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modeling doas in EnergyPlus using dual duct terminal

asked 2014-10-08 13:30:01 -0500

mleach gravatar image

updated 2015-04-13 13:38:41 -0500

I'm using the dual duct terminals to model DOAS so that I can correctly do DCV (with other terminals, the OA rate will reduce but the total fan flow and power will not). It's mostly working except for some strange artifacts. On random dates, but always near hvac startup or shutdown, the OA rate spikes up to the maximum fan flow value. The return and relief air nodes, and the mixed air mass flow rate output variable, always show the correct flow (no spikes). The OA fraction reads larger than one (the ratio of the OA rate to the return air rate), and the supply outlet node flow has the spikes. Again, this happens RANDOMLY. There is no pattern that repeats weekly. Additionally, multiple air loops that share the same inputs show DIFFERENT behavior, reinforcing the indication that this behavior is random. I've tried all combinations of adding and removing the min OA schedules in the outdoor air controller and design specification outdoor air objects and nothing fixed the problem (in fact, adding the MinOA schedule to the design spec oa objects made it worse in some cases). Is this a bug or am I doing something wrong?

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Are you using AirTerminal:DualDuct:VAV:OutdoorAir configured that way it is in example file DOASDualDuctSchool.idf? Difficult to diagnose something like this without seeing the idf file. I would suggest posting a ticket to with idf attached (or send to

MJWitte gravatar imageMJWitte ( 2014-10-10 17:08:35 -0500 )edit

@mleach. If there is some "gotcha" here, please loop back and post the fix. @Everyone. In general, if there are questions here that end up on a Helpdesk (EnergyPlus or other) because the answer cannot be given without the model, please summarize the general lesson in a response. Perhaps with a comment to the effect of "to debug something like this, you really need to see the model."

__AmirRoth__ gravatar image__AmirRoth__ ( 2014-10-11 13:48:21 -0500 )edit

@mleach. In the meantime, please retag with 'dual-duct-terminal'. I bleieve we have adopted the convention of '-' for ' '.

__AmirRoth__ gravatar image__AmirRoth__ ( 2014-10-11 13:49:08 -0500 )edit

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answered 2014-10-14 14:52:22 -0500

Are the spikes of long enough duration and frequent enough that you won't be getting the correct answer from the modeling results at a holistic level?

It will depend on what you are trying to get out of the model -- if you are trying to optimize other parts of the building, maybe these spikes don't change the larger recommendation.

However if the analysis is specifically pertaining to the zone airflow and OA quantity, then I think you need to review the parameters as suggested by Mike Witte, and/or investigate the possibility of a bug.

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answered 2014-12-05 12:06:54 -0500

mleach gravatar image

Sorry for the late response on this. I actually wrote an answer previously but it looks like it didn't take. I found out that the issue corresponded to the HVAC solver hitting maximum iterations because of a separate control problem. When the solver was unable to solve, the flow rate would default to 100% flow rather than tracking the correct DCV flow. FYI, the solving problem was related to what appears to be a bug in ERV (HeatExchanger:AirToAir:SensibleAndLatent) frost control (MinimumExhaustTemperature method).

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Asked: 2014-10-08 13:30:01 -0500

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Last updated: Dec 05 '14