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Best output:variable for zone heating / cooling load

asked 2015-11-24 05:15:11 -0600

Jim Dirkes gravatar image

updated 2015-11-24 10:48:15 -0600

I thought I knew what I was doing.... but now I'm not sure.

I want an hour-by-hour representation of the total heating load and total cooling load for DOE reference buildings. It seemed to me that "Zone Predicted Sensible Load to Setpoint Heat Transfer Rate" would be perfect; add up all the positive values for heating load and all negative values for cooling load. (The definition is shown at the bottom of this post.)

This approach, after reviewing results, is suspect. I'm looking at the Hospital results and see that the Operating Rooms, which are on an exterior wall and the 2nd floor, NEVER have a cooling load. In fact, they have a few heating load hours, but show zero load for the vast majority of hours. This is true despite having substantial lighting, plug and people loads.

Can you provide a suggestion for:

  • what I may be missing in my understanding of the variable? or
  • a better approach?

Thanks in advance.

Zone Predicted Sensible Load to Setpoint Heat Transfer Rate [W] This is the predicted sensible load in W required to meet the current zone thermostat setpoint. A positive value indicates a heating load, a negative value indicates a cooling load. This is calculated and reported from the Predict step in the Zone Predictor-Corrector module. For nearly all equipment types, the Predictor-Corrector evaluates the active heating and/or cooling setpoints, determines if the zone requires heating or cooling or is in the deadband, and then passes this single load to the equipment. This value is not multiplied by zone or group multipliers.

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1

I am using Zone Predicted Sensible Load to Cooling Setpoint Heat Transfer Rate for school reference building - seems to be giving non-zero loads for cooling (since I am doing a summer day). Did you see how much OA the operating room has - depending on the climate, maybe it is meeting your load? You must have already checked the thermostat schedule. You could run at the detailed time step to see what happens - maybe there is lot of cycling? Can't think of anything else.

Rohini gravatar imageRohini ( 2015-11-24 11:17:48 -0600 )edit

Also check the heat balance table in the allsummary report for clues on the migration of heat to and from the zone.

Jean Marais gravatar imageJean Marais ( 2015-11-24 13:57:48 -0600 )edit

I have done some more investigation and put some thoughts in a small Google Sheets document. It's link is: https://docs.google.com/a/buildingper... I'm still at a loss for trying to make sense of the variable, but am hoping that the sheet may spark some ideas.

Jim Dirkes gravatar imageJim Dirkes ( 2015-11-25 12:30:05 -0600 )edit

Jim, you may have to change the permissions on the Google Sheet so that anyone can view it. Currently I have to request access.

Neal Kruis gravatar imageNeal Kruis ( 2015-11-25 16:33:24 -0600 )edit

I changed permissions. Sorry about that.

Jim Dirkes gravatar imageJim Dirkes ( 2015-11-28 11:35:24 -0600 )edit

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answered 2015-11-24 14:04:36 -0600

updated 2015-11-24 14:09:36 -0600

The Hospital reference building model has some bad schedules that don't zero out lights and equipment for winter design days. This causes the heating loads to be zero and the reheat coils to have zero capacity for some zones. Because there is no heating capacity for those zones, the temperatures are floating well below the setpoint and throwing off the predicted load.

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Neal, I am amazed that you know the reference buildings in such detail! I saw some of that variation from setpoint in the hourly results, but the overall scheme about how the output:variable "thinks" is still unclear.

Jim Dirkes gravatar imageJim Dirkes ( 2015-11-25 12:32:57 -0600 )edit
2

answered 2017-08-03 13:32:52 -0600

Molly Curtz gravatar image

updated 2017-08-03 13:42:48 -0600

Jim, I realize your question is now fairly old, but I was just looking at this myself, and found your post, so I thought I'd share my thoughts. I am also looking at the three output variables Zone Predicted Sensible Load to Heating Setpoint Heat Transfer Rate, Zone Predicted Sensible Load to Cooling Heat Transfer Rate, and Zone Predicted Sensible Load to Setpoint Heat Transfer Rate. I reported these at the "Detailed" level, which might be helpful for clearly seeing what is going on. In my model I have a heating setpoint (23C) and a cooling setpoint (24C) with a 1C deadband between the two. I also reported the Zone Air Temperature. When I compare the zone air temp to the three predicted load variables it seems to make sense in this way:

  1. When the zone temp is in the deadband: the Zone Predicted Sensible Load to Setpoint Heat Transfer Rate is zero. (The Zone Predicted Sensible Load to Heating Setpoint Heat Transfer Rate and Zone Predicted Sensible Load to Cooling Heat Transfer Rate have non-zero values reflecting what it would take to get from the deadband zone temp to the respective setpoint temps.)

  2. When the zone temp is above the cooling setpoint: the Zone Predicted Sensible Load to Setpoint Heat Transfer Rate has the same value as the Zone Predicted Sensible Load to Cooling Heat Transfer Rate. (Zone Predicted Sensible Load to Heating Setpoint Heat Transfer Rate also has a non-zero value reflecting what it would take to drop the zone temp all the way down to the heating setpoint.)

  3. When the zone temp is below the heating setpoint: the Zone Predicted Sensible Load to Setpoint Heat Transfer Rate has the same value as the Zone Predicted Sensible Load to Heating Setpoint Heat Transfer Rate. (Zone Predicted Sensible Load to Cooling Heat Transfer Rate also has a value reflecting what it would take to heat the zone all the way up to the cooling setpoint.)

It sounds like there were some control issues in the model you were looking at, but in any case comparing the predicted load variables with the zone air temperature should be helpful.

It is also important to note that these values don't reflect the impact of air delivered to the zone, as explained in the response to another question I asked. So, for example, when meeting these loads in simulation, if your system/terminal minimum flow settings and supply air temp limits result in overcooling - that can create an additional load and the additional heating required to counteract it would not be included in these reported values.

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Molly, I'm too slow to respond. Sorry! 1) The key which you have provided, I think, is to use the Detailed results for those variables. For some reason, although I report many output:variables for every model, I've never tried "Detailed". I'll put that into the tool kit. 2) Thanks for your insights. They make perfect sense to me now that you've explained them so nicely, but did not occur to me earlier.

Jim Dirkes gravatar imageJim Dirkes ( 2017-08-29 11:47:41 -0600 )edit
2

answered 2015-11-24 10:59:54 -0600

Jean Marais gravatar image

Sounds correct. It should report the average for the reporting period specified, i.e. hourly. Perhaps these are better for duel setpoint thermostats with deadband:

Zone Predicted Sensible Load to Heating Setpoint Heat Transfer Rate [W]

Zone Predicted Sensible Load to Cooling Setpoint Heat Transfer Rate [W]

You could try...

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I tried each of these, but no new insights.I put a link in a comment to my original question which takes you to a Google Sheets document that shows some results and also some of my thinking...

Jim Dirkes gravatar imageJim Dirkes ( 2015-11-25 12:36:40 -0600 )edit

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Asked: 2015-11-24 05:15:11 -0600

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Last updated: Aug 03 '17