[Equest-users] DCV for Me, Not for Thee

[Bishop, Bill]

Hey All,

Just a tip/FYI for modeling DCV. Comments, anecdotes and non-sequiturs encouraged!
I have modeled, or at least thought I modeled, demand controlled ventilation many times in eQUEST by setting the SYSTEM:MIN-OA-METHOD to DCV-RETURN-SENSOR  or DCV-ZONE-SENSORS, making sure I enter the correct number of people in the spaces, using an occupancy schedule that varies, and specifying the people-based ventilation using ZONE:OA-FLOW/PER. However, I recently discovered what I believe are additional steps.
I am modeling a variable 100% OA MAU serving a laundry by entering the total OA requirement using people-based ventilation. There are 9 dryers, which I modeled as 9 people, and zone OA-FLOW/PER of 220 giving a total OA requirement of 1980 cfm for the MAU. The occupancy schedule is a good proxy for dryer use, so it should work out in principle.
I modeled the system as PSZ packaged single zone and set the SYSTEM:FAN-CONTROL to FAN-EIR-FPLR and used the ASHRAE App. G fan curve. However, the SYSTEM:MIN-FLOW-RATIO defaults to 1.0 even after setting the fan to variable control. If you adjust the SYSTEM:MIN-FLOW-RATIO to something else (let's say 0.2) and make no other changes, the system operates at that minimum flow at all hours (and should run at higher flow to meet the thermostat setpoint if I had a higher T-stat setpoint). However, it does NOT simulate DCV.
In order to get a PSZ system to simulate DCV, you need to do all of the above PLUS set the ZONE:MIN-FLOW-CTRL to DCV-RESET-UP to allow the hourly flowrate to go above the MIN-FLOW-RATIO based on the people-based ventilation rate and schedule. You will not be able to select it from the drop-down menu, so you have to enter #SI("DCV-RESET-UP","ZONE","MIN-FLOW-CTRL") as a user expression for that zone (or zones). Now it WILL simulate DCV, as verified in the SS-L report for system fan electric energy use.
Before I figured that out, I tried modeling the system as PVAV. For the PVAV system, any value for SYSTEM:FAN-CONTROL other than CONSTANT-VOLUME gives a SYSTEM:MIN-FLOW-RATIO of no_default (no value - an empty box). CONSTANT-VOLUME gives a 1.0 MIN-FLOW-RATIO which won't allow DCV. Regardless of which SYSTEM:MIN-OA-METHOD setting you use, including DCV-RETURN- or DCV-ZONE-SENSORS, the ZONE:MIN-FLOW-CONTROL defaults to FIXED/SCHEDULED, which will NOT simulate DCV. To simulate DCV, you have to set the ZONE:MIN-FLOW-CONTROL to DCV-RESET-DOWN. Alternatively, you can enter a ZONE:MIN-FLOW-RATIO less than 1 (say, 0.2) and set the ZONE:MIN-FLOW-CONTROL to DCV-RESET-UP.
I have not tried modeling DCV with a VAV system lately (not PVAV), but it probably requires similar inputs to the PVAV system at the SYSTEM and ZONE(s).
Any of the options above can be modified by setting the SYSTEM:MIN-AIR-SCHEDULE, which you should do regardless in most situations to model zero OA when the system is in night-cycling mode, except for a MAU which should always be 100% OA.

TL;DR version: Read the above next time you want to be confident you're actually modeling DCV.

~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer
[Logo, company nameDescription automatically generated]      T: (585) 698-1956                  www.pathfinder-ea.com<http://www.pathfinder-ea.com>
[IconDescription automatically generated][cid:image009.jpg at 01D95C04.B8F9E630]bbishop at pathfinder-ea.com<mailto:bbishop at pathfinder-ea.com>
134 South Fitzhugh Street
Rochester, NY 14608

Buffalo Office: 100 Corporate Parkway, Suite 116, Amherst, NY 14226

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