Four Pipe Fan Coils
This equipment is modeled as and in-room forced-convection hydronic unit. The hydronic heating coil may be replaced with an electric heating coil. Typically these units are small (200 – 1200 cfm) and self-contained. They are mostly used in exterior zones, usually in hotels, apartments, or offices. They may be connected to ducted outside air, or have a direct outside air vent, but they do not have outside air economizers. Units with outside air economizers are marketed (in the United States) as unit ventilators. Unit ventilators are typically bigger than fan coils and are widely used in classrooms or other applications where ventilation is a priority.
The heating or cooling output of the fan coil is controlled by varying the air flow rate, the water flow rate, or both. Air flow rate can be controlled by cycling the fan on/off or with a variable speed fan drive. The most common setup is a two or three speed fan with the speed selected by hand. The fan then cycles on/off to control heating / cooling output. The controls are often a wall mounted thermostat with hand selection of heating/cooling and fan speed (off/low/medium/high).
These controls may also be mounted on the unit.
Fan coil units with hydronic heat can instead be modeled using an electric heating coil if desired (i.e., replace the hydronic heating coil with an electric heating coil).
Note: Standard TRACE library members cannot be modified or deleted but they can be copied and edited.
Nominal Heating Capacity
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Default: Auto Size box checked
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Typical Range: 2 to 65 Mbh
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: % Htg Capacity; W; kW; W/m2; W/ft2; tons; MW; Btuh; MBh; tons/ft2; Btuh/ft2
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This checkbox indicates whether or not the capacity used for the selected heating coil will be auto sized by the program or referenced from the user defined input.
Nominal Cooling Capacity
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Default: Auto Size box checked
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Typical Range: 2 to 65 Mbh
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: % Clg Capacity; W; kW; W/m2; W/ft2; tons; MW; Btuh; MBh; tons/ft2; Btuh/ft2
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This checkbox indicates whether or not the capacity used for the selected cooling coil will be auto sized by the program or referenced from the user defined input.
Supply Fan
Supply Fan Placement
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Default: Blow Through
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This selectable input field defines how the supply fan is arranged. There are two choices, Blow Through or Draw Through. The Blow Through choice indicates that the fan is located upstream of the coils. The Draw Through choice indicates that the fan is located downstream of the coils. These are the only configurations that are available and the HVAC system nodes used as inlets and outlet in the components must be arranged consistent with the flow path.
Capacity Control Method
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Default: Multi-speed Cycling Fan with Constant Water Flow
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This field is used to determine how the unit fan is to be controlled. There are four possible choices:
1. Multi-speed Cycling Fan with Constant Water Flow: The fan speed is chosen so that the unit capacity is greater than or equal to the heating / cooling load and the fan is cycled to match unit output with the load. The fan number of fan speeds is limited to a handful (1 – 3), as compared to the multiple speeds used in the Multi-speed Fan Cycling Speeds with Constant Water Flow control option.
Note: This capacity control method should only be used when the Supply Fan type selected is Constant Volume.
2. Constant Flow Fan Variable Water Flow: The fan speed is held constant to produce a fixed air flow rate whenever the unit is scheduled on. The hot water or chilled flow rate is varied so that the unit output matches the zone heating or cooling requirement.
Note: This capacity control method should only be used when the Supply Fan type selected is Constant Volume.
3. Variable speed Fan Variable Water Flow: Both air and water flow rates are varied to match the load.
Note: This capacity control method should only be used when the Supply Fan type selected is Variable Volume.
4. Variable speed Fan Constant Water Flow: The water flow rate is at full flow and the fan speed varies to meet the load.
Note: This capacity control method should only be used when the Supply Fan type selected is Variable Volume.
5. Multi-speed Fan Cycling Speeds with Constant Water Flow: The water flow rate is at full flow and the fan cycles between multiple set speeds to meet the load.
Note: This capacity control method should only be used when the Supply Fan type selected is Constant Volume.
6. ASHRAE 90.1 Control: The fan air flow rate is reduced according to the low speed supply air flow ratio when the zone sensible load is less than the zone sensible load multiplied by the low speed supply air flow ratio. The water coil flow rate, or the electric heating coil part-load ratio, is modulated to meet the zone load. If the zone sensible load is greater than the zone sensible load multiplied by the low speed supply air flow ratio, then the air and water flow rate is increased to meet the load. If the zone load is greater than the design sensible load, the fan air flow rate is maintained at the maximum value while the water flow rate is further increased to the maximum available while electric heating coils are maintained at the maximum output.
Special Note: when the ASHRAE 90.1 VAV Fan is selected as the Supply Air Fan, if the Minimum Supply Air Temperature in Cooling/Heating Mode inputs are not specified, the simulation must include zone sizing to calculate the zone design sensible cooling and heating load used to modulate the fan speed and, for water coils, the water flow rate, or for electric heating coils, the part load ratio.
