[Equest-users] Appendix G VAV Minimum Flow per G3.1.3.13 - eQUEST inputs

[Bishop, Bill]

Hey All,
I was incorrect yesterday, and Nick Caton called me on it offline.

I have always assumed that when zonal inputs are entered for outside air, that the hourly total system outside airflow would not drop below the cfm indicated on the SV-A reports (SUPPLY CFM * OA ratio)  unless specifically controlled to do so such as by specifying a MIN-AIR-SCH or using DCV for the MIN-OA-METHOD.
Nick tried to verify this yesterday following the posts below, and discovered THIS IS NOT THE CASE.

It turns out that eQUEST gives priority to MIN-FLOW-RATIO values entered for the systems and/or the zones. This can result in simulations with hourly outside air flowrates below their intended values. Nick verified that zonal EXHAUST-FLOW inputs do take priority over zone MIN-FLOW-RATIO. I have created an input file (attached) to meet the 90.1-2013 VAV minimum flow requirement of G3.1.3.13. Minimum hourly zone flowrates are the greater of 30% of peak flow or the outdoor airflow rate.

The expressions in the input file do the following:

-        create EXHAUST-FLOWs equal to zone outside air inputs

-        set AIRFLOW-TRACKING to TRACK-EXHAUST so that hourly zone flow is the greater of exhaust (OA), MIN-FLOW-RATIO, or that required for space conditioning

-        set EXHAUST-FAN-SCH equal to the system FAN-SCHEDULE

-        set EXHAUST-STATIC to zero to avoid adding fan energy

-        set zone MIN-FLOW-RATIO to 0.30 per G3.1.3.13 for Systems 5 and 7

I have tried the input file successfully in eQUEST 3.64 models and in 3.65 running both DOE-2.2 and DOE-2.3. Use File -> Import File… to give it a try!
You still need to enter a SYSTEM:MIN-AIR-SCH to model zero outside airflow during unoccupied fan cycling. Best to verify the hourly OA is correct by creating custom hourly reports for system(s) and tracking supply cfm and OA%.
There are many simulation changes and options for system sizing in eQUEST 3.65 running DOE-2.3 that I am just starting to explore, so the file may need edits.

Feedback welcomed!
Thanks,
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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From: Bishop, Bill
Sent: Thursday, February 15, 2018 9:43 AM
To: 'equest user' <equestuser00 at gmail.com>
Cc: equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] USER EXPRESSION

Hi Tejas,
I believe that eQUEST/DOE-2 always gives priority to outside air flow, so that hourly outdoor air requirements are always satisfied, consistent with the zone and/or system OA cfm values and control method. Therefore, the “minimum outdoor air flow rate” requirement in ASHRAE 90.1-G3.1.3.13 is automatically met.
So you enter 0.30 for the SYSTEM:MIN-FLOW-RATIO and you meet the 30% minimum flow requirement. (You can also enter minimum flow at the zones using ZONE:MIN-FLOW-RATIO, but the default expression copies the system-level value if it is entered.)
HOWEVER, if you still wanted an expression that sets the MIN-FLOW-RATIO to the greater of 30% or SYSTEM:MIN-OUTSIDE-AIR, you could use this expression for SYSTEM:MIN-FLOW-RATIO:
if (#RV(#L("MIN-OUTSIDE-AIR")) == 0)
              then MAX(#L("MIN-OUTSIDE-AIR"),0.30)
              else 0.30
              endif

Regards,
~Bill

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of equest user via Equest-users
Sent: Thursday, February 15, 2018 7:17 AM
To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [Equest-users] USER EXPRESSION

Hi all Modelers,

This question is for all those 'user expressions' prodigies.

Can anyone guide me on how can user expression be used for MIN-FLOW RATIO in Air side sytem as ASHRAE states to keep MIN-FLOW RATIO as 0.30 or equal to Design outdoor air value whichever is greater.

I am in need of a user expression which i should be able to put in MIN- FLOW RATIO Keyword such that it
1) takes the Value as '0.30' when outdoor design ratio is less than 0.30 and
2) takes the Value same as design outdoor ratio when design ratio value is greater than 0.30

Thanks and regards,
Tejas
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