OpenStudio | Multiple terminal units providing outdoor air to 1 zone

asked 2021-05-18 15:36:42 -0500

Elisa Matar gravatar image

updated 2021-05-18 15:43:13 -0500

I need to model an office building in Open Studio, served by a VRF system. Each room has about 20 terminal units and the outdoor air is provided through each VRF terminal, not through a dedicated outdoor air system. The terminal units in each room are not all the same type, the capacities and flow rates differ between some of them.

In Open Studio, I can connect these terminal units to the outdoor air system using the AirTerminal:SingleDuct:Mixer component. However Open Studio doesn't allow me to have more than 1 Mixer for each zone.

Can I simply sum capacities and flow rates of these 20 terminal units to consider a 'virtual' terminal unit per zone in my simulation and connect it to the Mixer, even though not all of their capacities/flow rates are the same?

I saw a post here suggesting this strategy of a 'virtual' terminal unit (solution #1 in the second answer here:, but I was not sure since my terminal units are not all equal.

If I can't do this, does anyone know a better way to model this system in Open Studio?

Thank you!

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If all of your terminal units operate at the same time at their rated capacities/flowrates and serve the same thermal zone, even if they have different rated/capacities or flow rates, you should be able to sum their individual, different capacities and flowrates for the total capacity and flowrate of your "virtual" unit.

If the terminal units each follow their own control scheme (i.e. do not turn on/off together), then you will need a different approach, because at some points in time a fraction of the 8 could be on while the others are off.

sashadf1 gravatar image sashadf1  ( 2021-05-21 04:48:41 -0500 )edit

In the solution by @ here, emphasis should be placed on (assumes that they all operate together)

sashadf1 gravatar image sashadf1  ( 2021-05-21 04:50:05 -0500 )edit

Right, it makes sense, thank you!

In this case each of them has a sensor to adjust the refrigerant and air flows to what is needed, but they are connected to the same system, following the same setpoint.

So I guess it should work to sum their capacities then?

Elisa Matar gravatar image Elisa Matar  ( 2021-05-21 06:59:54 -0500 )edit

If they are serving the same thermal zone and that TZ has a single setpoint that they are all responsive to, then their air flows and capacities should modulate as a group to meet the load and maintain the setpoint (in the real world).

In the modeling “environment” I would sum their rated/total capacities because in the event of large loads on the thermal zone, all the terminal units would be running at max capacity simultaneously to meet load.

If anyone who has experience with VRFs wants to correct me or chime in, feel free.

sashadf1 gravatar image sashadf1  ( 2021-05-21 07:13:06 -0500 )edit

Yes, I am not familiar with VRFs either, but for me now it makes sense to group them. Thank you!

Elisa Matar gravatar image Elisa Matar  ( 2021-05-21 11:39:42 -0500 )edit