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VRF Outdoor Air Autosizing Problem

asked 2017-06-02 06:09:13 -0500

updated 2017-08-06 07:35:18 -0500

I am modeling a DOAS with VRF system. The space has VAV box provides OA and then go through the VRF terminal unit with plenum air.

From the measure VRFwithDOAS system, in the VRF terminal sizing, the Outdoor Air Flow Rate is set to 0, is this correct ? Because I found it will give different cooling energy and heating energy if those three values are set to 0 or auto sizing.

From the Input out reference, it says "This input field is set to zero flow when the VRF terminal unit is connected to an AirTerminal:SingleDuct:Mixer object." So when it is connected to a VAV box, should these three parameters set to 0 ? Thank you image description

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I think this should set Autosized, otherwise the OA flow is 0 when there is heating/ cooling demand from the VRF terminal. Actually, when there is cooling/heating demand, the OA will modulate. Any comments ?

Bo_Lin gravatar image Bo_Lin  ( 2017-06-02 08:26:31 -0500 )edit

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answered 2017-06-02 10:26:24 -0500

kwalkerman gravatar image

From the EnergyPlus Documentation: image description

So the Outdoor Air Flow Rate fields for the VRF unit will be set to 0.

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@kwalkerman But AirTerminal:SingleDuct:Mixer is different from VAVBox with NoReheat, correct ? Does this rule apply to VAVBox wtih NoReheat as well ?

Bo_Lin gravatar image Bo_Lin  ( 2017-06-02 10:32:48 -0500 )edit

And also, in actual operation, the VRF terminal do receive OA from the DOAS system. If it is set zero, the VRF will have no OA in cooling or heating condition which is not true in reality. (I ran two models and compared)

Bo_Lin gravatar image Bo_Lin  ( 2017-06-02 10:34:59 -0500 )edit

AirTerminal:SingleDuct:Mixer is a special type of air terminal that allows you to provide air to a ZoneHVAC:Equipment object, instead of providing air directly to the zone. The other flavors of AirTerminal:SingleDuct only provide air directly to the zone.

There are two ways to set up the mixer. In my experience, the method that is applied in real buildings is that pictured above, with the mixer supplying outdoor air mixed with zone air to the supply side of the terminal unit. The mixer receives air from an airloop (usually outdoor air) and mixes it with the return air from the zone.

kwalkerman gravatar image kwalkerman  ( 2017-06-02 10:52:05 -0500 )edit

.. continued from above

If you look at the node connections between the two objects, I think it'll make more sense. Without a DOAS, the VRF terminal unit will receive outdoor air directly from it's own outdoor air mixer. In this case, if you looked at hourly variables, you'll see an airflow at the outdoor air inlet node. With a DOAS, the outdoor air is mixed in prior to the VRF inlet node, so you'd need to look for the outdoor air farther upstream.

kwalkerman gravatar image kwalkerman  ( 2017-06-02 10:55:31 -0500 )edit

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Asked: 2017-06-02 06:09:13 -0500

Seen: 302 times

Last updated: Jun 02 '17