Demand Controlled Ventilation

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I am working on a school building model where i have matched the utility
bills for elec and gas consumption. I am using the parametric runs to
perform ECMs on the building, two of which will be demand controlled
ventilation on different units. Currently i have not gotten the DCV to work
and show energy savings, so i am wondering if anyone has some pointers and
things to look for concerning DCV while I am trouble shooting it myself?

--
Rob Hudson

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If you have an economizer, it might be getting priority over the DCV. That?s where I?d start. If there?s no economizer, then I?d create an hourly report to compare zone occupancy, hourly OA flow rates, and min OA flow specified.

Thanks,

DAKOTA KELLEY

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Hi All,

I have an existing building with a multi-zone air handling system with
chilled water and hot water coils. The system consists of the air handling
units, a centrifugal chiller, cooling tower, and gas fired boiler. I am
trying to introduce a water-side economizer into the system. In the detailed
mode, I notice that a water-side economizer is not allowed with a multi-zone
system. The spreadsheet input simply has an inaccessible "n/a" shown for the
economizer and does not allow access.

Am I missing something?

Thanks for your help.

Jeffrey G Ross-Bain, PE, LEED, BEMP

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Jeff,

if you have chilled water coils through your unit MZ unit and the chilled
water is coming from a chiller, then the water size economizing function
comes from the cooling tower that is connected to the chiller which supplies
the chilled water coils. If you are talking about economizer --in general
terms, then maybe you are trying to air-economize which should be able to be
input in the air-side tab-cooling for your MZ AHU.

Look at the plant & cooling tower for your water econo inputs.

Pasha

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Just set up a new chiller in your primary chilled water loop, and set it
to type "waterside economizer". Then create the associated pumps /
power consumption, and eQuest will automatically use this chiller (in
lieu of any powered chillers) whenever there is a possibility for
waterside economizer based on ambient conditions and required
temperature of chilled water loop to meet loads.

James Hansen, PE, LEED AP

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Jeff,

Note however that the waterside economizer chiller will be used only as a 'all or nothing' where it is used only when it can meet all of the load, not partial loads hence possibly under estimating the energy savings. Also it is important to have only one cooling tower assigned when using water side economizer chiller.

I have pasted the help section below that describes this -

Chiller Water-Economizer

The program assumes a water-side economizer is used in an "all or nothing" mode. If the water-side economizer can handle the entire cooling load, then it will be used. Otherwise, the loop will use its other chillers. A water-side economizer cannot precool the return flow prior to entering the chillers.

The "all or nothing" mode precludes the use of an EQUIP-CTRL sequence to coordinate the operation of an economizer chiller with other chillers. If you list a water-side economizer chiller in an EQUIP-CTRL sequence, it will be ignored.

There must be a one-to-one correspondence between a CHW loop, a water-side economizer, and a CW loop. A CHW loop may be supplied by only one water-side economizer, and a CW loop may supply only one water-side economizer. In other words, a plant may have any number of chilled water loops having water-side economizers, but those economizers cannot draw from the same CW loop. The program will generate an error if this condition is not met.

If a CW loop supplies a water-side economizer, it may also supply chillers serving the same CHW loop as the economizer. However, it cannot supply chillers serving other CHW loops. The program will generate an error if this condition is not met.

A CW loop supplying a water-side economizer should have only one cooling tower. More than one cooling tower is allowed, however only the first tower defined will be checked to see if sufficient free cooling capacity is available. Once free cooling is allocated to a CW loop, the program may use any combination of towers to satisfy it. The program may underpredict the water-side economizer savings if more than one cooling tower serves a given CW loop. To minimize the underprediction, define the largest tower first.

If there is any water-cooled packaged equipment in the SYSTEM command, this equipment will be ignored when determining whether the tower has sufficient free cooling capacity. This may cause the program to overpredict water-side economizer savings.

For maximum effectiveness, the CHW loop should have COOL-SETPT-CTRL = LOAD-RESET. This allows the CHW temperature setpoint to rise during periods of low loads, maximizing the effectiveness of the water-cooled economizer.

Amarpreet Sethi, M.Sc., LEED? AP

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I am having some trouble with setting up DCV in my model. if i select DCV
in Zone Sensors for the Min OA Control Method and a min outside Air Ratio,
do i need any Zone level OA inputs?
Any help or insight as to what inputs are necessary for DCV would be great!

