Radiant Surface - Variable Flow

Similar to the constant flow radiant surface but control of the variable flow radiant surface is accomplished by throttling the hot and/or chilled water flow to the unit.

Note An important advantage of the EnergyPlus variable flow radiant surface over the constant flow equivalent is that it is fully autosizable.

Name

This is a read-only label that is automatically generated by the software and which incorporates the name of the zone in which the radiant surface is located.

Type

There are 3 types of radiant surface available:

 

Temperature control type

This option is only available when using the 2-Detailed HVAC Detailed HVAC Activity data

 

Along with setpoint (control) and water schedules, this setting allows you to specify how the radiant surface is to be controlled. The temperature denoted in the setpoint schedule can refer to one of five different temperatures: the zone mean air temperature, the zone mean radiant temperature, the zone operative temperature, the outdoor dry-bulb temperature, or the outdoor wet-bulb temperature. The choice of temperature is controlled by the temperature control type. The user must select from the following options:

 

 

Operative temperature for radiant surface controls is the average of Mean Air Temperature and Mean Radiant Temperature. See the control temperature schedule settings below for more information.

Tube Settings

Hydronic tubing inside diameter

This is the inside diameter of the tubes through which water is circulated (in m or in). The inside diameter should be entered in metres and is used to determine the convective heat transfer from the water to the inside surface of the hydronic tubing.

Hydronic tubing length

This is the total length of embedded pipe (in m or ft). The length of the tube is used to determine the effectiveness of heat transfer from the fluid being circulated through the tubes and the tube/surface. Longer tubing lengths result in more heat that will be transferred to/from the radiant surface to the circulating fluid. This data is auto-sizable.

Number of circuits

This input allows you to choose between modelling each surface in the radiant system as a single hydronic circuit or to allow the program to divide the surface into multiple parallel hydronic circuits based on the Circuit length (below). The corresponding options are:

 

 

It is recommended that 2-Calculate from circuit length be chosen for new models. The default is 1-One per surface for backward compatibility with older versions of DesignBuilder.

Circuit length

The length (in m or ft) of each parallel hydronic circuit in a surface. This data is only used when the Number of circuits (above) is set to 2-Calculate from circuit length. The default is 106.7 meters (350 feet), which is the maximum circuit length allowed in Title 24.

Heating

Has heating element

If this radiant surface is connected to a hot water loop for heating then this checkbox should be checked.

Note: Either the Has cooling element or the Has heating element (or both) must be checked.

Heating design capacity method

There are 3 ways to define the heating capacity of the unit as selected from the following list of options:

 

Heating design capacity

This autosizable field defines the convective electric nominal heating capacity (in W or Btu/h). It is only available when the Heating design capacity method is set to 1-Design capacity.

Heating design capacity per floor area

Enter the heating capacity per unit floor area (in W/m2 or W/ft2) of the unit. This data is only available when the Heating design capacity method is 2-Capacity per floor area.

The program calculates the heating capacity from floor area of the zone served by the unit and the heating capacity per unit floor area value specified here.

Fraction of autosized heating design capacity

Enter the heating capacity as a fraction of the autosized heating capacity for convective electric baseboard unit. This data is only available when the Heating design capacity method is 3-Fraction of autosized capacity. EnergyPlus calculates the heating capacity from the design autosized heating capacity and this fraction. The default value is 1.0.

Maximum hot water flow rate

This is the maximum flow rate of hot water through the radiant surface (in m3/s or gal/min). The controls will vary the flow rate of hot water through the surface(s) using zero flow and the maximum flow rate specified in this field as the lower and upper bounds, respectively. This value is auto-sizable.

Tip: See Checking Sizing Results for Design below for advice on how to access the autosizing results for Heating design capacity and Maximum hot water flow rate.

Control

Heating control temperature schedule

This option is only available when using the 2-Detailed HVAC Detailed HVAC Activity data

 

The Heating control temperature schedule specifies the heating setpoint or control temperature for the radiant surface (in °C only). Used in conjunction with the Throttling range, it defines whether or not the system is running and the current flow rate. Water flow rate to the system is varied linearly around the setpoint temperature based on the Throttling range and the Maximum heating flow rate parameters (see above). It should be noted that this control schedule will allow different setpoint temperatures throughout the year for heating. The control of the radiant surface is based solely on the values in this schedule, and the Temperature control type described above. The radiant surface will not use the HVAC zone thermostat that might be used by other systems serving the zone in which the component resides.

Throttling range

This is the range of temperature (°C) which the radiant surface throttles from zero flow rate up to the maximum defined by the Maximum hot water flow rate described above. The throttling range parameter is used in conjunction with the control temperature to define the response of the system to various zone conditions. The heating control temperature schedule specifies the setpoint temperature where the flow rate to the system is at half of the maximum flow rate. For example, if the heating control temperature setpoint is currently 15°C and the heating throttling range is 2°C, the water flow rate to the system will be zero when the controlling temperature (see Temperature Control Type below) is at or above 16°C and the maximum flow rate when the controlling temperature is at or below 14°C. This represents a throttling range of 2°C around the setpoint of 15°C. In between 14°C and 16°C, the flow rate to the radiant system is varied linearly.

 

 

The minimum throttling range is 0.5°C.

Cooling

Has cooling element

If this radiant surface is connected to a chilled water loop for cooling then this checkbox should be checked.

Note: Either the Has cooling element or the Has heating element (or both) must be checked.

Cooling design capacity method

There are 3 ways to define the cooling capacity of the unit as selected from the following list of options:

 

Cooling design capacity

This autosizable field defines the convective electric nominal cooling capacity (in W or Btu/h). It is only available when the Cooling design capacity method is set to 1-Design capacity.

