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10
Unit Operation
Low Temperature Detection
The low temperature detection
diagnostic protects the heat
exchanger by using an analog
leaving water temperature sensor to
protect refrigerant circuit 1 and a
binary low temperature detection
device to protect refrigerant circuit
2. Each individual refrigerant circuit
is disabled when the low
temperature condition exists for that
circuit.
For two compressor units, the
controller responds to low
temperature detection by allowing
the fan to operate, while disabling
the compressor for the faulty circuit.
The compressor for the normal
circuit continues to operate. The
outdoor air damper also operates
normally.
All unit operation is disabled when
the heat pump shuts down both
circuits, due to low temperature
conditions. See Table 3 for more
information.
Table 3: ZN510 response to low temperature detection diagnostic
Description
Fan
Operation
Compressor Operation
Damper
Operation
Low Temperature Detection
(Circuit 1)
Enabled
Circuit 1-Disabled
Circuit 2-Normal Operation
Normal
operation
Low Temperature Detection
(Circuit 2)
Enabled
Circuit 1-Normal Operation
Circuit 2-Disabled
Normal
operation
Low Temperature Detection
(Circuits 1 and 2)
Disabled
Circuit 1-Disabled
Circuit 2-Disabled
Closed
Note:
z The low temperature detection device automatically resets when the heat exchanger temperature returns to
normal. However, you must manually reset the low temperature detection diagnostic to clear the diagnostic and
restart the unit. Refer to page 28 on how to reset a unit.
z If BOP 6 is configured as a generic binary output, the state of the output is not affected by the low temperature
detection diagnostic or by other diagnostics.
Condensate Overflow
A condensate overflow switch
detects the condensate condition.
The condensate overflow switch is a
normally closed device. This switch
is physically connected to the binary
input 2 (BI 2). When the
condensation reaches the trip point,
the binary input detects the
diagnostic condition. A condensate
overflow signal generates a
diagnostic which disables the fan,
disables all compressors, and closes
the 2-position outdoor air damper
(when present). The condensate
overflow diagnostic does not affect
the generic binary output (when
present).
Note:
The condensate overflow switch,
located in the condensate pan, auto-
matically resets when the conden-
sation returns to normal levels.
However, you must manually reset
the controller’s condensate over-
flow diagnostic to clear the diagnos-
tic and restart the unit. Refer to page
28 on how to reset a unit.
Occupancy
ZN510 uses the occupancy binary
input for two occupancy-related
functions. For standalone
controllers (any unit not receiving a
communicated occupancy request,
typically from a building automation
system), the occupancy binary input
determines the unit’s occupancy
based on the hardwired signal.
Typically, the signal is a dry set of
binary contacts which is either
connected to a switch or timeclock
contacts.
When a hardwired occupancy signal
is open, the unit switches to
occupied mode (if the occupancy
input is configured as normally
open). When a hardwired
occupancy signal is closed, the
controller switches to Unoccupied
mode.