DCV for Me, Not for Thee

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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.


William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer
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Bill Bishop's picture
Joined: 2012-02-25
Reputation: 7

Bill & All;

Not to conscript your question ? but an offshoot thereof. With PSZ Systems regardless of the fan and min-flow settings we have not seen the hourly airflows vary; DCV or not. Setting the fan to Constant Volume, Variable, or Fan EIR FPLR with the Min-Flow-Ratio and Min-Fan-Ratio set to 0.3 yields same hourly airflows and <1% difference in energy consumption. Checking with submetering and hourly reports. Maybe the DCV helps ?force? the PSZ to vary its fans?

We?ve gone to using a PVAV with an aggressive supply air temp reset (to avoid zone heating since there typically is none in this type of system). The aggressive reheat forces all the heating and cooling to the system (Where it actually is) and minimizes unmet hours.

Maybe there is an input that is being overlooked to get PSZ fans to vary, so I am all ears. I?ve cracked this up to ?maybe they never intended for PSZ systems to have variable fans? since eQuest/DOE has some age to it.

Any others have similar experience or workarounds ?


Energy Modeling Technical Manager | Energy Analyst | Associate
Venue Sector
Daric.Adair at hendersonengineers.com

Daric Adair's picture
Joined: 2017-10-21
Reputation: 0


My understanding is that and PSZ system will default to a SYSTEM:MIN-FLOW-RATIO of 1.0. I believe setting the fans to variable control will modulate the fan speed and therefore flow but is not the keyword responsible for opening and closing the OA damper. If the system MIN-OA-RATIO is 1.0 for 100% OA I don't see how the system can supply less than 100% OA whenever the fans are running. Is this still a problem if the unit had an in-zone reheat coil? Is it possible to model the 100 OA% MAU as a DOAS in eQuest and specify it as the DOAS unit under your PSZ? I believe DCV is the default option when a system receives air from a DOAS.

Ethan Seaman
Sustainable Design Analyst
t: 617.520.9288

eseaman413's picture
Joined: 2019-05-07
Reputation: 0

You can set the minimum ventilation rate using SYSTEM:MIN-OUTSIDE-AIR, but it is overridden by setting any of the ZONE OA keywords. I'm not sure what problem you are referring to with zone reheat coils.
Yes, I could have modeled a DOAS in my example below, but I think it would have been an unnecessary complication. DCV is not the default option for a system supplied by a DOAS. The default is FRAC-OF-DESIGN-FLOW, which models constant OA unless the MIN-AIR-SCH is specified. You still have to use one of the two DCV options for MIN-OA-METHOD. The DOE-2 help text for MIN-OA-METHOD is very detailed regarding modeling DCV with or without DOAS.

Bill Bishop's picture
Joined: 2012-02-25
Reputation: 7