Unitary Heating

HVAC tab in model data - Compact HVAC

Heating capacity

This data is only visible at zone level and only when the Manual sizing model option is 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.

Heating Plant

A single set of heating plant data is required per building and all unitary zones in the building refer to this data when plant fuel consumption is calculated.

Unitary heating fuel

This data is only visible at building level (even for Unitary single zone systems), that is all unitary heating systems in the building must use the same fuel. Select the type of fuel used to generate the heating energy - choose from:

You should select 1-Electricity from grid you wish the results displayed in DesignBuilder to agree with those calculated by EnergyPlus directly and displayed on the Summary tab of the Simulation screen.

Heating coil CoP

This data is only visible at building level, that is all Unitary heating systems have the same CoP. The CoP can only be entered for non-electric heating coils. Electric heating coils are assumed to a have a CoP of 1.

The Unitary heating system Coefficient of Performance is only 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.

You should enter a value of 1 if you wish the results displayed in DesignBuilder to agree with those calculated by EnergyPlus directly and displayed on the Summary tab of the Simulation screen.

Unitary distribution loss

This data is only visible at building level, that is all unitary heating systems have the same distribution loss. The distribution loss is an additional loss accounting for the loss of heat to outside due to distribution of hot fluids through the building. It acts in combination with the CoP to increase the fuel consumption based on the heating load.

You should enter a value of 0 if you wish the results displayed in DesignBuilder to agree with those calculated by EnergyPlus directly and displayed on the Summary tab of the Simulation screen.

Radiator/Baseboard heating (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.

With unitary single zone 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:

 

• 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.

Mode

This option is only available for single zone unitary systems:

 

• 1-Supplements main heating - where the radiator/baseboard coils provide additional backup heating in cases where the heating setpoint temperature has not been satisfied by the main heating system.
• 2-Replaces main heating - where the radiator/baseboard coils are used instead of the main unitary single zone heating coils.This option allows you to model radiant heating 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

1. 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.
2. 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. 
3. 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.

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.

Operation

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.