, ventilation may be required even when the space is unoccupied. Standard 62 states: “When spaces are unoccupied, ventilation is not generally required unless it is needed to prevent accumulation of contaminants injurious to people, contents, or structure.” (Section 6.1.3.1)
In this case, the term “contaminants” seems to include humidity, due to the reference to a contaminant which is “injurious to contents or structure,” as well as those listed in Tables 1, 3, B-1, B-2, B-3, and B-4 of the standard. The unoccupied ventilation rate is determined by the contaminant emission rate. Indoor humidity can be measured and controlled by mechanical systems, but many of the other contaminants listed cannot be easily or reliably monitored. This leaves the designer with limited options, the most basic of which is time-of-day scheduling of ventilation during unoccupied periods.

See

https://www.trane.com/commercial/Uploads/PDF/520/ISS-APG001-EN.pdf

page 14

Does the room contain Hydrocarbon/flammable or toxic inventory?
NFPA 30 provides guidance on ventilation requirements from the chemical process plant perspective.

Dejan IVANOVIC
Process Engineer, MSChE

 

[Adnan86]

Thanks Ammanuel, Can you mention which version of Ashrae 62 you are looking at. I have Ashrae 62.1-2007 and I dont find section 6.1.3.1 there.

 

[EmmanuelTop]

62-2001. The standard is superseded but the reasoning does make a lot of sense.

Dejan IVANOVIC
Process Engineer, MSChE

 

[urgross]

I think it depends on if your control scheme includes demand control ventilation. Refer to paragraph 6.2.7.1.1, ASHRAE 62.1-2013.

 

[dbill74]

International Mechanical Code specifies a CFM/SF for select spaces. Refer to your governing mechanical code if not IMC. Ventilation rates must meet both; however ASHRAE is generally more stringent and includes spaces not in IMC.