Heating

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.

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.

 

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.

Heating Plant

Fuel

Select the type of fuel used to generate the heating energy - choose from:

 

Heating system CoP (Simple HVAC only).

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 any boiler fan energy, boiler inefficiency, control equipment etc. It does not include energy involved in air or water distribution (fans or pumps) or control gear, which is accounted for in the Auxiliary energy.

 

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 generator CoP (Compact HVAC only).

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.

Heating distribution loss (%)

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.

Radiator/Baseboard heating

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. You can use radiator/baseboard heating with these Compact HVAC systems:

 

 

When used with a unitary single system you have the option for the radiator/baseboard heating to either supplement the main heating coil or to replace it. With the other system types the radiators/baseboard heater always supplements the main heating though you can operate the system without terminal reheat and/or AHU heating coils and heat purely by baseboard heating.

 

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

Type

Choose between:

 

Mode

This option is only available for single zone unitary systems:

 

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

VAV Outside Air Preheat Coils

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:

 

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.

VAV and CAV Central Heating Coils

The central AHU heating coil 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:

 

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:

 

 

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 main 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

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:

 

 

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.

 

 

Note: you should switch heating 'on' if you want the zone to be heated to the heating setpoint temperature specified on the Activity tab.

Local Heating Units

Heating type (Simple HVAC only)

There are currently two heating types:

 

1-Convective

When using Simple HVAC  you can specify the heating supply air conditions of the air.

Supply air delivery temperature

The constant dry-bulb air temperature of the air supplied for heating the zone.

Supply air humidity ratio

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. You should be careful to enter a value that is a valid condition, i.e. it is under the 100% saturation line on the Psychrometric Chart.

2-Radiant/Convective Units

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.

EnergyPlus 3.1 no longer supports this radiant/convective option combined with Simple HVAC cooling systems so if you need radiant heating and cooling then you should switch to Compact HVAC and use a Unitary single system with hot-water radiator/baseboard heating.

Heating radiant fraction

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.

Control method

Radiant units can be controlled in one of three ways:

 

 

This option overrides the overall simulation control radiant fraction option.

 

Note: Operative control can be useful for calculating realistic heating energy because heating systems controlled using the operative temperature continue to heat the building until comfort conditions are met (just that they probably 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.

Radiant distribution

The Radiant distribution allows you to control how the radiant heat from the Radiant unit is distributed around the room.  Options are:

 

Technical Note

VAV

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.

Unitary Multizone

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.

Unitary Single zone

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.

Zone setpoint schedule

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.