Radiant Surface - Constant Flow

The constant flow radiant surface keeps flow rate constant via a local circulation pump and varies the water temperature that is sent to the surface. This is accomplished with a mixing valve that is controlled by a sensor. The constant flow type has a built-in local secondary loop, which re-circulates flow coming out of the system and mixes this with flow from the loop to arrive at the desired inlet temperature to the surface (note that this radiant surface model has the temperature sensor after the local secondary pump to ensure proper inlet temperature to the surface). The local loop also contains a pump which is assumed to be upstream of the component and after the mixing valve. So, the local loop can have some recirculation. The flow from the main loop may also bypass the component if more than enough flow is available and the main loop is also a constant flow system.

General

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 two 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 heated floor 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 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.

Note: With the constant flow radiant surface, this length is not autosizable.

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.

Flow Settings

Rated flow rate

This is the maximum flow rate of water through the heated floor (in m3/sec or gal/min). This flow rate is held constant by the local component pump, but you have the option of varying this flow rate via a schedule (see Pump Flow Rate Schedule). The constant flow system will accept this flow rate and control the inlet temperature based on the control and water temperature schedules defined below.

Pump Settings

Pump flow rate schedule

This schedule modifies the maximum flow rate of water through the radiant surface (in m3/s only). Note that the values for this schedule must be between zero and one.

Rated pump head

This is the pump rated head (Pa or ftH20).

Rated power consumption

This is the pump rated power consumption (W).

Motor efficiency

This is the pump efficiency in decimal form (0 = 0%, 1 = 100%).

Fraction of motor inefficiencies to fluid stream

This is the fraction of the pump power lost to the fluid.

Operation

Availability schedule

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

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.

Control

Heating high water temperature schedule

This schedule specifies the high water temperature (in °C only) for the temperature control of the constant flow heated floor. Water and control temperatures for heating work together to provide a linear function that determines the water temperature sent to the heated floor. The current control temperature (see Temperature Control Type above) is compared to the high and low control temperatures at the current time. If the control temperature is above the high temperature, then the system will be turned off and the water mass flow rate will be zero. If the control temperature is below the low temperature, then the inlet water temperature is set to the high water temperature. If the control temperature is between the high and low value, then the inlet water temperature is linearly interpolated between the low and high water temperature values.

 

Heating low water temperature schedule

This schedule specifies the low water temperature (in °C only) for the temperature control of the constant flow heated floor. For more information on its interpretation, Heating high water temperature schedule above.

Heating high control temperature schedule

This schedule specifies the high control temperature (in °C only) for the temperature control of a constant flow heated floor. For more information on its interpretation, see Heating high water temperature schedule above.

Heating low control temperature schedule

This schedule specifies the low control temperature (in °C only) for the temperature control of a constant flow heated floor. For more information on its interpretation, see Heating high water temperature schedule above.

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.

Control

Cooling high water temperature schedule

This schedule specifies the high water temperature (in °C only) for the temperature control of a constant flow chilled ceiling. Water and control temperatures for cooling work together to provide a linear function that determines the water temperature sent to the chilled ceiling. The current control temperature (see Temperature control type above) is compared to the high and low control temperatures at the current time. If the control temperature is above the high temperature, then the inlet water temperature is set to the low water temperature. If the control temperature is below the low temperature, then system will be turned off and the water mass flow rate will be zero. If the control temperature is between the high and low value, then the inlet water temperature is linearly interpolated between the low and high water temperature values.

 

 

Cooling low water temperature schedule

This schedule specifies the low water temperature (in °C only) for the temperature control of the constant flow chilled ceiling. For more information on its interpretation, see Cooling high water temperature schedule above.

Cooling high control temperature schedule

This schedule specifies the high control temperature (in °C only) for the temperature control of a constant flow chilled ceiling. For more information on its interpretation, see Cooling high water temperature schedule above.

Cooling low control temperature schedule

This schedule specifies the low control temperature (in °C only) for the temperature control of a constant flow chilled ceiling. For more information on its interpretation, see Cooling high water temperature schedule above.

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 chilled ceiling 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 valid with the 1-Simple off condensation handling algorithm (see Condensation control type above). 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 chilled ceiling shuts down. This setting can be any positive, negative, or zero value. When this parameter is zero, the chilled ceiling 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 chilled ceiling 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.