--
Rob Hudson

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Rob,

I recently modeled DCV successfully for the first time. (I showed
savings anyway.) You need to enter zone-level OA as OA-FLOW/PER and/or
OA-FLOW/AREA. The OA flow/per (person) is what will get you the savings,
based on your occupancy schedule(s). I also created a MIN-AIR-SCH,
coinciding with the fan schedule, set to zero when fan is off and -999
when fan is on.

Regards,

William Bishop, EIT, BEMP, LEED(r) AP

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How can i tell if the system is using a lower OA CFM during the less
occupied times?

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Rob,

You need to set up an hourly report. See attached.

Bill

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Hello all,

I also have a similar question related to minimum OA settings for DCV,
particularly with OA-FLOW/PER and OA-FLOW/AREA. Say I am using ASHRAE
62.1 for my OA rates, there are minimum requirements for cfm/person and
cfm/area. However, the way eQuest calculates min zone OA is by taking
the higher value of either OA-FLOW/PER or OA-FLOW/AREA, not the combine
total. To correctly model the OA requirement, I have to calculate the
combine OA rate (i.e. {[cfm/per * max # of person] + [cfm/area * floor
area] } / floor area) and input that value in OA-FLOW/AREA. Now, the
problem comes in when we include DCV into the model, at which eQuest
cannot calculate the savings because the OA-FLOW/PER value is not
entered. I would greatly appreciate it if anyone can clarify this.

VICTOR YEUNG, BASc, LEED(r) AP

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Otto,

Thanks for the reply. The problem I see with dividing the total OA cfm
by the number of people and inputting that number into OA-FLOW/PER is
that this method doesn't address the space cfm requirement. For
example, if the building is only 10% occupied at a particular hour, then
the OA supplied will be 10% occupant requirement and 10% of space
requirement, not 10% occupant and 100% area as it should be.

Victor

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"However, the way eQuest calculates min zone OA is by taking the higher
value of either OA-FLOW/PER or OA-FLOW/AREA, not the combine total. To
correctly model the OA requirement, I have to calculate the combine OA
rate (i.e. {[cfm/per * max # of person] + [cfm/area * floor area] } /
floor area) and input that value in OA-FLOW/AREA."

Are you sure about that Victor? Last time I checked, when I input 5 CFM
/ person and 0.06 CFM / square foot, the minimum zone OA was the SUM of
these two values. In which case you can model DCV without having to use
that option in the system tab. eQuest will automatically size the 5 CFM
/ person portion of the outside air based on the occupancy diversity.
Am I wrong here?

James Hansen, PE, LEED AP

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James,

This is from the eQuest help file:

OA-FLOW/AREA

This keyword allows the specification of a minimum zone outside air (OA)
flow rate per unit of zone floor area. The use of OUTSIDE-AIR-FLOW will
over ride the value of this keyword. The maximum of values resulting
from OA-FLOW/AREA, OA-FLOW/PER, and OA-CHANGES will be used unless an
over-ride value for OUTSIDE-AIR-FLOW is specified.

I've also tested this out by varying the OA-FLOW/PER in a model I've
worked on. The total energy consumption didn't change, which suggested
that my cfm/area rate was producing the larger cfm requirement and the
program ignored my cfm/person rate.

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What I had ended up doing was to use both the numbers /person and
/sqft. but the per person number is slightly higher than the actual
per person number based on the area of the zone and the cfm/area. You
can use a equation in this cell to get the equivalent number.
So during the occupied time, if the occupancy is zero any hour, the
/sqft value gets used, and rest of the time /person value multiplied
with the actual number of people gets used. It is not exactly what you
want but it is better than one or the other.
I hope the equest team include sthe ASHRAE ventilation calcs in their
next version - the current version of calculating OA probably works
for Title 24?
-Rohini

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Rohini said, "the current version of calculating OA probably works for
Title 24?"

The answer is yes. T24 requires 15 cfm per expected number of
occupants, with a minimum value of 0.15 cfm/sq-ft (this value depends on
occ type). For a regular (non-DCV) system, your ventilation will be
constant and be based on the higher of the area and people based
calculations. For a system with DCV, your ventilation amount will reset
from the aforementioned value down to the minimum area-based value.
eQuest is built for such.

Greg Collins

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Thanks Rohini. Depending on the number you use for the cfm/person, your
method will end up over/underestimating the space cfm requirement. I
guess I have to conclude by saying this is a limitation of eQuest at the
moment. What I have been thinking about doing is to calculate the avg
number of person per zone per hr based on the occupancy schedule, and
use that avg to calculate my effective total OA rate. It will be
extremely tedious, but at least it should be accurate.

Thanks everyone for their input.

Victor

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