HVAC tab in model data
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.
When using Simple HVAC you can specify the cooling supply air conditions of the air. This data is used for simulations only - cooling design calculations have a specific equivalent setting.
The constant dry-bulb air temperature of the air supplied for cooling the zone.
The constant humidity ratio (mass of water per mass of dry air) of the cold supply air to be delivered when cooling is required. The default humidity ratio is 0.009.
Select the type of fuel used to generate the cooling energy - choose from:
The cooling system coefficient of performance is used to calculate the fuel consumption required to meet cooling demand. The value represents the total seasonal efficiency of the entire cooling system and should include the effect of all energy consumption associated with building cooling such as fan and pump energy, chiller inefficiency, control equipment etc.
Cooling system CoP data is specified by zone so you can model different cooling efficiencies in each zone. But generally you should access this data from the building level and allow zones to take on building default.
The chiller coefficient of performance is used to calculate the fuel consumption required to meet cooling demand. It represents the total seasonal efficiency of the chiller excluding losses/consumption due to external pumps and fans but including all energy consumed by such ancillary devices within the chiller.
Chiller CoP data is specified for the whole building.
The cooling distribution loss is the loss of heat (cooling energy) due to the distribution of cold water/air around the building. It is used to increase the cooling load prior to calculating chiller energy consumption.
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 3-Warmest for constant volume systems and 1-None for other system types.
When using the 2-Unitary multizone 5-CAV and 3-VAV Compact HVAC system types, the AHU cooling coil operation is defined by AHU cooling coil operation data.
Note: only one set of zone thermostatic control data is used per Unitary multizone system - this is the zone for which Thermostatic control zone for unitary system is selected.
Cooling operation schedules are set at zone level and there is no central plant apart from the chiller.
Note: for all systems, cooling setpoint temperatures are defined on the Activity tab.
This schedule data is used in conjunction with the cooling setpoint temperatures on the Activity tab to define the cooling demand in the zone by creating a cooling setpoint schedule. The 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.