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

[Grando, John]

I think you can just delete the walls and ceiling and be okay


John Grando LEED AP





________________________________
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 3:58 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? =)

[cid:163040020 at 25092009-27E3]

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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