[Bldg-sim] ASHRAE 62.1 unoccupied space minimum outdoorairsupply

Nick Caton ncaton at smithboucher.com
Thu Nov 12 11:07:57 PST 2009


I may be wrong, but it appears a number of people aren't examining the
equation in 6.2.2.1 as I'm suggesting.  If you'll check that out I'm
certain you'll find this is a much simpler question/answer than it's
being made out to be.

 

Both sets of figures provided in Table 6-1 (OA/person and OA/SF) are
used to determine the design minimum OA rate, using 6.2.2.1's equation.
Occupancy has no effect on the OA/SF requirement, and likewise zonal
area has no effect on the OA/person requirement.  Additionally, the
occupancy figure used is the maximum occupancy of that zone.  Quoting
6.2.2.1:  "Pz = zone population: the largest number of people expected
to occupy the zone during typical usage..."  It is not the hourly
percentage occupancy and is not zero unless the space is never normally
occupied (as with a storage room like Karen suggested).

 

The design minimum OA rate is only applicable when the ventilation
system is operating.  Unless I'm missing something major, the
ventilation system is not required by ASHRAE 62.1 to operate 24 hours
per day  (again, refer to Section 5.4, which I've copied entirely
below).

 

 

I'll try to break down the current questions/statements on the table:

 

The reason I had the question was because I think OA/sqft technically
should not be dependent on occupancy...

That's a fair judgement a designer is allowed to make.  The 62.1
Standard is just that - not a code, but a set of suggestions provided by
some of those best qualified to provide them.  To play the devil's
advocate, the counter-argument is that Occupancy cannot be ignored by an
energy-conscious ventilation system designer.

 

...If that was the case, it would suffice to just increase the OA/sqft
and have only one number to take care of.

Someone doing that would be ignoring the equation Table 6-1 was built
around (re: 6.2.2.1).  I'd agree that would be a misinterpretation of
the standard.

 

What I am not sure of is whether OA/sqft is used only during the typical
occupied hours irrespective of whether it is actually occupied or not or
is it used throughtout the day/weekend/vacation etc.

OA/SF is always used alongside OA/person to determine the "breathing
zone outdoor airflow." (Which I've been calling the design minimum OA
rate).  The question of when these ventilation rates apply is tied to
when the zone is occupied.  To restate my position (Again Ref: Section
5.4):  ASHRAE 62.1 does not require that ventilation systems operate
during unoccupied times.

 

What do the hvac designers specify for the control of OA?

Not sure whether a concise answer can be provided to this broad
question... OA ventilation can be controlled by any combination of
scheduling/occupancy sensing/timer control/temperature/humidity/VOC,
CO2, CO or other air quality sensors... all depending on the project
budget, climate, and type of system being designed.  If this question
had a simple answer, then we wouldn't need HVAC designers =).

 

what happens when you have a space that is almost NEVER occupied, but
has a cfm/sf requirement - like a storage room with a requirement of
0.12 cfm/sf. 

See my answer above.

 

So do you need this ventilation rate only when the building is occupied,
only when the space is occupied, or all the time?

Strictly speaking, only when the zone (space) is occupied.  If you have
a one-zone system that doesn't distinguish between occupancy rates, then
whenever the building is occupied.

 

If you only need the ventilation when the space is occupied, and it is
almost never occupied, why install the ventilation in the first place?

This is a broad-scope question, so I'll add another quote under Section
1, entitled "PURPOSE:"

"1.1        The purpose of this standard is to specify minimum
ventilation rates and other measures intended to provide indoor air
quality that is acceptable to human occupants ant that minimizes adverse
health effects."

If a space is never occupied, then ASHRAE doesn't care about air
quality.  If a space is "almost never occupied," then they do.  To
continue your example, if you have a storage warehouse with no
ventilation installed, and it's almost never occupied, someone might
eventually store something in that space that without ventilation would
eventually cause a health hazard to the next occupant entering the
space.  In my college years, this was a commonly observed effect caused
on a small scale by my roommates forgetting food in the microwave =).

 

~Nick

 

~Nick

 

 

NICK CATON, E.I.T.

PROJECT ENGINEER

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

Check out our new web-site @ www.smithboucher.com 

 

From: Karen Walkerman [mailto:kwalkerman at gmail.com] 
Sent: Thursday, November 12, 2009 12:28 PM
To: Brahme, Rohini UTRC
Cc: Nick Caton; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] ASHRAE 62.1 unoccupied space minimum
outdoorairsupply

 

Ok, so here's a question - what happens when you have a space that is
almost NEVER occupied, but has a cfm/sf requirement - like a storage
room with a requirement of 0.12 cfm/sf.  This is a pretty high outdoor
airflow, especially if you're just storing old files.

So do you need this ventilation rate only when the building is occupied,
only when the space is occupied, or all the time?

If you only need the ventilation when the space is occupied, and it is
almost never occupied, why install the ventilation in the first place?


--
Karen



On Thu, Nov 12, 2009 at 1:11 PM, Brahme, Rohini UTRC <
BrahmeR at utrc.utc.com> wrote:

Thanks Nick. The reason I had the question was because I think OA/sqft
technically should not be dependent on occupancy. If that was the case,
it would suffice to just increase the OA/sqft and have only one number
to take care of. What I am not sure of is whether OA/sqft is used only
during the typical occupied hours irrespective of whether it is actually
occupied or not or is it used throughtout the day/weekend/vacation etc.
What do the hvac designers specify for the control of OA?

-rohini

 

From: Nick Caton [mailto:ncaton at smithboucher.com] 
Sent: Thursday, November 12, 2009 12:22 PM
To: Brahme, Rohini UTRC; bldg-sim at lists.onebuilding.org
Subject: RE: [Bldg-sim] ASHRAE 62.1 unoccupied space minimum
outdoorairsupply

 

I havn't read the standard in detail, but isn't there a requirement of
some OA/sqft in addition to OA/person. 

