[Equest-users] DOAS Dummy Warnings - no problem?

[Nick Caton]

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

 

 

 

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: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Jacob
Goodman
Sent: Friday, December 18, 2009 3:24 PM
To: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] DOAS Dummy Warnings - no problem?

 

I saw your question on the warnings from your DOAS model and didn't see
any replies. I have some of those same warnings. 

 
**WARNING***************************************************************
*******
             EL1 Sys1 (PVVT) (G.SW1)          has a MIN-AIR-SCH,
OA-CONTROL
             other than FIXED and/or a MIN-OA-METHOD other than
FRACTION-OF-DESIGN
             along with having a specified OA-FROM-SYSTEM. This may
cause incorrect
             OA load/flow calculation for its OA-FROM-SYSTEM.

Shows up for each zone the DOAS is supplying and so does:


 
**WARNING***************************************************************
*******
             SYSTEM EL1 Sys1 (PVVT) (T.C12)          has zero outside
air for design calculations

the second one seems normal since I put 0 cfm OA in the DD wiz before I
went to detailed edit mode, but I'm not sure about the first one. 

Hope your model turned out well, for both of our sakes.

Thanks,

-- 

Jacob Goodman , LEED AP, Green Building Specialist 
v:509.747.2179  f:509.747.2186  i:www.lseng.com <http://www.lseng.com> 
L&S  Engineering Associates, Inc. 
'High Performance Design' 

 

 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Nick
Caton
Sent: Friday, September 25, 2009 2:58 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] DOAS Dummy Warnings - no problem?

 

Hi everyone,

 

I've got what I believe to be a working preliminary model of a DOAS
system.  I'm following the DOAS concept (prescribed by previous archived
discussions) of creating a "dummy zone/shell" to be served by the DOAS
equipment.  All terminal units are tied to the DOAS via OA-FROM-SYSTEM.
My DOAS is based on the system type SZRH ("Single Zone Air Handler with
HW reheat").  

 

This DOAS is assigned to a 1ft-cube "dummy shell" with a single zone,
adiabatic ceiling/floor constructions, wall constructions of U=.0001,
and zeroed-out internal loads.  Conceptually, I believe the airflow for
the DOAS should be calculated as the hourly sum of all the terminal
unit's calculated OA needs.  

 

I've got three basic questions, presented in order of (perceived)
difficulty:

 

QUESTION 1)

As I've set my "dummy" zone's walls to U=0.0001, and of physically tiny
dimensions, I don't think there's much to worry about regarding heat
gains/losses.  In the interest of doing the intuitively "right" thing
however, is there a way to set their "surface type" as adiabatic in the
same way as roof and floor surfaces?  Is perhaps the cleanest way to
deal with all three to create "adiabatic" constructions, and if so how
do you get around the error (see attached image) when you input U=0?

 

QUESTION 2)  

Every system (DOAS and terminal units) currently has the MIN-OA-METHOD
set as FRACTION-OF-DESIGN, per the warning below.  In reality, the
hourly OA supplied by the DOAS will be calculated by summing what is
being called for by all of the terminal units, which will be based on
local DCV sensors in the return air path of each unit.  Unfortunately,
setting any of the terminal units' MIN-OA-METHOD to the intuitive
DCV-RETURN-SENSOR results in the following warning (typical for each
system):

 

**WARNING***************************************************************
*******

             EL1 Sys1 (FC) (G.N1)             has a MIN-AIR-SCH,
OA-CONTROL

             other than FIXED and/or a MIN-OA-METHOD other than
FRACTION-OF-DESIGN

             along with having a specified OA-FROM-SYSTEM. This may
cause incorrect

             OA load/flow calculation for its OA-FROM-SYSTEM.

 

I've gathered that one approach to this problem is to manually define
the design airflow of the DOAS, based on summing the critical case of OA
required by digging through the reports (not sure where to begin there),
but is there any way of tricking eQuest into correctly summing the
hourly OA CFM required by all terminal units tied to the DOAS, and then
sizing the DOAS system based on that critical sum?

 

QUESTION 3)

The following are the remaining 3 warnings, all referring to this "dummy
zone:"

 

**WARNING***************************************************************
*******

             Zone: EL2 Zn (G.1)                     has a design cooling

             temperature differential of only  1.0F.  This

             may result in an extremely large design airflow.

 

 
**WARNING***************************************************************
*******

             Zone: EL2 Zn (G.1)                     has a design heating

             temperature differential of only  -1.0F.  This

             may result in an extremely large design airflow.

 

 
**WARNING***************************************************************
*******

             ZONE EL2 Zn (G.1)                    

             might have insufficient heating capability.

             Check that the SYSTEM or ZONE HEATING-CAPACITY plus this

             ZONEs BASEBOARD-RATING is adequate to maintain the ZONE

             specified DESIGN-HEAT-T for the calculated peak ZONE load

             (see LS-A or LS-B for the ZONE peak load.)

 

Which (if any) of these I should be addressing/evaluating, considering
the modeling function of this "dummy zone?"  In other words, can I
ignore these and sleep well at night? =)

 



 

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 

 

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