Cooling design capacity per floor area

Enter the cooling capacity per unit floor area (in W/m2 or W/ft2) of the unit. This data is only available when the Cooling design capacity method is 2-Capacity per floor area.

The program calculates the cooling capacity from floor area of the zone served by the unit and the cooling capacity per unit floor area value specified here.

Fraction of autosized cooling design capacity

Enter the cooling capacity as a fraction of the autosized cooling capacity for convective electric baseboard unit. This data is only available when the Cooling design capacity method is 3-Fraction of autosized capacity. EnergyPlus calculates the cooling capacity from the design autosized cooling capacity and this fraction. The default value is 1.0.

Maximum cold water flow rate

This is the maximum flow rate of cold water through the radiant surface (in m3/s or gal/min). The controls will vary the flow rate of hot water through the surface(s) using zero flow and the maximum flow rate specified in this field as the lower and upper bounds, respectively. This value is auto-sizable.

Tip: See Checking Sizing Results for Design below for advice on how to access the autosizing results for Cooling design capacity and Maximum cold water flow rate.

Control

Cooling control temperature schedule

This option is only available when using the 2-Detailed HVAC Detailed HVAC Activity data

 

This schedule specifies the cooling setpoint or control temperature for the radiant surface (in °C only) when in cooling mode. Used in conjunction with the Throttling range, it defines whether or not the system is running and the current flow rate. Water flow rate to the system is varied linearly around the setpoint temperature based on the Throttling range and the Maximum cooling flow rate parameters (see above). This control schedule will allow different setpoint temperatures throughout the year for cooling. The control of the radiant surface is based solely on the temperature values in this schedule, and the Temperature control type listed above. The radiant surface will not use any thermostats defined on the HVAC zone dialog that might be used by other systems serving the zone in which the component resides.

Tip: Some users have found it helpful to give the radiant surface priority by setting slightly lower cooling setpoint temperatures than the setpoints on the HVAC zone dialog used to control a supplementary air system.

Throttling range

This is the range of temperature (in °C or °F) over which the radiant surface throttles from zero flow rate up to the maximum defined by the Maximum Cold Water Flow Rate described above. The throttling range parameter is used in conjunction with the control temperature to define the response of the system to various zone conditions. The cooling control temperature schedule specifies the setpoint temperature where the flow rate to the system is at half of the maximum flow rate. For example, if the cooling control temperature setpoint is currently 25°C and the cooling throttling range is 2°C, the water flow rate to the radiant system will be zero when the controlling temperature (see Temperature Control Type below) is at or below 24°C and the maximum flow rate when the controlling temperature is at or above 26°C. This represents a throttling range of 2°C around the setpoint of 25°C. In between 24°C and 26°C, the flow rate to the radiant system is varied linearly.

 

 

The minimum throttling range is 0.5°C.

Condensation Control

Condensation control type

With a chilled ceiling, there is the possibility that condensation will occur on the surface that is being cooled. This is due to the fact that the surface temperature may drop below the dew-point temperature of the space. When this occurs, condensation on the surface will occur. There are two options for handling this situation:

 

 

If you choose the 2‑Off option, EnergyPlus will not do anything other than produce a warning message when condensation is predicted to occur. The simulation will simply proceed; no moisture will be removed from the zone air and there will be no adjustment of the surface temperature as a result of the condensation. When the 1‑Simple off option is selected, EnergyPlus will predict cases where condensation will occur and shut-off the radiant surface to avoid this situation. With this latter option, you also have the opportunity to adjust when the system will shut down. This is specified with the Condensation Control Dew-point Offset, below.

Condensation control dew-point offset

This offset temperature (in °C or °F) is only available with the 1-Simple off condensation control type. It establishes the difference between the calculated dew-point temperature of the space and the allowed surface temperature to which the surface can drop before the radiant surface shuts down. This setting can be any positive, negative, or zero value. When this parameter is zero, the radiant surface will shut down when the surface temperature drops to the dew-point temperature or below.

 

When this parameter is positive, the radiant system will shut down when the surface is this value above the dew-point temperature. This allows some extra safety to avoid condensation. When this parameter is negative, the radiant surface will shut down when the surface temperature is this value below the dew-point temperature. While not recommended, this strategy allows the user to simulate a situation where small amounts of condensation are tolerable.

Note: This value is an offset not an absolute temperature. A typical value might be 0-2°C.

Operation

Availability Schedule

This is the schedule that determines whether or not the component is available for each timestep of the simulation. A schedule value greater than 0 (usually 1 is used) indicates that the unit can be on during the timestep. A value less than or equal to 0 (usually 0 is used) denotes that the unit must be off for the timestep.

Checking Sizing Results for Design

When Design heating/cooling capacity, Maximum hot/cold water flow rates and tubing lengths are to be autosized, you can find the Component Sizing Summary results on the Summary output generated by EnergyPlus. It is important to note that the "Design Size Heating Design Capacity", "Design Size Maximum Hot Water Flow", "Design Size Cooling Design Capacity", "Design Size Maximum Cold Water Flow" and "Design Size Hydronic Tubing Length" values reported are the sum for all the active surfaces linked to the zone radiant surface object. So these results cannot be used directly to calculate the fluid velocity. Fluid velocity, must be hand calculated by the user for the number of active surfaces when checking pressure drop and fluid turbulence. To help with that you can find the flow fraction for each EnergyPlus surface in the ZoneHVAC:LowTemperatureRadiant:SurfaceGroup data associated with each ZoneHVAC:LowTemperatureRadiant:VariableFlow object.