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How to Model Makeup Air Unit in OpenStudio

asked 2014-10-15 20:12:46 -0500

dannyllim gravatar image

updated 2014-10-16 09:25:52 -0500

I am using OpenStudio 1.4 to model the HVAC system of a multi-story office building. The cooling requirements of the spaces on each floor are served by AHUs (for office spaces) and FCUs (for common areas). Fresh air or makeup air is provided by rooftop AHU units with heat pipe to the AHUs and spaces with FCUs to meet the ventilation requirements.

I understand that this configuration can be implemented in EnergyPlus using the AirTerminal:DualDuct:VAV:OutdoorAir object for the AHUs and AirTerminal:SingleDuct:SupplySideMixer object for the FCUs but these objects are not yet available in OpenStudio.

My question is if there is a workaround to set this up in OpenStudio and perhaps complete the setup in EnergyPlus if necessary. My point is to do the connections in OpenStudio which is easier to do than in EnergyPlus. Suggestions will be most appreciated.

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@dannylim. May want to add a topic tag like 'makeup-air' or 'outdoor-air' for search/routing purposes.

__AmirRoth__ gravatar image __AmirRoth__  ( 2014-10-16 09:08:58 -0500 )edit

I added "doas" (dedicated outdoor air system) since it has been used already on this site.

Julien Marrec gravatar image Julien Marrec  ( 2014-10-16 09:26:24 -0500 )edit

For this particular application, is the DOAS constant volume or variable volume?

MJWitte gravatar image MJWitte  ( 2014-10-16 10:26:34 -0500 )edit

The DOAS is variable volume for this application.

dannyllim gravatar image dannyllim  ( 2014-10-16 19:04:18 -0500 )edit

Note that current EnergyPlus does not support DOAS conditioned air feeding an AHU. In EnergyPlus v8.2.0 DOAS air can be supplied directly to a zone, to an FCU and to a zonal water-to-air heat pump unit.

MJWitte gravatar image MJWitte  ( 2014-10-17 08:33:11 -0500 )edit

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answered 2014-10-16 09:27:35 -0500

You are correct that the dual duct terminals are not available in OS, but beginning in 1.5 we have the AirTerminalSingleDuctSupplySideMixer. I know you said your model is version 1.4, so upgrading might not be an option for you. If however you want to migrate to 1.5, then to use the SupplySideMixer you add the terminal like any other, then you can drop the FCU on the terminal outlet node. All of the connections will be made for you. You must have the supply side mixer in place before you try to drop the FCU otherwise it will not be allowed inside the AirLoopHVAC topology.

For the dual duct terminal I suggest that you use one of the single duct terminals in OS. Then apply an EnergyPlus Measure to swap out the place holder terminal with the dual duct terminal. Since I expect you only need one leg of the dual duct the connections should all be in place, and the EnergyPlus measure should only need to swap one object for another.

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Hi Kyle, thanks for the suggestions. I will upgrade my model to 1.5 because I also need the 2-speed cooling tower for the baseline model. Thanks also for the link to the Measure Writing Guide.

dannyllim gravatar image dannyllim  ( 2014-10-16 19:28:24 -0500 )edit

I have a gas fired Kitchen MAU supplying OA directly to a zone that already has a dedicated roof top unit. Would the above approach be the best for this scenario? If so, can you further explain how to use the air terminal single duct supply side mixer object. gravatar image  ( 2015-04-03 13:54:50 -0500 )edit

answered 2014-10-16 09:05:44 -0500

I would recommend upgrading to OS 1.5... it has added a number of features which improve the program, and they've also added a number of additional HVAC objects that weren't in 1.4.

You'll have to build up your AHUs, or start from one of the template HVAC systems. You'll need an OA controller, coils for your heating and cooling, and a fan box, constant or variable, as well as one (or more) setpoint managers to make up the supply side of the AHU. Then on the demand side, you'll assign the thermal zones as well as a VAV box to serve each zone. The splitter/mixer objects are created automatically - they are represented by the branch split, and on the other side where the branches join again.

Again, my suggestion is to try version 1.5, you might find what you are looking for there.

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Thanks for the suggestions Benjamin. I already did the AHUs and demand side. What I need to do next is model the makeup air unit or DOAS to feed the AHUs and FCUs with pre-cooled ventilation air. I will upgrade to version 1.5.

dannyllim gravatar image dannyllim  ( 2014-10-16 19:47:30 -0500 )edit

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Asked: 2014-10-15 20:12:46 -0500

Seen: 1,222 times

Last updated: Oct 16 '14