HVAC tab in model data
The data required to define the heating system will depend on whether the HVAC model option is set to Simple or Compact and if set to Compact the type of Compact HVAC system selected.
The heating capacity can either be entered by hand or can be Autosized using the Heating Design Calculations. By default, if immediately prior to a Simulation the heating capacity has not been entered (neither by hand nor from a previous Heating design calculation) then a Heating design autosizing simulation is started to calculate the capacity. The Plant sizing building model option controls the way this works.
You can change heating 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.
If the HVAC system is Unitary multizone then the central heating coil capacity is calculated as the sum of the individual zone heating energy requirements.
Select the type of fuel used to generate the heating energy - choose from:
The heating system coefficient of performance is used to calculate the fuel consumption required to meet heating demand. The value represents the total seasonal efficiency of the entire heating system and should include the effect of all energy consumption associated with building heating such as fan and pump energy, boiler inefficiency, control equipment etc.
Heating system CoP data is specified by zone so you can model different heating efficiencies in each zone. But generally you should access this data from the building level and allow zones to take on building default.
Heat generators are devices such as boilers and heat pumps used to heat water for distribution to heating coils, radiators and DHW systems. The heat generator coefficient of performance is used to calculate the fuel consumption required to meet heating demand. It represents the total seasonal efficiency of the boiler excluding losses/consumption due to external pumps and fans but including all energy consumed by such ancillary devices within the boiler.
CoP is entered as a fractional value (not % value). So a boiler might have a CoP of 0.89.
Heat generator CoP data is specified for the whole building, i.e. a single hot water circuit (served by one boiler) is simulated.
Part-load boiler/heat pump performance cannot be calculated in DesignBuilder v.1.
The heating distribution loss is the loss of heat due to the distribution of hot water/air around the building. It is used to increase the heating load prior to calculating boiler heating energy consumption.
AHU preheat data is specified for the whole building and is not accessible at zone level.
The preheat coil is located in the outside air stream, upstream of the outside air mixing box and tempers the outside air. If no preheat coil is used in the VAV system, then the option 1-None should be specified here. Otherwise this indicates the type of preheat coil. It is unlikely that both a heating coil and a preheat coil would be used at the same time. The options are:
1-None - no preheat.
2-Electric - preheat coils powered by electricity (assumed 100% efficient, 0 distribution loss).
3-Gas - gas-fired preheat coils (assumed 80% efficient, 0 distribution loss).
When preheat is specified, this data defines the temperature of the air coming off the preheat coils assuming idealised control. Preheat coils are automatically sized by EnergyPlus before the simulation to provide this off coil temperature given the flow rates in the air handling unit.
The central AHU heating coil data is specified for the whole building and is not accessible at zone level.
The main central heating coil is located in the supply air stream, upstream of the cooling coil and after the outside air mixing box. If no central heating coil is used in the VAV system, then the option 1-None should be specified here. Otherwise this indicates the type of heating coil. It is unlikely that both a heating coil and a preheat coil would be used at the same time. The options are:
1-None - no heating.
2-Electric - heating coils powered by electricity (assumed 80% efficient, 0 distribution loss).
3-Hot water - heating coils supplied with hot water generated by a boiler.
4-Gas - gas-fired heating coils (assumed 80% efficient, 0 distribution loss).
If you select 3-Hot water main heating coils, the program generates data to simulate the boiler and the pumps required to serve the heating coil.
When main heating coils are specified, this data defines the temperature of the air coming off the coils assuming idealised control. Heating 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 heating supply air temperature for CAV and VAV system. The choices are:
1-None – no reset, use the Off-coil air temperature set point in conjunction with the Zone setpoint schedule.
2-Outdoor air temperature reset – reset the heating supply air temperature based on the following default rules. When the outdoor dry bulb temperature (ODB) is at or below –6.7C the setpoint is the Off-coil air temperature set point. When the ODB is at or above 10.0C the setpoint is 20.0C. In between, the setpoint is varied linearly. These control parameters are from ASHRAE 90.1 Appendix G.
The default is 1-None.
Note: you should be aware when using 2-Outdoor air temperature reset that the effective off-coil setpoint for the AHU heating coil used in the simulation will be between 20C and the value you enter for the Off-coil air temperature set point. If you also have an AHU cooling coil you must make sure that it does not operate simultaneously with the AHU heating coil by making the appropriate AHU coil operation schedules.
Reheat data is specified for individual zones.
