Commercial Defrost Control - Carrier 50HCQ Installation Instructions Manual

The CTB does not provide any microprocessor control; it
is simply a basic multifunction
configuration.

Commercial Defrost Control

The Commercial Defrost
coordinates thermostat demands for supply fan control, 1
or 2 stage cooling, 2 stage heating, emergency heating and
defrost control with unit operating sequences. The DFB
also provides an indoor fan off delay feature (user
selectable). See Fig. 28 for board arrangement.
Fig. 28 − Defrost Control Board Arrangement
The DFB is located in the 50HCQ's main control box (see
Fig. 30). All connections are factory−made through
harnesses to the unit's CTB, to IFC (belt−drive motor) or
to ECM (direct−drive motor), reversing valve solenoids
and to defrost thermostats. Refer to Table 5 for details of
DFB Inputs and Outputs.
Reversing valve control — The DFB has two outputs for
unit reversing valve control. Operation of the reversing
valves is based on internal logic; this application does not
use an "O" or "B" signal to determine reversing valve
position. Reversing valves are energized during the
cooling stages and the defrost cycle and de−energized
during heating cycles. Once energized at the start of a
cooling stage, the reversing valve will remain energized
until the next heating cycle demand is received. Once
de−energized at the start of a Heating cycle, the reversing
valves will remain de−energized until the next cooling
stage is initiated.
Compressor control — The DFB receives inputs indicating
Stage 1 Cooling, Stage 2 Cooling and Stage 1 Heating from
wiring terminal
Control Board (DFB)
DIP
Switches
Speed-Up
Jumpers
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the space thermostat or unit control system (PremierLinkt
controller or RTU Open controller); it generates commands
to start compressors with or without reversing valve
operation to produce Stage 1 Cooling (one compressor on
08−09 systems, compressor unloaded on 07 2−stage system),
Stage 2 Cooling (both compressors run on 08−09 systems,
compressor full load on 07 2−stage system) or Stage 1
Heating (both compressors run on 08−09 systems; the 07
systems have only one compressor).
Auxiliary (Electric) Heat control — The 50HCQ unit can
be equipped with one or two auxiliary electric heaters, to
provide a second stage of heating. The DFB will energize
this Heating System for a Stage 2 Heating Command
(heaters operate concurrently with compressor(s) in the Stage
1 Heating cycle), for an Emergency Heating sequence
(compressors are off and only the electric heaters are
energized) and also during the Defrost cycle (to eliminate a
"cold blow" condition in the space).
Defrost — The defrost control mode is a time/temperature
sequence. There are two
continuous run period and the test/defrost cycle period.
The temperature component is provided by Defrost
Thermostat 1 and 2 (DFT1 and DFT2 [08−09 only])
mounted on the outdoor coil.
The continuous run period is a fixed time period between the
end of the last defrost cycle (or start of the current Heating
cycle) during which no defrost will be permitted. This period
can be set at 30, 60, 90 or 120 minutes by changing the
positions of DIP switches SW1 and SW2 (see Fig. 29 and
Table 6). The default run periods are 30 minutes for unit size
07 and 90 minutes for unit sizes 08−09.
Fig. 29 − DIP Switch Settings — Defrost Board
Shorting the jumpers for a period of 5 to 20 secs bypasses
the remaining continuous run period and places the unit in
a Forced Defrost mode. If the controlling DFT is closed
when this mode is initiated, the unit will complete a
normal defrost period that will terminate when the
controlling DFT opens or the 10 minute defrost cycle limit
is reached. If the controlling DFT is open when this mode
is initiated, the Defrost cycle will run for 30 secs. Both
modes end at the end of the Defrost cycle.
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time components: The
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