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VAV rooftop unit + VAV DOAS in the same zone

asked 2017-07-24 07:33:19 -0500

Jim Dirkes gravatar image

updated 2017-07-24 09:06:32 -0500

Dear UMH forum, I'm thinking about the best way to model a supermarket in EnergyPlus which uses:

  • a VAV rooftop unit and
  • a DOAS with hot gas reheat for humidity control in the same zone (the grocery)

My first idea is to assign the RTU as the zone's air loop and add a ZoneHVAC:OutdoorAirUnit. I'm uncertain / haven't checked whether the OutdoorAirUnit can handle the special DX coil for hot gas reheat, so I'm being somewhat lazy by asking the forum, but also wondering if there's a better way to configure the system.

My fallback solution will be to model the DOAS with a standard DX coil, standard natural gas heating, standard relative humidity control ... then a custom meter to remove heating energy when cooling is also active....

Thanks in advance for your insights.

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answered 2017-08-02 17:19:37 -0500

mleach gravatar image

You could try with the ZoneHVAC:OutdoorAir Unit as you mentioned but it looks a bit like a black box (heating, cooling, and heat recovery seem to be just built in) so I'm not sure you could do the humidity control that you want.

Alternatively, you could try to model the DOAS and RTU together as one E+ Airloop. You could put the DOAS components on the outdoor air path (between the OA node and the OA Mixer). If the DOAS serves multiple zones you won't be able to capture part load performance exactly (the many small DX coils representing the central DX coil will likely all be running at different part load ratios; same for fans), but it should be reasonably close. If the DOAS only serves that zone, even better! Note that I have done this successfully in the past for DOAS heating and cooling but not for DOAS fan power. In my case, it was a constant volume system so I represented fan power using a separate process load (and ignored fan heat), but since you have a variable speed fan, you'll want to model the fan explicitly. My understanding is that there is nothing stopping you from putting a variable speed fan on the OA path. I would try it and see what you get. You should be able to apply DCV or whatever other control that you have that results in variable OA and get the right flow rate (and associated fan power) for the DOAS fan.

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That's helpful. Thanks! I decided to create a hybrid DOAS + RTU air loop, much as you describe. It's a single zone, both fans are VAV ... and it turns out the ZoneHVAC:OA unit can't accept a hot gas reheat coil.

Jim Dirkes gravatar imageJim Dirkes ( 2017-08-03 06:57:06 -0500 )edit

I'm trying to do something similar, would it be possible for you to post a screenshot of the new hybrid air loop or provide a description of your new setup including the setpoint managers used and which nodes they are at? I'm having trouble with the control of my DOAS that is leading to unmet load hours.

anchapin gravatar imageanchapin ( 2018-08-02 06:46:31 -0500 )edit

It's a standard air loop with: * minimum OA controlled to maintain the DOAS OA airflow * EER / IEER adjusted to represent a blend of the attributes for both units

Jim Dirkes gravatar imageJim Dirkes ( 2018-08-04 10:03:32 -0500 )edit

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Asked: 2017-07-24 07:33:19 -0500

Seen: 190 times

Last updated: Aug 02 '17