Heating

HVAC tab in model data - Simple HVAC

The data required to define the heating system will depend on whether the HVAC model option is set to Simple or Detailed and if using Detailed the type of Detailed HVAC system selected.

Note: for all systems, heating setpoint temperatures are defined on the Activity tab.

Heated

If the zone is to be heated then check this checkbox. When it is checked the rest of the items described on this page become available.

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

Heating 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 for District heating and displayed on the Summary tab of the Simulation screen.

Heating system CoP

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.

Note: When comparing the heating fuel consumption results with the data displayed on the Summary tab, it is important to realise that DesignBuilder applies a fuel type and a CoP to the idealised heating systems used in EnergyPlus when using Simple HVAC. On the other hand EnergyPlus displays the unprocessed loads as District heating in the tables on the Summary tab. This is because the idealised systems in EnergyPlus are considered to be met directly without use of HVAC equipment through a district heating type system.

Local Heating Units

Heating type

There are two heating types available:

• 1-Convective - the space is heated by an air system and controlled to the air temperature set point.  The system is modelled using the EnergyPlus "Ideal Loads" system.
• 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'.

1-Convective

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

Maximum supply air delivery temperature

The maximum dry-bulb air temperature of the air supplied for heating the zone (in °C or °F).

Maximum 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 0.0156. 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.

Heating limit type

There are 3 ways that EnergyPlus ideal loads systems can limit zone heating capacity:

• 1-Limit flow rate - means that the heating supply air flow rate will be limited to a flow rate value calculated based on the autosizing model option settings. When using the 3-Autosize HVAC sizing method the flow rate limit is calculated by EnergyPlus sizing calculations. Otherwise if the DesignBuilder sizing method is used the flow rate is calculated based on the required heating capacity calculated by DesignBuilder and the Maximum supply air delivery temperature (above).
• 2-Limit capacity - means that the sensible heating capacity will be limited to a capacity value calculated based on the autosizing model option settings. When using the 3-Autosize HVAC sizing method the capacity is calculated by EnergyPlus sizing calculations. Otherwise if the DesignBuilder sizing method is used the flow rate is calculated based on the required heating capacity calculated by DesignBuilder and the Maximum supply air delivery temperature (above).
• 3-Limit flow rate and capacity - means that both flow rate and capacity will be limited to the above values.

Technical description

 

EnergyPlus uses the following 4 settings to define the maximum heating input for ideal loads systems:

 

• Maximum Heating Supply Air Temperature

• Maximum Heating Air Flow Rate,

• Maximum Sensible Heating Capacity

• Heating Limit Type

 

When using the default 3-Autosize HVAC sizing Model option the "Maximum Heating Supply Air Temperature" and "Heating Limit Type" IDF fields are set using corresponding data from the HVAC tab, and the "Maximum Heating Air Flow Rate" and "Maximum Sensible Heating Capacity" fields are calculated by EnergyPlus before the simulation in the design sizing calculations.

 

Alternatively, when using either of the 2-Manual or 4-Autosize when not set HVAC sizing Model options, the "Maximum Heating Supply Air Temperature", "Maximum Sensible Heating Capacity" and "Heating Limit Type" IDF fields are set using corresponding data from the HVAC tab, and DesignBuilder calculates the corresponding "Maximum Heating Air Flow Rate" based on the user-defined Heating capacity and Maximum supply air delivery temperature data. In this case all of the 4 related IDF fields are specified in the IDF by DesignBuilder.

 

Or when the 1-Adequate HVAC sizing Model option is selected, DesignBuilder sets the EnergyPlus "Heating Limit Type" IDF field to "NoLimit" which means that the heating system can meet any load. In this case the other 3 fields are not needed and so are not provided in the IDF.

 

In all of the above cases, apart from when the 1-Adequate HVAC sizing Model option is selected, EnergyPlus holds values for each of the 4 settings used to define the maximum heating input for ideal loads systems, and the way these are used in the simulation is defined by the Heating limit type per the above descriptions.

Note: In practice this setting should not make a noticeable difference to the thermal heat balance of the zone during heating since the maximum heat delivery to the zone is already defined before the simulation and this setting simply tells EnergyPlus exactly how to apply the limit.

2-Radiant/Convective Units

When the Simple HVAC, 1-Room ventilation Mechanical ventilation and 1-DesignBuilder Simple HVAC autosizing method model options are set you can model radiant heating systems. Set the heating system type as 2-Radiant/convective units (above) and enter the heating radiant fraction and the way radiant heat is distributed through the space.

Important: Recent versions of EnergyPlus no longer support the radiant/convective option combined with Simple HVAC cooling systems, so if you need radiant heating and cooling in the same zone then you should switch to Detailed HVAC.

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.

Radiant distribution

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.

Technical Note

DesignBuilder uses the EnergyPlus 'HIGH TEMP RADIANT SYSTEM' to model radiant heating systems when in Simple HVAC mode. This system type is intended for modelling 'high-temperature' radiant sources but it can equally well be used for low temperature radiant sources.

Note: for all Simple HVAC systems, heating setpoint temperatures are defined on the Activity tab.

Sizing Zone Equipment

By default EnergyPlus does not account for mechanical and natural ventilation (outdoor air) loads in Heating design calculations so the options below are provided to add an extra load for Winter design day only.

Mechanical ventilation load

Define how mechanical ventilation loads should be dealt with in Heating design calculations. Options are:

• 1-Met by zone equipment where mechanical ventilation loads are met entirely by zone equipment and so the capacity of the zone heating systems must account for mechanical ventilation loads.
• 2-Exclude where the mechanical ventilation loads are not met by zone heating equipment so zone heating sizing calculations can safely ignore them.
• 3-Partially met by AHU where the mechanical ventilation loads are partially met by heating coils in the AHU but there is still some residual load to be met by zone heating equipment. In this case the supply air temperature must also be entered. See below.

Note: The default Mechanical ventilation load setting is 1-Met by zone equipment. For DOAS systems which introduce air that is partially or fully pre-heated, this setting should be changed to 2-Exclude or to 3-Partially met by AHU to ensure that zone equipment is not oversized.

Note: This setting applies only to Heating design calculations. For Detailed HVAC simulations the DOAS settings on the HVAC zone group dialog are used instead to account for any additional loads due to mechanically supplied fresh air. For Simple HVAC the full outdoor air load is assumed when sizing heating and cooling systems, no preheat/precool is assumed when sizing zone equipment.

System supply temperature

When Mechanical ventilation load is set to 3-Partially met by AHU then you must enter the supply air temperature to enable the residual additional zone loads required to be accounted for.

Exclude heat recovery from zone heating sizing

If the savings provided by heat recovery can be safely included in the heating sizing calculations then leave this checkbox unchecked. Otherwise for a safer calculation where zone heating systems are sized based on no heat recovery being available then check this option.

Natural ventilation load

If natural ventilation is activated then you can define how natural ventilation loads should be dealt with in Heating design and simulation autosizing simulations. Options are:

• 1-Met by zone equipment the default option where natural ventilation loads are met entirely by zone equipment and so the capacity of the zone heating systems must account for natural ventilation loads. Note that in Heating design calculations, when this option is selected, natural ventilation will be applied without regard for schedule or mechanical ventilation setpoint temperature settings.
• 2-Exclude where the natural ventilation loads are not met by zone heating equipment so zone heating sizing calculations can safely ignore them.

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.

Note: the zone heating operation schedule is also used in Detailed HVAC with the default 1-Simple HVAC Detailed HVAC Activity data setting.