Setting up a DOAS with a Floating Supply Temp

asked 2015-07-07 10:39:38 -0600

pflaumingo

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updated 2015-07-11 14:47:49 -0600

__AmirRoth__

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What's the best way to set up a DOAS in OpenStudio with a floating (deadband) supply temperature that supplies air between 60F and 75F with the heating and cooling coil not operating between these temperatures? I believe that the dual deadband is for just waterside equipment, right? I was thinking the simplest way would to have a scheduled setpoint manager of 60F after the heating coil and 75F after the DX coil, but this has the problem of not accounting for the fan heat. Moreover, with an ERV is how would you control it to temper the air when outside of the limits, but bypass otherwise?

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Solution Using a Work Around

I would first read the discussion with @Kyle Benne in the comments of his answer to understand how we got here. Seeing as the SetpointManagerScheduledDualSetpoint object is averaging the high and low setpoint I decided to use a slight workaround. My initial take was to have a SetpointManager:Scheduled after the furnace and cooling coil set at 55F and 75F respectively with the ERV and mixed air node using a SetpointManager:MixedAir object to reference the SetpointManager just after the furnace because the model is in a predominantly cooling environment and I don't want to unnecessarily overheat the air. I then ended up shifting the SetpointManager:Scheduled object on the cooling coil to the node after the fan and added SetpointManager:MixedAir object at the cooling coil to reference the setpoint after the fan. I did this because I wanted to supply room neutral air when cooling rather than slightly warm air. I wasn't too worried about the fan heat provided when tempering the air.

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Oh thats sorta sweet. And such a hack! We're adding duct so I guess that will make this 1% less of a hack. I feel like your second paragraph should be converted to an answer and I should remove my answer. The new answer should highlight what we discussed in the comments about how dual setpoint behaves on air system and why the hack is necessary.

Kyle Benne ( 2015-07-08 11:06:56 -0600 )edit

I've posted a final answer below. I would probably keep your initial answer because the conversation in the comments may be useful for others.

pflaumingo ( 2015-07-08 11:56:10 -0600 )edit

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answered 2015-07-08 11:55:03 -0600

pflaumingo

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updated 2015-07-08 12:03:29 -0600

The order of this thread started with me posting the question, which was then followed by Kyle's answer and our conversation in the comments of his answer. I then appended my original post to include the workaround I have attempted and the final solution is in the paragraph below.

After suggestions from @Kyle Benne I placed a dummy object downstream of the fan (any object with capacities zeroed out) to give an extra node to add a SetpointManager:Scheduled to. The final results is there is a fan and immediately after it there is a SetpointManager:Scheduled with a constant value of 55F, there is then a dummy object and after this object there is another SetpointManager:Scheduled with a constant value of 75F. The ERV, mixed air node and node just after the furnace use a SetpointManager:MixedAir to reference the SPM at 55F and the cooling coil uses the same SPM class to reference the 75F SPM.

The issue this solves is being able to use a DOAS with a floating supply temperature so that the zone systems handle most of the load. The initial attempts to use a SetpointManager:Scheduled:DualSetpoint object didn't work because when used on an airside system the SPM will just average the high and low setpoints (a constant average setpoint), which is not the desired effect.

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Hello, for a DOAS system I've found a simple alternative (I think). You can use a SetpointManager:OutdoorAirReset with 55F at 55F or below and 75F with 75F or above. In between, it is linearly interpolated so you set the same setpoint as outdoor air, getting a deadband. You use this SM at the outlet of the AHU and use 2 SetpointManager:MixedAir to reference it from the cooling, heating and mixed air nodes.

ecoeficiente ( 2016-07-04 11:44:54 -0600 )edit

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answered 2015-07-07 12:42:18 -0600

Kyle Benne
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SetpointManagerScheduledDualSetpoint is allowed on air systems. I'd say go with that.

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Thanks @Kyle Benne, I sort of wrongly assumed it was for plantloops based on the way EnergyPlus documentation words it. Would you put the SetpointManagerScheduledDualSetpoint on the supply outlet node and then have SetpointManager:MixedAir objects on the mixed air node, and each node after the furnace and DX coil that reference the SetpointManagerScheduledDualSetpoint?

pflaumingo ( 2015-07-07 13:11:55 -0600 )edit

Yes, you will need to use the SetpointManagerMixedAir on all of the component outlet nodes to account for the fan heat. OpenStudio will do this automatically on idf creation if you have components without SPMs on their outlets.

Kyle Benne ( 2015-07-07 13:33:01 -0600 )edit

If I run it with the SetpointManagerScheduledDualSetpoint then the supply temperature out of the unit (the node past the fan) is the average of both the high and low setpoint. I.e if the high setpoint is 75F and the low setpoint is 55F then it will hold it at 65F. I've tried adding scheduled setpoints of 55F off the furnace and 75F off the cooling coil, but this doesn't work either. Any thoughts?

pflaumingo ( 2015-07-07 15:52:35 -0600 )edit

Line 3781 of SetPointManager.cc is: Node( NodeNum ).TempSetPoint = ( Node( NodeNum ).TempSetPointHi + Node( NodeNum).TempSetPointLo ) / 2.0;

Which is what it seems to be doing on my air loop.

pflaumingo ( 2015-07-07 16:14:02 -0600 )edit

Well that's unfortunate. And good to know. I feel like I should amend my answer or you should provide one with this information. Now what to do? Your first thought isn't so bad. Do you really care if you're off by a little fan heat? You're not going to be controlling the zone load without some trim at the terminal anyway. My second thought is but some non participating dummy components downstream of the fan, just to get some nodes in there. Drop your scheduled managers on there, then setup mixed air managers on the comps upstream of the fan. Total hack!

Kyle Benne ( 2015-07-08 09:13:12 -0600 )edit

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