ASHRAE 62.1 unoccupied space minimum outdoor air supply

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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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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" Sent by: bldg-sim-bounces at lists.onebuilding.org
11/12/2009 06:09 AM

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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

Shawn.Lee at fluor.com's picture
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The OA schedule should be constructed so that the OA is off during
unoccupied hours.

Chris Jones

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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

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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 CATON, E.I.T.

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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

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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 CATON, E.I.T.

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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

Brahme, Rohini            UTRC's picture
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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

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Karen,

In this case your storage room, while listed in table 6-1, isn't
classified as "occupiable space". The definition of occupiable spaces
specifically excludes those spaced that are "only occupied occasionally
and for short periods of time". Only regularly occupied areas (occupied
spaces) require ventilation air. Your storage space is not regularly
occupied and therefore requires no ventilation air.

Seth Spangler, LEED(r) AP

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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 CATON, E.I.T.

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Ok, this makes more sense. What about corridors that have only transient
and very low occupancy rates? Can this also be classified as unoccupied?

Thanks,

--
Karen

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Note that Table 6-1 has "-" under the CFM/person values for ("General")
corridors, storage spaces, electrical rooms and similar spaces. I
consider this ASHRAE's indirect guidance on spaces you can consider
"typically unoccupied" (Pz = 0) for the purposes of determining the
minimum OA rate.

NICK CATON, E.I.T.

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Hi Karen
I had similar questions as you do. However, typically the HVAC design at unoccupied hours is to enable the air handling unit, heat or cool and recirculate the air to maintain the night setback temperature setting .
As the Storage rooms, ventilation will be provided during the occupied hours . I think the reason to provide ventilation for storage room is to avoid stale air or dust accumulation. Probably the storage rooms will have occupancy occasionally during regular work hour.

Ming Zeng, PE, LEED(r) AP

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Nick,

I agree that both a per-person and per-area rate must be calculated to
determine the total OA. I have two comments though:

1. In many jurisdictions, ASHRAE 62.1 is not suggested practice,
rather a code that must be met. For example, all buildings built in
Ontario must be designed in accordance with 62.1.

2. I don't think you could ever effectively argue that there are any
unoccupied spaces in a building, other than say mechanical shafts (I
clearly just contradicted myself...). Although a storage room is not
regularly occupied, somebody will eventually have to store something in
it and at some point retrieve it.

Regardless though, Pz in equation 6.2.2.1 applies only to the per-person
rate, which in the case of storage rooms and corridors is zero anyway,
so the zone outdoor airflow Vbz reduces to just the per-area rate x
area. Therefore, I think the per-area rate must always be supplied to
all spaces in the building during regularly occupied hours. At night,
the ventilation system in the office building can be turned off, but
during normal office hours it should supply the total OA, which includes
OA for non-regularly occupied areas. For multi-zone systems the total
OA is usually lower than the sum of all Vbz, to account for occupant
diversity and other system parameters.

I think in reality, not all spaces are directly provided with ducted
ventilation air, and transfer air is often introduced via grilles in the
doorway. Granted, one could question the effectiveness of that delivery
given the lack of exhaust fans to induce the air into the room as would
be the case in washrooms. Nonetheless, I've seen it done before.

Cheers,

Luka Matutinovic, B.A.Sc., LEED(r) AP

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In the equation in section 6.2.2.1, "Az" is defined as "zone floor area: the
net occupiable floor area of the zone"

and "occupiable space: an enclosed space intended for human activities,
excluding those spaces intended primarily for other purposes, such as
storage rooms and equipment rooms, that are only occupied occasionally and
for short periods of time."

I think it is pretty clear that storage rooms that are rarely occupied don't
need to be provided with ventilation air. Yes, if the room is storing
something that emits or smells, I would hope that the designer would use
common sense and provide ventilation air, but if a space is only occupied
for very short periods of time, and not very often, it doesn't make sense to
go to the expense of installing a ventilation system, and spend $$ on
conditioning the ventilation air.

I am still not sure about corridors. These are spaces that people commonly
go through, but actual average occupancy is very low. In addition, the
ventilation requirements for corridors is low. For a building that have
separate heating and cooling equipment in each zone, and no central air
system, does it really make sense to pipe 30 cfm of ventilation air to the
corridor? What about naturally ventilated buildings? Often the corridor
doesn't meet the requirements for natural venilation as it is in the core of
the building. Are you going to add an entire ventilation system just for
the corridor?

--
Karen

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I guess I'm more used to seeing buildings with all-air systems where the
30 cfm of extra ventilation air is a drop in the bucket. Why not duct
some extra OA from the individual zonal systems to the corridor? Or in
the case of naturally ventilated buildings, duct raw air with a small
unit heater, if needed. Compared to the rest of the building, I can't
expect the operational cost of conditioning the air to be that high
given the small air quantities, although the capital cost for such a
small system might not be economical.

At the end of the day, if this is a code issue, a building department
official should be able to provide some guidance.

Luka

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Luke and I are in full agreement, but I feel I should clarify a point
I've been trying to convey today.

I do feel you can argue a corridor for example is an "unoccupied space"
in the context of determining the OA requirements for a system. More
directly stated, Pz=0 for the equation in 6.2.2.1. We're all in
agreement that the CFM/SF values still apply to a corridor, of course.
When I say "unoccupied space" I'm not saying "zero outside air."

While I've done projects around the country, I have had limited
experience with plan reviewers demanding to review my OA calcs (those
projects have been mainly in Arizona and California, to my
recollection). I've yet to personally encounter someone actively
enforcing the 2007 version of 62.1, but I do rely on its suggested
practices whether the standard is adopted as a local code or not. I
have successfully argued (maybe a better term is "presented a civil
discussion") for designing around more up-to-date ASHRAE 62.1 standards
where an older version is officially adopted.

NICK CATON, E.I.T.

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I believe there is a minimum outdoor air requirement/sq.ft. When there
is no occupancy, we can eliminate the CFM/person and keep only CFM/sq.ft.
Can any one else give some more advice on this please?

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This is not my understanding. ASHRAE 62 defines ventilation is for
acceptable air quality. When the space is unoccupied, there is no
requirement for ventilation.

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To the best of my knowledge, the requirement for OA is ONLY when the
zone is occupied. And yes, it includes both a CFM per person component
and a CFM per Sq. Ft. component. But they ONLY APPLY when the zone is
occupied. Please note that for systems serving zones in normal
applications, this is during times when the building is occupied, like
normal business hours or whenever the HVAC system is running for
building conditioning. There are special cases, like after-hours heat
removal when, if nobody is in the building, you don't need outside air,
but that is an unusual case.

Also please note that in some places, like California, there is an
absolute minimum, even with CO2 monitors, based on square footage, but
that isn't in 62.1.

--
Robert P. Wichert, P.Eng.

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