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Unitary System: where to add the supply fan?

asked 2016-03-02 10:00:23 -0600

It's more generally an E+ question, but I am using OpenStudio (worth noting because I know there are some "issues" with where to place setpoints on inner nodes (read here))

I want to model a rooftop unit with water-cooled DX units (no heating, no reheat). I'm therefore planning to use an AirLoopHVAC:UnitarySystem with a Coil:Cooling:WaterToAirHeatPump:EquationFit. I also want to use a Fan:VariableSpeed.

My understanding is that I can put my fan either inside the unitary system, or I can put it on the AirLoopHVAC supply side by itself. First, is that correct?

Second, if correct, which way should I use? Is there any advantage/disadvantage of putting it inside versus outside?

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answered 2016-03-02 10:40:11 -0600

Your understanding is correct in that the fan may be placed inside or outside the parent UnitarySystem object. The subtlety is in sizing the system and how you use the scalable sizing inputs (i.e., Cooling Supply Air Flow Rate Method) in the unitary system since the fan may not be able to size using the same method if placed outside the UnitarySystem object. The UnitarySystem object is new and has been tested with many variations in configuration. However, you might find that you have selected a certain configuration that was not tested thoroughly enough and issues may arise.

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I get the feeling that I'll be better if I leave it inside the UnitarySystem, right? Will that not adversely affect any fan operation to deliver outside air to the zone? (including DCV)

Julien Marrec's avatar Julien Marrec  ( 2016-03-02 11:05:44 -0600 )edit

I would certainly start there. Then try the other just to see if it works correctly. The outside air system should manage the OA as necessary in conjunction with the air loop operation (i.e., amount of return air to the mixer).

rraustad's avatar rraustad  ( 2016-03-02 12:03:53 -0600 )edit

One more thing. When using a variable speed fan with the UnitarySystem, VAV boxes are required to set the air loop flow rate.

rraustad's avatar rraustad  ( 2016-03-02 12:04:45 -0600 )edit

Any one of the VAV air terminal? Reheat, No reheat doesn't matter as long as I don't use a single duct uncontrolled? I'll certainly try it both ways. For the record, once I added an additional airloop with a unitary system having only the water to air coil, a vsd fan and an outdoor air system, my simulation runtime seemed much slower... it seems that HVAC sims had trouble to converge. Not sure if this is expected, I plan on implemented at least twenty of these...

Julien Marrec's avatar Julien Marrec  ( 2016-03-02 16:18:41 -0600 )edit

Correct. The air loop furnaces and heat pumps actually set their inlet flow rate so they can cycle to meet the load. These models do not allow VS fans. The new UnitarySystem allows all fan types and in the case of the variable volume fan, which will use set point based control, the terminal unit selects the flow rate. If not using set point based control, TU's are not a required component (i.e., the UnitarySystem will control it's own flow rate).

rraustad's avatar rraustad  ( 2016-03-03 11:10:39 -0600 )edit

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Asked: 2016-03-02 10:00:23 -0600

Seen: 255 times

Last updated: Mar 02 '16