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.