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AHU control with Night Cycle

asked 2016-04-25 04:31:45 -0600

Urado gravatar image

I have an AHU which is turned off during night and weekends. I want to add a control to turn it on if the temperature in my zone drops below 14°C.

Until now I was using the Availability Schedules of my objetcs (coils, air terminal, outdoor air controller...) to stop them. Then I found the HVAC Operation Schedule and Night Cycle option. Is this the right way to use it ? :

  • Set the Availability Schedules of all HVAC objects to always on.
  • Set the HVAC Operation Schedule to turn off HVAC during nights and weekends.
  • Set a thermostat to the setpoint (14°C for me) during nights and weekends.
  • Use Night Cycle: Cycle on Full System if Heating or Cooling Required.

If this is correct, I have a question: Is it possible to control the behavior of the system during Night Cycle ? If my temperature drops below 14°C I want to heat until it reaches 16°C and then turn it off again.

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I found EnergyPlus has more options than OpenStudio to control it (at least without measures). However, we can only choose the Cycling Run Time and not a Temperature.

For information, OpenStudio set this time to 3600s, it should be useful to have the possibility to change it in the GUI.

Urado gravatar imageUrado ( 2016-04-25 04:49:27 -0600 )edit

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answered 2016-04-25 05:33:14 -0600

Chandan Sharma gravatar image

That is the correct approach. In step 2 above, 'HVAC Operation Schedule' should be used for fan only and not for any other parent coil object. Night cycle availability manager internally overrides the fan schedule only. There are plenty of example files which use night cycle manager which can be very helpful.

If your setback thermostat temeprature is 14 C and thermostat tolerance is 0.2, the night cycle will turn on when control zone temperature is 13.9 C [i.e. 14-{0.2/2}]. Once the system is on it will remain on for the Cycling Run Time specified by the user. AvailabilityManager:HighTemperatureTurnOff is generally used for solar collector control but you can try using it in combination with night cycle manager in availability manager assignment list and carefully review the results. High temperature turn off can sense zone node temperature and turn off the system if it exceeds 16 C. Of course, this will work for only one control zone. For multiple zones, EMS can be an option to turn off the heating coil schedule at 16C.

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Thanks for your answer, I will try the AvailabilityManager:HighTemperatureTurnOff if it can be used in combination with night cycle.

I saw that the HVAC Operation Schedule is only for fan but when the fan is off, the coils are also turned off (even with always on schedule). I checked this with the output Heating Coil Heating Energy which is null during nights and weekends when T°zone > 14°C, as expected.

Urado gravatar imageUrado ( 2016-04-25 05:45:11 -0600 )edit

Agree, when the fan is off, the loop is also off and there is no heat transfer. The limited point above was that night cycle manager doesn't modify the original schedule. It only overrides the fan schedule internally. If coils use the same schedule as fan and are off at night, even if the night cycle signals the fan to be on, there won't be any heat transfer since the coil is scheduled off.

Chandan Sharma gravatar imageChandan Sharma ( 2016-04-25 05:59:06 -0600 )edit

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Asked: 2016-04-25 04:31:45 -0600

Seen: 543 times

Last updated: Apr 25 '16