This control strategy is very similar to the reverse acting VAV control methodology used in TRACE™ 700. A schematic of this control strategy is provided below:
Note: This capacity control method can be used when the Supply Fan type selected is either Constant Volume or Variable Volume.
Supply Fan Type
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Default: Constant volume
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This is the type of supply fan that is associated with the library member. Two options are available: Constant or Variable Volume.
Supply Air Fan
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Default: Direct Drive Plenum Fan
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This input selection refers to the name of the fan library member that is used in this object.
Fan Full Load Energy Rate
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Default: 0.0002 kW/(cfm); Auto Size box is unchecked
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Typical Range: 0.0001 to 0.0004 kW/(cfm)
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: W; kW; W/(m3/sec); W/(m3/sec)-Pa; W/(cfm); W/(L/sec); W/(L/sec)-Pa; kW/(cfm); kW/(L/sec); kW/(cfm)-in H2O; kW/(L/sec)-Pa; W/(cfm)-in H2O; bhp
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This checkbox indicates whether or not the full load energy rate used for the selected fan will be auto sized by the program or referenced from the user defined input.
Pressure Rise
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Default: 4 inches H2O
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Typical Range: 0.25 to 5 in H2O
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: inches H2O; lb/ft2; psig; Pa; kPa; mm Hg; mm H2O; bar; in Hg; atm; mm CE; m CE; m H2O; mbar
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The value will be populated by default based on the fan library member selected. You can override this value and the units of measure.
Low Speed Air Flow
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Default: 33%
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Typical Range: N/A
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Min Max: 0 to 100%
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Units: %
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This numerical field specifies the low speed flow rate to the maximum supply air flow rate as a percentage. This value should be less than the Medium Speed Air Flow value. Leave this field blank if the capacity control method selected is not Cycling Fan.
Medium Speed Air Flow
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Default: 66%
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Typical Range: N/A
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Min Max: 0 to 100%
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Units: %
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This numerical field specifies the medium speed flow rate to the maximum supply air flow rate as a percentage. This value should be greater than the Low Speed Air Flow value but less than 100%. Leave this field blank if the capacity control method selected is not Cycling Fan.
Dead Band Airflow
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Default: Auto Size box checked
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Typical Range: 200 to 1200 cfm
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: m3/sec; cfm; cfm/ft2; (m3/s)/m2; % Clg Airflow; % Htg Airflow; m3/hr-m2; L/s; m3/hr
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This numeric field defines the supply air flow rate leaving the fan coil when neither cooling nor heating is required (i.e. the cooling and heating coils are not operating). This field is only used when the fan coil supply air fan operating mode schedule specifies continuous fan operation.
If the air conditioner’s supply air fan operating mode schedule specifies continuous fan operation and this value is set to zero or this field is left blank, then the model assumes that the supply air flow rate when no cooling/heating is needed is equal to the supply air flow rate when the cooling or heating coil was last operating (for cooling operation or heating operation).
Unitary Cooling
Unitary Cooling Type
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Default: Water (Detailed)
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This field specifies the type of chilled water cooling coil to model for this fan coil unit. Only two options are available to choose from: Water (Detailed) and Water (Simple)
Unitary Cooling
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Default: 10 Row – Copper Fins – Prima Flo
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This field allows you to select the cooling coil library member that will be used for this four pipe fan coil equipment.
Unitary Cooling Design Airflow
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Default: Auto Size box checked
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Typical Range: 200 to 1200 cfm
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: m3/s; cfm; cfm/ft2; (m3/s)/m2; % Clg Airflow; cfm/ton; (m3/s)/(kW of Cooling Capacity); (m3/hr)/(kW of Cooling Capacity); (m3/s)/(W of Cooling Capacity); m3/hr ; m3/hr/m2; L/s; (L/s)(kW of Cooling Capacity)
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This input refers to the design cooling airflow value associated with the four pipe fan coil library member. This input can be used to scale the size of the coil as well.
Unitary Heating
Unitary Heating Type
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Default: water
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This field specifies the type of heating coil to model for this fan coil unit. Only two options are available to choose from: Water and Electric
Unitary Heating
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Default: Water Heating Coil
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This field allows you to select the heating coil library member that will be used for this four pipe fan coil equipment.
Unitary Heating Design Airflow
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Default: Auto Size box checked
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Typical Range: 200 to 1200 cfm
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: cm3/s; m3/s; cfm; cfm/ft2; (m3/s)/m2; % Htg Airflow; cfm/ton; (m3/s)/(kW); (m3/hr)/(kW); (m3/s)/(W); m3/hr ; m3/hr/m2
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This input refers to the design heating airflow value associated with the four pipe fan coil library member. This input can be used to scale the size of the coil as well.