HVAC Operation with Schedules and Days

Summary

  • A building's HVAC operation depends on when the building is on or off, what the system is and what the setpoints are. 
  • The Space use tab offers zone by zone control of setpoints and on/off controls. 
  • A project can have different space use templates applied to different zones which means that a central system can be supplying some zones that are "on" and some zones that are "off" at the same time.
  • A zone's is considered to be "occupied" if HVAC is "on" in a given timestep.
  • Natural Ventilation can cause parts of the HVAC system to be off at different times.
  • This article explains how this all works.

Controls Overview - What turns the HVAC components on or off

For each zone, there are important controls that determine whether the HVAC system is running and how. These are set in the Space Use tab and can be saved to templates applied to specific zones.

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HVAC Schedule

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The operating hours determine:

  • Whether the temperatures the building is conditioned to are "setpoints" or "setbacks"
  • Whether the ventilation is on
  • Whether windows can open
  • Whether we consider the building "occupied" for any calculation that looks at occupancy

The setback to setpoint ramp up time determines how many hours before the start time we make the HVAC strive to achieve the setpoint range instead of the setback temperature range. 

In the example above,

  • The building is occupied between 6am and 10pm
  • Ventilation is on between 6am and 10pm
  • The heating and cooling system strives to achieve the setpoint range from 5am to 10pm
  • The heating and cooling system strives to achieve the setback range from 12 midnight to 5am and from 10pm to midnight.

 Design Temperatures

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Setpoint temperatures

  • These are the temperatures the building is trying to maintain when the HVAC system is on and also before the HVAC system comes if there is a ramp-up time specified.
  • Windows are assumed to operate such that these temperatures are maintained. This is true even if they're operated when the building is "unoccupied"

Setback temperatures

  • These are the temperatures the building tries to maintain when the HVAC system is off and when it is not in ramp-up mode. Note: this means heating and cooling can operate outside of HVAC operating hours.
  • A high difference between setpoints and setback may require more ramp up time and more capacity but will use less energy. 

Making your own "Unconditioned", "uncooled" or "unheated" Space

  • Setting both setback and setpoint temperatures to the limits of their extents is a way to create an "unconditioned" space. 
  • Turning down the lower end will eventually switch off all heating. Raising the upper end of the temperature range will eventually switch off cooling. When these systems switch off will depend on climate, glazing extent and material properties.
  • Doing this increases the chance of EnergyPlus errors due to convergence failures. If this happens, trying relaxing the differences or using ignored for unconditioned zones.
  • Don't try to do this in a naturally ventilated space. This strategy only works for buildings without natural ventilation.

 Day Schedules

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The day schedules determine when the internal loads and HVAC systems are "on".

Internal loads work as follows

  • Days when internal loads are applied will use the the diversity schedule to determine the hourly % of load applied to the internal loads.
  • If there are days of the week internal loads are not applied, Sefaira will apply a 5% diversity every hour during these days for equipment, lighting and people. 

HVAC system operation works as follows

  • Days of the week the HVAC system is "On" will have the HVAC "On" based on the HVAC schedule notes above during hours specified for it to be on.
  • Days of the week the HVAC system is "Off" will operate exactly the same as hours when the HVAC system is off (see HVAC schedule above).

The days each component are on will always match if possible (eg if both say 5-days they will be the same 5-days, if 1 says 5-days and another says 6-days they will have 5 days per week in common).

 

Here are some schematic examples of "On" days and "Off" days

 

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Notes about Natural Ventilation

Some potentially counter-intuitive aspects of natural ventilation for some users:

  • Windows open and close to maintain setpoint temperatures. Do not make extreme setpoint ranges to try and mimic natural ventilation. Use the setpoints to control the window operation.
  • Having windows open when the windows are unoccupied will only have them open and close to maintain the setpoint range, not the setback range. This is to prevent buildings cooling too much overnight and then needing energy to heat back up again.
  • Other natural ventilation controls turn components of the HVAC system on or off as well and these controls override the space use controls.
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    Leland Curtis

    It appears that a single diversity schedule controls Lighting, Equipment, and People loads. Is there any way to provide unique diversity schedules for each?