Heating / Cooling coil variable speed heat pump, one analog output with reversing valve control (HCclHpu11)

Required elements and how to configure them:

Function

The figure below shows BACnet objects associated with this application function. Primary signal flow is summarized as follows:

The demand signal(s) are received (HCclHReq / HCclCReq / HCclDhuReq) and processed into an analog output command signal for a variable speed heat pump (HpuVarSpd).

Basic function: Accept request signal from associated room controller and pass it through a device mode logic switch; Output the result as a command to the BACnet object that controls the device.

Sequence - heating, cooling and dehumidification control

If heating, cooling or dehumidification is used, the accepted request signal (HCclHReq, HCclCReq, HCclDhuReq) will be converted into an analog output signal for the variable speed heat pump (HpuVarSpd), and a binary output signal (EnHpuSpd) that can be connected and used as an enable/disable input to the compressor (optional). A binary output commands the reversing valve position. (see Compressor commanding)

An additional binary output is available for source control.

Note
This AF does not perform loop control; its main purpose is to run commands. Loop control processes (for this AF) are managed in the "room" AF(s) that collaborate with this AF to command the end device.

Device mode (HCclDevMod): The input signal for device mode is a multistate value. HCclDevMod supports the following states:

• Off
• Control mode
• Fully open, heating
• Fully open, cooling

Availability & Request signals (heating, cooling and dehumidification)

Heating availability: When the following conditions are true, the binary output signal for "Heating / Cooling coil available for heating" (HCclAvlH) will be Yes (available):

• Device mode = Control mode
• Manual control mode parameter (CtlModMan) = Automatic or Heating
• Present dehumidification request is 0%
• OA temperature not below a configurable limit (optional - see EnLockHTOaLo in Configuration)

When HCclAvlH is Yes, and the heating request signal is greater than the switch-on point, the heating request signal will be accepted.

Cooling availability: When the following conditions are true, the binary output signal for "Heating / cooling coil available for cooling" (HCclAvlC) will be Yes (available):

• Device mode = Control mode
• Manual control mode parameter (CtlModMan) = Automatic or Cooling
• Present dehumidification request is 0%

When HCclAvlC is Yes, and the cooling request signal is greater than the switch-on point, the cooling request signal will be accepted.

Dehumidification availability: When the following conditions are true, the binary output signal for "Heating / cooling coil available for dehumidification" (HCclAvlDhu) will be Yes (available):

• Device mode = Control mode
• Manual control mode parameter (CtlModMan) = Automatic or Cooling

When HCclAvlDhu is Yes, and the dehumidification request signal is greater than the switch-on point, the dehumidification request signal will be accepted.

Reversing valve operation

When the device mode (HCclDevMod) is off the reversing valve is inactive. Otherwise, the position of the reversing valve depends on the configuration of the Boolean parameter RvrVlvActv (Reversing valve is activated by). Compressor commanding during reversing valve switchover depends on the configuration of the Boolean parameter CprStaRvrVlv (Compressor state during switchover reversing valve). Both parameters must be set according to the operation type of heat pump being used. See Configuration.

Changeover delay for heating/cooling
Parameter ChovrDlyHC (Changeover delay for heating/cooling) represents the minimum delay time before reversing valve switchover can occur.

If compressor must be off during reversing valve switchover (CprStaRvrVlv = Off), the reversing valve may only be changed after the compressor remains off longer than ChovrDlyHC.
If compressor must be on during reversing valve switchover (CprStaRvrVlv = On), the reversing valve may only be changed after the change request persists longer than ChovrDlyHC.

Heat pump state (HpuSta)

HpuSta is a MCalcVal object that indicates the operating state of the heat pump (Off, Heating, or Cooling). If device mode (HCclDevMod) is off, HpuSta will be off. The value of HpuSta also depends on the configuration parameter RvrVlvActv (Reversing valve is activated by) and the state of the binary output object HpuRvrVlv (Heat pump reversing valve). HpuRvrVlv can be 0:Inactive or 1:Active.

When device mode is not off,

• If the reversing valve is activated by cooling (RvrVlvActv = Cooling) and HpuRvrVlv is Active, then HpuSta is Cooling.
• If the reversing valve is activated by heating (RvrVlvActv = Heating) and HpuRvrVlv is Active, then HpuSta is Heating.

Compressor commanding (HpuVarSpd, EnHpuSpd)

The 0-100% demand signal (heating, cooling or dehumidification) is converted to an analog output signal for the variable speed compressor (HpuVarSpd). The variable speed compressor operation has a minimum on / minimum off time of 3 minutes (3 minutes on, 3 minutes off, the times are configurable) applied to the AO and the optional BO (EnHpuSpd) at priority 5.

HpuVarSpd will also be limited by configurations for min and max output voltages (see Configuration). EnHpuSpd will be off whenever demand is so low that the AO signal drops to its minimum speed output level.

