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McQuay Enfinity vertical water source heat pumps are
available with a factory mounted and tested Alerton BACnet
unit controller as a special. The unit controller is factory pro-
grammed and tested with all the logic required to control the
unit, and is designed to communicate over a BACnet MS/TP
communications network to an Alerton BACtalk building
automation system (BAS). The controller operates the com-
pressor, fan, and reversing valve as required to maintain the
space temperature within the current setpoints. Data
regarding equipment status, water and air temperatures,
and fault conditions can be monitored by an Alerton BACtalk
BAS. Setpoints and other system preferences may be
changed remotely using an Alerton BACtalk workstation or
Alerton service tool software.
The controller makes operational data and commands
available on the Alerton BACtalk network using BACnet
objects and properties. Each heat pump controller connects
to the BACtalk network using a BACnet MS/TP LAN, which
is a simple twisted-pair communications connection that
operates at up to 76.8 Kbps. DIP switches on the controller
enable the MS/TP MAC address to be set in the range 0-
127. A status LED on the unit indicates communication
activity on the MS/TP LAN.
Each BACnet-compliant unit includes discharge air and
leaving water temperature sensors, as well as all safety
sensors, signals, and switches. Wall-mounted room sen-
sors are available from Alerton to control heating and cool-
ing operation. Available sensors include tamper-resistant
stainless steel wall sensors with optional push-button for
status override; wall-mounted sensors with tenant setpoint
adjustment lever and timed-override button; wall-mounted
sensors with LED status, timed-override button, tenant set-
point adjustment buttons, password-protected field service
access to operational data, and optional humidity sensor;
and wall-mounted sensors with LCD and programmable
operation.
Figure 18. BACnet
®
WSHP unit controller
Each BACnet-compliant controller has the following operat-
ing features:
●
Start-up – The unit will not operate until all the inputs
and safety controls are checked for normal conditions.
●
Fan operation – Fan operation can be customized in
software to run continuously during occupied mode, or to
cycle ON or OFF appropriately on a call for heating and
cooling.
●
Cooling mode – On a call for cooling, the compressor
and fan start immediately. Compressor run-time is cal-
culated as a percent of full cycle time (17 minutes) using
proportional-integral control to maximize efficiency.
●
Heating mode – On a call for heating, the compressor
and fan start immediately, and compressor run-time is
calculated as a percent of full cycle time (17 minutes)
using proportional-integral control to maximize
efficiency.
●
Short Cycle Protection and Random Start – A start
delay of 180 seconds plus the compressor’s MAC
address in seconds prevents short-cycling and simulta-
neous start-up. A minimum 2-minute on time and 5-
minute off time for the compressor further ensures short-
cycle protection.
●
Occupied Mode – A simple software control signal from
the building automation system or a wall-mounted unit
puts the unit into occupied mode. The unit controls com-
pressor and fan operation to maintain occupied set-
points. High and low limits for occupied setpoints are
software configurable.
●
Unoccupied Mode – A simple software control signal
from the building automation system or a wall-mounted
unit puts the unit into unoccupied mode for night setback
operation. The unit controls compressor and fan opera-
tion to maintain unoccupied heating and cooling set-
points, which are also software configurable.
●
After-hours Override Mode – A simple software control
signal from the building automation system or a wall-
mounted unit can initiate after-hours heating or cooling
in half-hour increments. Maximum override time is soft-
ware configurable up to 9.5 hours. This feature can also
be disabled in software.
●
Reversing valve delay – When the compressor turns
off after heating mode, the reversing valve remains ener-
gized for 60 seconds before it returns to the normal cool-
ing position to eliminate swishing. The reversing valve
energizes 10 seconds before the compressor.
●
Load Shed – Load shedding can be orchestrated by the
building automation system using the occupied/unoccu-
pied command in software.
●
Brownout Protection – An onboard sensor measures
input voltage and suspends compressor and fan opera-
tion if the supply voltage drops below 82% of the normal
line voltage for a minimum of 10 seconds, creating an
alarm available in software. The alarm automatically
resets when the supply voltage returns to above 90% of
normal.
BACnet
®
WSHP Unit Controller