 

You are correct.  Refer to the equation of Section 6.2.2.1 as I
referenced below.

 

Let's assume that an office is assumed normally occupied from 8 am to 6
pm -  Then during unoccupied hours (6.01 pm to 7.59 am) can the OA be
zero or should it be OA per sqft?

 

It sounds like you're asking the same question I just answered, so I may
not be understanding completely...  By Section 5.4 I do not believe an
unoccupied space is required to have any OA supplied.  The equation in
6.2.2.1 does include a component "per square foot," but that equation is
for determining the design minimum OA rate.

 

More tricky, let's say we have a occupancy sensor and know that the
office is unoccupied from 12.00 pm to 3 pm (during the regular office
times). What is the OA at this time - 0 or OA/sqft?

 

Applying the same logic and the same section, 5.4, if there are controls
that automatically determine a space is occupied and turn the associated
system on to provide the required OA, then there's no reason those
systems aren't allowed to turn off while unoccupied.  Short answer for
your example:  0.

 

I'll point out, I'm merely an E.I.T. (with my head in the books) sharing
my interpretations, so it may be worth holding out for a second opinion
=).

 

~Nick

 



 

NICK CATON, E.I.T.

PROJECT ENGINEER

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

Check out our new web-site @ www.smithboucher.com
<http://www.smithboucher.com>  

 

From: bldg-sim-bounces at lists.onebuilding.org [mailto:
bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Brahme, Rohini UTRC
Sent: Thursday, November 12, 2009 11:14 AM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] ASHRAE 62.1 unoccupied space minimum
outdoorairsupply

 

Since we are on this topic, I had a question on definition of occupancy.

I havn't read the standard in detail, but isn't there a requirement of
some OA/sqft in addition to OA/person. 

Let's assume that an office is assumed normally occupied from 8 am to 6
pm -  Then during unoccupied hours (6.01 pm to 7.59 am) can the OA be
zero or should it be OA per sqft?

More tricky, let's say we have a occupancy sensor and know that the
office is unoccupied from 12.00 pm to 3 pm (during the regular office
times). What is the OA at this time - 0 or OA/sqft?

Thanks

-rohini

 

From: bldg-sim-bounces at lists.onebuilding.org [mailto:
bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Thursday, November 12, 2009 11:36 AM
To: Guin, Michael; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] ASHRAE 62.1 unoccupied space minimum
outdoorairsupply

 

No.

 

Refer to Section 5.4: 

"Mechanical ventilation systems shall include controls, manual or
automatic, that enable the fan system to operate whenever the spaces
served are occupied.  The system shall be designed to maintain the
minimum outdoor airflow as required by Section 6 under any load
condition."  

 

In summary, you must provided the minimum ventilation rate as prescribed
in Chapter 6 only when (A) the space is occupied and (B) there's a
thermal load condition (or manual control - see Section 5.4) triggering
the operation of the ventilation system.

 

Your original question is confusing to at least a few of us because of
stating an office ceases to be a "zone" at night...  It's a terminology
thing, but 62.1 and other standards will become more digestible to you
when you recognize that's not the case.  The clarification we're making
here is that where a zone occupancy (i.e. "office") is decided and a
constant over time, something like "occupancy rate" can be variable.

 

I'd like to also point out, based on your first inquiry, that the
equation at 6.2.2.1 uses maximum zone occupancy to establish the minimum
design OA rate, not the hourly occupancy rate (which may be zero).

 

~Nick

 

 



 

NICK CATON, E.I.T.

PROJECT ENGINEER

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617

Check out our new web-site @ www.smithboucher.com
<http://www.smithboucher.com>  

 

From: bldg-sim-bounces at lists.onebuilding.org [mailto:
bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Guin, Michael
Sent: Thursday, November 12, 2009 8:44 AM
To: Shawn.Lee at fluor.com
Cc: bldg-sim-bounces at lists.onebuilding.org; 
bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] ASHRAE 62.1 unoccupied space minimum outdoor
airsupply

 

is there any requirement to provide minimum outdoor fresh air wihin the
office space when it is unoccupied according to ASHRAE 62.1

 

Do you think it it clear enough? 

Regards

Michael

________________________________

From: Shawn.Lee at fluor.com [mailto:Shawn.Lee at fluor.com] 
Sent: 12 November 2009 13:58
To: Guin, Michael
Cc: bldg-sim at lists.onebuilding.org; 
bldg-sim-bounces at lists.onebuilding.org
Subject: Re: [Bldg-sim] ASHRAE 62.1 unoccupied space minimum outdoor air
supply


Michael, 

Technically answering your question may not help you much.  You need to
make the question clearer and more meaningful.  Tell people what you are
trying to do, what is in your mind, and what you are trying to
accomplish.  Then this group may help you more on the target. 

Thanks. 

Shawn Lee, PhD, PE 

	"Guin, Michael" <Michael.Guin at WSPGroup.com> 
Sent by:  bldg-sim-bounces at lists.onebuilding.org 
11/12/2009 06:09 AM 

To

<bldg-sim at lists.onebuilding.org> 

cc

	
Subject

[Bldg-sim] ASHRAE 62.1 unoccupied space minimum outdoor air supply 

 

		

 

Dear all, 

ASHARE 62.1 2007 deifned zone as an occupied space. Table 6-1 provides
the calculation for different spaces based on occupancy and floor area
of the occupied space. 

If we consider an unoccupied office space at night (i.e. not strictly a
"zone  anymore" ); is there any requirement to provide minimum fresh air
wihin the office space. 

Thanks in advance for your help 
Michael   

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