How is Demand Controlled Ventilation applied without a Controller:MechanicalVentilation object?

asked 2020-01-27 08:56:46 -0500

jbatt gravatar image

updated 2020-01-27 09:15:42 -0500

I am looking at the Large office, post-1980 construction DOE Commercial Reference Building and was going to assess the impact of implementing demand controlled ventilation. To my surprise, though, when I looked at the annual outdoor air profile, it seemed that DCV was already being applied. If you look at the attached picture, outdoor air volume is clearly tracking occupancy to some extent (emphasized by the dip during lunchtime and tapering into the evening); however, it is doing so imperfectly (you can see slight variation in profile from day to day). There is no Controller:MechanicalVentilation object in the model (which is the only place I thought that DCV could be applied - my methodology was going to be to introduce this object to implement DCV). These observations have led me to the following questions:

  1. How is DCV applied in the absence of a Controller:MechanicalVentilation object?
  2. What, other than occupancy, is governing the outdoor air volume delivered by the system (note: NoEconomizer is selected)?
  3. Is it reasonable to assume that these older buildings are really employing DCV to begin with? I've noticed that the New Construction versions of these models do not employ DCV (despite DCV being selected, ventilation rates are entered on a per-unit-area basis and as a result outdoor air volume does not vary with occupancy as it has no defined ralationship to it).

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I assume you got the file from here (a link is always nice, so we don't have to track it down). Then what did you do? Run it with v7.2? Use IDFVersionUpdater to bring it to 9.2? What variable are you plotting exactly?

Julien Marrec gravatar image Julien Marrec  ( 2020-01-27 10:14:00 -0500 )edit

Correct - that's where I got it. I used IDFVersionUpdater to bring it to 9.0 and am plotting Air System Outdoor Air Mass Flow Rate, so I believe this is showing total outdoor air for a typical floor. If I am understanding correctly, it seems that Controller:OutdoorAir defaults to using ZoneSum as it's outdoor air supply method (?)

jbatt gravatar image jbatt  ( 2020-01-27 10:33:28 -0500 )edit

The DesignSpecification:OutdoorAir objects in that model use 'Flow/Person' input, which are modified by the occupancy schedule. I think the combination of the blank Mechanical Ventilation controller and the autosized flow rate fields in the Controller:OutdoorAir mean the OA defaults to what the DS:OA requests.

Eric Ringold gravatar image Eric Ringold  ( 2020-01-27 16:49:32 -0500 )edit

I agree that this is what seems to be happening. Before looking into this I would have expected the OA supplied to correspond to the DS:OA at full occupancy (so a flat OA supply) for all hours that the system was on, since DCV was not selected. To me these results suggest that DCV is applied by default when no Mechanical Ventilation Controller object is included. My questions then are:

  1. Short of switching my DS:OA to an area-based or fixed flow, how do I remove this DCV action?
  2. Could the intent behind the reference buildings really be to have DCV in these older vintages??
jbatt gravatar image jbatt  ( 2020-01-27 17:03:56 -0500 )edit

Default applying DCV if you have no Controller:MechanicalVentilation seems very counter-intuitive to me. I would have expected the same as you: DSOA calculated for full occupancy using ZoneSum (ignore schedule, NumPeople * Flow/Person + Area * Flow/Area + ...). 1. To remove this DCV action, I suggest adding a Controller:MechanicalVentilation... 2. I doubt it.

Julien Marrec gravatar image Julien Marrec  ( 2020-01-28 02:53:21 -0500 )edit