I am revisiting this model and never received a response, and I see
another has run into the same warnings. Reposting/rephrasing for any
suggestions/input:
In hindsight perhaps I can phrase a simpler set of questions:
1. True or False: Is it simply impossible in eQuest to
simultaneously use OA-FROM-SYSTEM and either of the DCV ventilation
control methods with the same system. This appears to be the case.
2. Based on #1, I seem to be stuck being unable to modeling the
savings from DCV at the zonal level (instead forced to use fraction of
design or the like for my min-OA method)...
But I've got a crazy idea: For an alternative way to force DCV
behavior, might I define a set of MIN-AIR-SCHEDULE's, using the zonal
occupancy schedules as a template, and multiply each fractional
occupancy rate by the designed OA ratio? This would be tied to an
assumption that the C02 levels rise and fall instantaneously with the
occupancy, which I know is not true but may be an acceptable
approximation given #1.
Example: A classroom has a daily schedule that looks like this (this is
for evening classes, if this looks odd):
"General Classroom Occup Mon-Thu" = DAY-SCHEDULE-PD
TYPE = FRACTION
VALUES = ( 0, &D, &D, &D, &D, &D, 0.2, &D, &D, &D, &D, &D,
&D,
&D, &D, &D, 0.6, 1, &D, &D, &D, &D, 0.5, 0 )
..
With the design OA flow being 60% of the design Supply Air flow for that
terminal unit, I create a new MIN-AIR-SCHEDULE multiplying through:
"General Classroom MinAir Mon-Thu" = DAY-SCHEDULE-PD
TYPE = FRACTION
VALUES = ( 0, &D, &D, &D, &D, &D, 0.12, &D, &D, &D, &D, &D,
&D,
&D, &D, &D, 0.36, 0.60, &D, &D, &D, &D, 0.30, 0 )
..
For this approach I would tightly control the OA present at each zone
specifying MIN-OUTSIDE-AIR and clearing out the zonal-level OA inputs.
For reference - The system I am trying to model is:
* Dedicated outside air unit - HW/CHW coils, enthalpy recovery
wheel, variable supply which effectively sums the terminal fan coil's
call for OA over time. Conditioned OA is supplied to VAV's which
modulate based on the terminal fan-coil's DCV sensor.
* Terminal units of widely varying size: 4-pipe HW/CHW coils
which handle essentially all the internal/envelope loads, constant
volume, local thermostat controlled, conditioned OA from dedicated OA
units ties into supply ductwork (not seen by the terminal fan coil
units). OA is modulated down to zero based on a CO2 sensor in return
path.
For reference - I'm trying to model the above using:
* Single Zone Reheat (SZRH) for the DOAS, tied to a dummy zone
* 4-pipe Fan Coil (FC) for the terminal units, system per zone,
OA-FROM-SYSTEM
NICK CATON, E.I.T.
Question - 1: I believe that to be the case. What I had done ususally
is to use the DCV features and proportion the OA and related ERV, fans
etc. if any to the individual systems.
Question #1 - I have used this also but had run into some issue, but
cannot recollect whatthat was. It is a reasonable way to model. The
CO2 usually has a lag of 20-30 minutes, so you could shift your
schedule accordingly. But the DCV model in eQuest anyway does not
consider the lag - it is strictly number of people times the OA cfm
required.
-Rohini> I am revisiting this model and never received a response, and I see
I have used the DCV ventilation method with OA from the system and it
has worked great, as long as you have inputs in correctly under the zone
outside air & exhaust. If you have inputs under the zone incorrectly
then the OA control on the system will be overridden by the inputs under
the zone.
Inputs in the model would be something like this to get it to work:
-Air side HVAC System under outdoor air - vent & economizer:
-Under Min. outside air (ratio)- set at whatever, such as 0.10
for 10% OSA (if using an economizer the minimum must be set at least to
0.001, or the economizer feature will not work in the model).
-Under Min. OA Method - I always use DCV Return Sensor.
-On each Zone under the system under Outside Air & Exhaust:
-Under Outside Air Flow/Person - Input CFM required by code or
usually 15 to 20 CFM per person.
-Under Outside Air Flow - Leave this blank.
-Under Outside Air Changes - Leave this blank.
-Under Outside Air/Flow/Area (CFM/FT2) - Normally I input 0.06
here, which is typically the amount required by the code.
As long as you have your hourly occupancy schedule set up correctly with
the % of the number of people that are in each zone then these inputs
have always worked for me. The Outside Air entering the system will
vary down to as low as 0.06 CFM/FT2 of the area of the applicable zones
depending on how many people your occupancy schedule shows you have in
each of the applicable zones that you have under the HVAC air system.
The Outside air reduction will, of course, also always be limited by the
amount of exhaust CFM that you have input under the zone. Be careful
with what you enter under the zone regarding the CFM of the ventilation
and the exhaust, as the program will always override what you enter
under the system by what you have entered under the zone.
David A. Bastow