Page 4
DANGER
Do not attempt to repair this control. Unsafe
operation will result. If the control has failed,
replace the control.
6 – Defrost Relay K4
The defrost relay controls defrost. The relay is a 3PDT
relay powered 24 VAC from the thermostat. K4 is
enabled during both cooling and heating modes
(except emergency heat). It is only powered when the
defrost control is calling for defrost. When energized,
the reversing valve and indoor auxiliary heat are
energized. Simultaneously, the outdoor fan is
de-energized. K4 latches in for the duration of the
defrost period. Refer to unit wiring diagram and
operation sequence in the back of this manual.
7 – Defrost Control CMC1
The CMC1 defrost control (figure 4) is a solid state control
manufactured by Hamilton Standard. The control
provides automatic switching from normal heating
operation to defrost mode and back. The control
provides 14 minute defrost periods at 30, 60 or 90 minute
field changeable intervals. The control monitors
thermostat demand and “holds” the timer in place
between thermostat demand. A set of diagnostic pins are
also provided for troubleshooting the unit.
The control contains a solid state timer which switches
an external defrost relay through 1/4” male spades
mounted on the control’s circuit board. When the
defrost thermostat closes (call for defrost), the defrost
timer initiates a 30, 60 or 90 minute (depending on how
the control is preset) timing sequence. If the defrost
thermostat remains closed when the timing sequence
ends, the defrost relay is energized and defrost begins.
SOLID STATE DEFROST CONTROL CMC1
Timing
Jumper
Timing Pins
Troubleshooting Pins
Control Terminals
30
60 90
FIGURE 4
Defrost Control Components
1– Timing Pins 30, 60, 90
Each of these pins provides a different timed
interval between defrosts. A jumper connects the
pins to circuit board pin W1. Table 3 shows the
timings of each pin. The defrost interval can be field
changed to 30, 60 or 90 minutes. The defrost period
(14 minutes) cannot be changed. To change the
interval between defrosts, simply remove the
jumper from the pin it is connected to and
reconnect the jumper to one of the other available
pins (see figure 5).
TABLE 3
INTERVAL BETWEEN DEFROSTS
WITH JUMPER CONNECTED TO:
30
60
90
NORMAL
OPERATION
“TST” PINS
JUMPER
30 + 3
60 + 6
90 + 9
14 + 1.4
7 + 0.7
14 + 1.4
21 + 2.1
3.3 + 0.3
MIN.
MIN.
MIN.
MIN.
SEC.
SEC.
SEC.
SEC.
TOGETHER
CMC1 DEFROST
CONTROL
TIMINGS
DEFROST
TIME
1– Turn off all power to the unit to avoid circuit board damage.
2– Grasp wire connector firmly with fingers.
3–
Gently pull connector from pin.
4– Select new timing pin. DO NOT SELECT A “TST” PIN.
5– Gently push connector onto desired pin (see Table 3 for timings).
6– Turn on power to unit.
FIGURE 5
WARNING – AVOID CONTACT WITH OTHER CON-
TROL TERMINALS OR CONTROL COMPONENTS.
WARNING – DO NOT CONNECT
TIMING JUMPER TO EITHER
“TST” PIN.
TO CHANGE CONTROL TIMINGS:
DEFROST CONTROL TIMING CHANGES
2– Timing Jumper
The timing jumper is a factory installed jumper
on the circuit board used to connect pin W1 to
one of the three timing pins. The jumper may be
connected to any one of the timing pins but must
never be connected to either of the “TST” pins.
See Caution below.
CAUTION
3– “COM” Terminal
Terminal “COM” provides 24VAC Common.
4– “HLD” Terminal
Terminal “HLD” holds the internal timer in place
between thermostat demands and allows the unit to
continue timing upon resumption of thermostat
demand. Terminal “HLD” is connected directly to
thermostat demand.
NOTE – Hold function operates between thermostat
demands only when defrost thermostat is closed. This
is the only time that the timer is operating.