Equipment protection
The normal runtime priority for compressor commanding is 15 (Auto mode 6). During startup and changeover (reversing valve switchover) as well as for interlock functions such as airflow status, minimum On/Off timers and source state monitoring, the compressor(s) are commanded at priority 5 (Protection mode 2) to ensure equipment protection.

The system evaluates a set of conditions in sequential order. If (as soon as) one is found to be True, the compressor(s) are commanded On or Off at priority 5 and the evaluation sequence stops. See table.

If none of the conditions require protection, the heat pump compressor command at priority 5 is released.

Source commanding

When the present demand (heating, cooling or dehumidification) exceeds the switch-on point, a delay-off timer (DlyOffEnSrc) is activated to prevent compressor cycling and then the enable source command (EnHpuSrc) and source demand (HpuSrcDmd) will both be True.

Source available for monitoring

This feature indicates that media is available and heat pump coils can be activated. Source available indication can be from a central group function (Prio 15) or from a local BI sensor input (Prio14). If source available monitoring function is enabled (EnMonSrcAvl is Yes) and source is unavailable, device mode is set to off at Prio4 and each temperature/dehumidifier controller is turned off.

Source state monitoring

The source state indicates that media is flowing and heat pump coils can be activated.

The type of source state is configured to identify the location of the source (SrcStaMon). SrcStaMon is a multistate configuration parameter:

• None – The off command at equipment safety priority to device mode is disabled.
• Monitor enable source - When the source is commanded On, after a delay (DlyOnSrcSta) the compressor will be released; otherwise the compressor is set to off at equipment safety priority.
• Monitor source state - The source state (SrcSta) can come from a local BI sensor in the automation station. When there is no media flow the compressor is set to off at equipment safety priority.

Interlocks

Air volume flow status: When the switch indicates no airflow, the component sets compressors off at equipment safety priority (Prio 5, Protection mode 2). When the switch indicates airflow, the safety command to the compressors is released.

Air volume flow requests (heating, cooling, dehumidification)

Airflow heating request: When the present heating request signal is greater than the switch-on point or the device mode is "Fully open, heating" and the manual control mode is Automatic or Heating, then after a delay the Air volume flow heating request will be True.

Airflow cooling request: When the present cooling request signal is greater than the switch-on point or the device mode is "Fully open, cooling" and the manual control mode is Automatic or Cooling, then after a delay the Air volume flow cooling request will be True.

Airflow dehumidification request: When the present dehumidification request signal is greater than the switch-on point, then after a delay the Air volume flow dehumidification request will be True.

Airflow hold requests (heating, cooling)

Airflow hold for heating: When the present demand signal is greater than the switch-on point and the reversing valve position is Heating, the Air volume flow hold for heating will be True. When the present demand signal is less than the switch-on point minus a hysteresis, then after a delay the Air volume flow hold for heating will be False.

Airflow hold for cooling: When the present demand signal is greater than the switch-on point and the reversing valve position is Cooling, the Air volume flow hold for cooling will be True. When the present demand signal is less than the switch-on point minus a hysteresis, then after a delay the Air volume flow hold for cooling will be False.

Signals in the table are internal signals, not BACnet objects. For this reason they are not visible in the tool, but the parameters associated with them are. See comment column for hints on parameters that affect interlock functionality.

Equipment output limits

To avoid ice and refrigerant problems, specifications may call for logic that turns off the heat pump when the outside air is very cold. The following heat pump lock out feature is not active by default. To enable it, EnLockHpuTOaLo must be set to 1:Yes.

When the measured OA temperature value is below a configured cut out temperature, device mode (HcclDevMod) is set to Off at equipment safety priority (Prio 4). When the OA temperature rises above the cut out temperature plus a configured deadband, HcclDevMod is released from priority 4 and returns to normal control.

Fault inputs

Various heat pump safety switches can be connected to shut off the equipment including these examples:

• Low suction pressure
• High compressor pressure
• High refrigerant pressure
• Low refrigerant pressure

Note
See the Hvac17 room segment AF for how to configure a condensate level monitor. Hvac17

Optional binary input object(s) pick up any Heat pump equipment alarm switch. When the switch indicates a fault, the component sets device mode to off at equipment safety priority (Prio 4). The "Available" signals for cooling and dehumidification also turn off. When the switch indicates no fault, the command to the device mode is disabled.

Heat pump compressor power

The nominal electric power usage of the compressor is provided for energy monitoring and coordination with other devices. The nominal power (NomElPwr) can be obtained from the heat pump name plate (from heat pump manufacturer).

If the Boolean parameter HpuPwr = 0:Calculated, then HpuElPwr is calculated as follows:
HpuElPwr = NomElPwr * (HpuVarSpd/100%)^3
(The calculation will be enabled / disabled based on the status of EnHpuSpd (Enable heat pump variable speed)).

If HpuPwr (Heat pump power) = 1:Measured, then an optional analog input (Pwr) must provide the measured value of heat pump electric power (HpuElPwr).

Configuration

Objects

Parameters