Re: subject: [EnergyPlus_Support]] Unmet Load Hours Linked to NightCycle Cycling Run Time?

Jean -

Thanks for the reply! The model is for LEED, so any amount of unmet hours over 300 is non-compliant for the baseline. I'm at about 700 for the zone i'm looking at.

The settings are a .2 deg C thermostat tolerance and a 3600 second cycle time for the availabilitymanager:nightcycle object. If it ultimately cycles off and lets the zone temp creep up before re-engaging, how do you keep the unmet load hours down for LEED?

I'm modeling the Appendix G System 7. I think that the HVACtemplate:VAV:System object can also do the type of nightcycle control that you described, although the fan mode is read from the overall availability schedule, and not the supply fan operating mode schedule.

Cheers,

On Thu, Dec 4, 2014 at 3:23 PM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Sounds right. I would expect it to cycle off at the end of the selected cycle time, especially if the zone is within setpoint. But the nightcycle object works with a temperature tolerance right? What are your settings?

Also, what are you modelling? A real design or a ASHRAE 90.1 system for compliance? I ask because Unitary equipment have a very handy fan object which can switch modes if you like on a schedule. You can define a schedule that fits to your occupation so that it is 1 during unoccupied times and zero during occupant times. You then let the fan mode read its mode from this schedule. 1=cycle to meet load; 0=continously on. Great for App. G. and better than nightcycle for many reasons (amounst others that as far as I remember nightcycle only works to cycle on for the heating setpoint, i.e. it only works for heating).

PS I may have gotten the 0 & 1 the wrong way round, so check me if you use that method.

Mit freundlichen Grüßen- Sent from my iPhone (excuse the brevity)

i. A.
Jean Marais
b.i.g. bechtold
Tel. +49 30 6706662-23

On 04.12.2014, at 20:53, "Jacob Dunn arch.dunn@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

Hi All -
I'm investigating the inability of my EnergyPlus model to maintain setpoint during unoccupied hours.
The short story:
For my centralized VAV system (modeled through template objects), it appears that the AvailabilityManager:NightCycle control (cycleonany) is working properly for the time input into the "Cycling Run Time" field (currently set for 3,600 seconds - 1 hour). However, at the end of the cycle, the equipment drops to zero for one timestep, before cycling back on for an hour. Thus, running the model with 6 timesteps, the cooling coil energy will have values for 6 timesteps, then a zero, then values again for 6 timesteps, then zero, etc. This causes a drop in the VAV damper position and an increase supply air temperature (cooling) for said timestep, which allows the zone temp to float above the setback. I have no idea why it is doing this.
Image of a Sunday and Monday - notice the cycling of the AHU-2 cooling coil energy and VAV terminal damper temperature (https://drive.google.com/file/d/0B6aA8ZsOIiibNGh5VWFSNjh0MFE/view?usp=sharing)
The long story:
100+ zone, 2-story school. I used the HVACTemplateSystem:VAV, HVACTemplateZone:VAV, and constituent plant objects to model 6 air handling units for the various thermal zones of the building.
The zone I'm analyzing is an office zone, and I double checked all internal loads to make sure nothing was out of whack.
I've looked at the chilled water temperature leaving the plant and entering the air handling units, and the water temperatures look good. Its only the air temperatures leaving the coil that oscillate with the cycling behavior.
I've tried to change the "Cycling Run Time field: in the AvailabilityManager:NightCycle, but the mysterious zero values still happen at the end. It's as if it takes a timestep for the system to realized it needs to come back on. I'm running the model at 6 timesteps, which is the only reason I saw the mysterious zero values in the first place.
Also, looking at the other 5 air handlers, they all exhibit the same cycling behavior - EXCEPT the gym zone. It is the only one that doesn't, but I'm fairly certain everything is modeled the same. Here's an image of all 8 air handling unit cooling coil energies, notice the cycling during the unoccupied times (https://drive.google.com/file/d/0B6aA8ZsOIiibZW80QUlaQXl3VWM/view?usp=sharing)
Link to the expanded .idf file - set up to run for 1 week in June, 6 timesteps (https://drive.google.com/file/d/0B6aA8ZsOIiibaV9hbUtvYTFQMU0/view?usp=sharing)
Link Hourly CSV File (https://drive.google.com/file/d/0B6aA8ZsOIiibbl9aQ3JVNTA3RHc/view?usp=sharing)
Cheers and thanks for any help!

Jacob Dunn - LEED AP

"Building services today are essentially mechanical compensations for the fact that buildings are bad for what they are designed for--human life" - Bjarke Ingels

Jacob Dunn - LEED AP
Architectural Simulation Specialist
University of Idaho Integrated Design Lab

"Building services today are essentially mechanical compensations for the fact that buildings are bad for what they are designed for--human life" - Bjarke Ingels