HVAC tab in model data - Compact HVAC
You can enter details on the cooling system including the maximum capacity (at zone level).
The Cooling capacity can either be entered by hand or can be Autosized using the Cooling Design Calculations. If, immediately prior to a Simulation the Cooling capacity has not been entered (neither by hand nor from a previous Cooling Design Calculation) then a Cooling design autosizing simulation is started to calculate the capacity. The Plant sizing building model option controls the way this works.
You can change cooling capacities by hand if you wish but you should bear in mind that, by default, if Model Options change, all heating and cooling capacities are reset to zero. Set the Plant sizing model option to '2-Manual' to avoid this happening.
Select the chiller to be used to provide cold water to cooling coils. Only one chiller can be specified for the whole building in Compact HVAC, i.e. there is only a single cold water circuit served by one chiller in Compact HVAC simulations.
Enter the chilled water setpoint temperature to be used for sizing the chilled water loop and associated equipment.
Enter the condenser water temperature to be used for sizing the condenser loop and associated equipment.
Select from:
The EnergyPlus equipment object types which are used are CoolingTower:SingleSpeed or CoolingTower:TwoSpeed.
The central AHU cooling coil data is specified for the whole building and is not accessible at zone level.
The cooling coil must be one of:
The default is 2-Chilled water.
Note: you can achieve 'free cooling' without a cooling coil by selecting 1-None but you will only get the cooling effect of the HVAC air on the zone if the outside air + any fan pickup is cooler than the zone air temperature.
The difference between switching cooling off altogether (by unchecking the Cooled checkbox) and setting cooling coil type to 1-None is that in the latter case controlled free cooling is possible due to relatively cool outside air being introduced to the building but when cooling is switched off altogether there is no possibility of controlled cooling through the HVAC system.
This data defines the temperature of the air coming off the coils assuming idealised control. Cooling coils are automatically sized by EnergyPlus before the simulation to provide this off coil temperature given the flow rates in the air handling unit.
Select the type of automatic reset control for the central cooling supply air temperature. The choices are:
The 2003 ASHRAE Applications Handbook discusses warmest reset control for constant volume systems (p.41.27) and calls it "Supply Air Temperature Reset for Constant Air Volume (CAV)". A requirement is set out in ASHRAE Std 90.1-2004, Appendix G, paragraph G3.1.3.12: Supply air temperature shall be reset based on zone demand from the design temperature difference to a 10°F temperature difference under minimum load conditions. Design air flow rates shall be sized for the reset supply air temperature, i.e., a 10°F temperature difference.
Tip: using Warmest or Outside air temperature cooling coil setpoint reset can drastically improve energy efficiency by reducing the cooling energy delivered to the airstream to the minimum required for occupant comfort.
The DesignBuilder HVAC template default is Warmest for constant volume systems and None for other system types.
When using Outdoor air temperature reset, this is the high outdoor temperature at and above which the Design off-coil set point temperature is applied.
Required when using Outdoor air temperature reset and Warmest options, above. When using Outdoor air temperature this data defines the temperature of the air coming off the coils when the outdoor temperature is at or below the outdoor high temperature below.
When using Warmest reset, this is the upper limit on off-coil cooling set point temperature.
In both cases, this value should be higher than the Design off-coil set point temperature.
Required when using Outdoor air temperature reset, this data is the low outdoor temperature at and below which the Min off-coil set point temperature is applied.
This schedule data is used in conjunction with the cooling setpoint temperatures on the Activity tab to define the time-varying cooling setpoint schedule in the zone. The operation schedule defines the times when full and setback setpoints should be met and the setpoint data on the Activity tab define the actual setpoint values. See Defining setpoint temperature schedules for more on this.
