VAV & CAV Heating

HVAC tab in model data - Compact HVAC

Heating in VAV and CAV systems can be provided through any or all of: AHU heating, zone reheat and radiators/baseboards

Heating capacity

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.

Heating Plant (Building Level)

Boiler

At building level you can select the boiler to be used to provide hot water to all heating coils. Only one boiler can be specified for the whole building, i.e. there is only a single hot water circuit served by one boiler in Compact HVAC simulations.

Outside Air Preheat Coils (Building Level)

AHU preheat data is specified for the whole building and is not accessible at zone level.

Coil type

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).

Off-coil air temperature set point

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.

AHU Heating (Building Level)

The central AHU heating data is specified for the whole building and is not accessible at zone level.

Coil type

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.

Heating coil setpoint reset type

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 to be lower than the design value when the outdoor temperature is milder according to the conditions defined below.

The default is 1-None.

Take care when using 2-Outdoor air temperature reset as the resultant heating off coil temperature may sometimes be higher than the cooling coil temperature in which case simultaneous heating and cooling can occur if both are switched on at the time. This will not be obvious from looking at the heating and cooling off-coil temperature set points in the interface. In particular, if you also have an AHU cooling coil you must make sure that it does not operate simultaneously with the AHU heating coil by settings the appropriate AHU coil operation schedules.

Design off-coil set point temperature

When AHU heating coils are specified, this data defines the design 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.

You should make sure that this value is lower than the AHU cooling coil off-coil temperature at all times to avoid simultaneous heating and cooling.

Corresponding outdoor low temperature

When using Outdoor air temperature reset, this is the low outdoor temperature at and below which the Design off-coil set point temperature is applied.

Min off-coil set point temperature

Required when using Outdoor air temperature reset, this data defines the temperature of the air coming off the coils when the outdoor temperature is at or above the outdoor high temperature below. This value should be lower than the Design off-coil set point temperature.

Corresponding outdoor high temperature

Required when using Outdoor air temperature reset, this data is the high outdoor temperature at and above which the Min off-coil set point temperature is applied.

VAV Terminal Reheat Coils (Zone level)

Reheat data is specified for individual zones.

Coil type

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.

Zone damper heating action

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.

Radiator/Baseboard heating (Zone Level) (Disabled in v3)

Radiator/baseboard heating is controlled to meet any remaining zone load not met by other equipment in the zone that have higher heating priority.Radiators/baseboards operate according to the same set of time varying zone heating temperature setpoints as the main heating system.

To use radiant/baseboard heating switch on the option under the Heating header on the HVAC tab and enter information on:

Type

Choose between:

1-Hot water - where the radiator/baseboard coils are served by hot water from the main heating system. In this case you can also enter a radiant fraction to model radiant heat (more below).

2-Electric - where the radiator/baseboard coils are heated by electricity and heating is considered to be entirely convective.

Radiant fraction

Radiant fraction specifies what fraction of the power input to the radiator/baseboard heater is actually transferred to the space as radiant heat. The fraction should be between 0 and 1. This is the portion of the total power that is modelled as radiant, the remaining energy is added to the zone as convective heat transfer. The portion that is radiant heat transfer from the baseboard heater is distributed to people and specific surfaces using the remaining fields.

This option is only available for hot-water radiator/baseboard systems.

Technical Notes

Entering a value greater than zero will cause a the EnergyPlus ZoneHVAC:Baseboard:RadiantConvective:Water system to be used instead of the default ZoneHVAC:Baseboard:Convective:Electric system as a post-process to the idf data.

The radiant heat is transferred to the floor, or if the floor is smaller than 1m2, then to the largest other surface in the zone.

A default of 10% of the radiant heat is assumed to fall directly on the zone occupants and this is subsequently convected into the zone.

Local Heating Control (Zone Level)

Heating operation schedule

This schedule data is used in conjunction with the heating setpoint temperatures on the Activity tab to define the time-varying heating 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.