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What is Proper HVAC Operation Schedule for LEED?

asked 2017-09-08 09:08:52 -0500

DW gravatar image

I am struggling to reduce my unmet hours for my LEED baseline building in OpenStudio 2.2. The building is occupied M-F 7:00-17:00. However, when I set my HVAC Operation schedule to follow this, I end up with 500 unmet hours during unoccupied periods. When I set the HVAC Operation schedule to "Always On", I end up with 0 unmet hours.

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The proposed building uses a VRV which states that: "The VRV shall enter an unoccupied mode - should the ambient space temperatures request conditioned air, the associated fan coil and condensing unit shall be started, and will operate to meet the unoccupied setpoint. The supply fan should be off during the unoccupied schedule." The proposed model has about 113 unmet hours whether or not the HVAC operation schedule is set to Always On or only on during occupied mode.

What are the correct settings to meet the 90.1 requirements for night cycling of the baseline system? If it is only supposed to be on during occupied periods, and if so, how do I reduce unmet hours?

Further notes on the model: The building is a medium office. I have modeled a Type 5 packaged VAV system. My tolerance is 2 F. The heating and cooling setpoints are 70 and 74 respectively with setbacks of 68 and 75. My design day setpoints are defined as constant 70/74 for heating and cooling. The air loop setpoint manager is of type "warmest" with a minimum of 54 and a maximum of 60. The zone sizing is based on "temperature difference" and that difference is set to 20 F. The unmet hours are for both cooling and heating, with twice as many cooling unmet hours. I am using the ddy file associated with this location that I downloaded from the energyPlus weather website.

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answered 2017-09-08 10:17:32 -0500

Eric Ringold gravatar image

updated 2017-09-08 10:19:59 -0500

I've always interpreted G3.1.2.5 (G3.1.2.4 in 90.1-2007) to mean "HVAC Operation Schedule" is set to the occupancy schedule, and "Use Night Cycle" set to "Cycle on Full System if Heating or Cooling Required".

This is how the openstudio-standards gem sets up baseline systems, in this method. As explained in the comments above the def, the code assumes 'occupied' for hours when the people occupancy schedule value is greater than 0.15.

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It's helpful knowing to set the operation schedule to be only occupied. However, that leaves me with many unmet hours. Can you think of any reason why when scheduled this way the HVAC system is failing to meet the heating and cooling requirements? (the setbacks are fairly small).

DW gravatar imageDW ( 2017-09-08 11:07:05 -0500 )edit

It could be a consequence of the system responding to the setback/setup, i.e. the system is responding slower than the thermostat change and the load. You might want to extend the thermostat hours (so, e.g. cooling sets down an hour before the occupied time). Find out what zones have unmet hours, when they occur and what the system is doing during those hours. Also see this answer for general troubleshooting steps.

Eric Ringold gravatar imageEric Ringold ( 2017-09-08 11:21:48 -0500 )edit

Thank you. I found out that the unmet hours were coming as a result of not looking at my occupancy schedules closely enough. The default medium office occupancy schedule is non-zero (has occupants) before 7:00 and after 17:00. Since my HVAC operation schedule was on only between 7:00-17:00, it was due to this discrepancy that I was getting so many occupied unmet hours. Thank you for the suggestions and insight.

DW gravatar imageDW ( 2017-09-08 12:51:37 -0500 )edit

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Asked: 2017-09-08 09:08:52 -0500

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Last updated: Sep 08