Use this data to either switch reheat off or select either electric or hot water reheat coils. The options are:
1-None - no heating.
2-Electric - heating coils powered by electricity (assumed 100% efficient, 0 distribution loss).
3-Hot water - heating coils supplied with hot water generated by a boiler.
If you select 3-Hot water main heating coils, DesignBuilder generates data to simulate the boiler and the pumps required to serve the reheat coil.
During heating operation, there are two control options for the damper controlling the air flow in the VAV terminal unit as the zone moves above or below the zone setpoint. With both control options, the damper is at the minimum air flow rate whenever the zone temperature is between the cooling and heating setpoints.
1-Normal - the damper will remain at the minimum air flow rate during heating operation. As the heating load increases, the water flow rate in the reheat coil will be increased to maintain temperature in the zone until the maximum water flow rate is reached. The system essentially acts like a constant volume system during heating.
2-Reverse - the air flow rate through the VAV box may increase above the minimum air flow when the heating demand cannot be satisfied unless greater air flow is provided. As the heating load increases, the unit starts at minimum air flow and minimum hot water flow. The hot water flow is increased until it reaches maximum flow, then the air damper starts to open to meet the load. This option is used if the minimum air flow rate is not adequate to serve the peak heating load.
Note: you should switch heating 'on' if you want the zone to be heated to the heating setpoint temperature specified on the Activity tab.
There are currently two heating types:
1-Convective - the space is heated by an air system and controlled to the air temperature set point. The system is modelled using EnergyPlus 'purchased air'.
2-Radiative/convective units - used for modelling general systems where a radiant heat is a factor. Allows modelling of convective systems, hot water radiator systems, underfloor heating systems, baseboard heating etc. using EnergyPlus 'High Temp Radiant System'.
When using Simple HVAC you can specify the heating supply air conditions of the air.
The constant dry-bulb air temperature of the air supplied for heating the zone.
The constant humidity ratio (mass of water per mass of dry air) of the warm supply air to be delivered when heating is required. The default humidity ratio is 0.01.
When the Simple HVAC model option is set you can model radiant heating systems. Set the heating system type as '2-Radiant/convective units' (above) and enter the heating radiant fraction, control method and the way radiant heat is distributed through the space:
The fraction of the power input to the radiant heater that is actually radiant heat transfer. The fraction should be between 0 and 1. In conjunction with the Radiant distribution option, it defines the breakdown of how the power input to the heater is distributed to the rest of the zone.
Radiant units can be controlled in one of three ways:
2-Operative - control the room operative temperature (MAT+MRT) / 2 to the heating setpoint temperature specified on the Activity tab.
3-MRT - control the room mean radiant temperature to the heating setpoint temperature specified on the Activity tab.
This option overrides the overall simulation control radiant fraction option.
Note: You should ideally use Operative control if you want to calculate realistic heating energy because heating systems controlled using the operative temperature continue to heat the building until comfort conditions are met (just that they are in the real building). Also, the default heating temperature set points from the Activity templates are derived from sources quoting Operative temperatures. With MAT control the room air temperature is controlled to the heating set point temperature, which (depending on internal radiant temperatures) may not necessarily be comfortable.
The Radiant distribution allows you to control how the radiant heat from the Radiant unit is distributed around the room. Options are:
1-Uniform - radiant heat is distributed uniformly around the zone.
2-Floor - heat is distributed uniformly across all floors in the zone. Use this for modelling floor heating systems.
3-Wall - heat is distributed uniformly across all walls in the zone. Use this for modelling hot water radiators and other radiant heaters close to wall surfaces.
Technical Note: DesignBuilder uses the EnergyPlus 'HIGH TEMP RADIANT SYSTEM' to model radiant heating systems. This system type is intended for modelling 'high-temperature' radiant sources but it can equally well be used for low temperature radiant sources.
When using the 1-VAV system type, you can set the operation schedule for the central air handling unit heating coils separately from the schedules for the local reheat coils. The operating schedule for the central heating coils is set under the AHU Heating Coil Operation header at building level. Local reheat coils are controlled using Operation data at zone level.
When using the 2-Unitary multizone system type, the operating schedule for the central heating coils is set under the AHU Heating Coil Operation header at building level.
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.
Heating operation schedules are set at zone level and there is no central plant apart from the boiler.
Note: for all systems, heating setpoint temperatures are defined on the Activity tab.
This schedule data is used in conjunction with the heating setpoint temperatures on the Activity tab to define the heating demand in the zone by creating a heating 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.