Defrost Control; Cooling Mode Operation - Lennox HP21 Unit Information

TABLE 10
AMBIENT COMPENSATION THERMISTOR
Ambient Temperature
°
F
32
77
100
T−Defrost Control CMC1
The CMC defrost control (figure 13) is a solid state control
manufactured by Ranco. The control provides automatic
switching from normal heating operation to defrost mode
and back. Once in defrost mode, the control times the de-
frost period. Defrost time varies dependent on the
temperature difference between the liquid line and ambient
(outdoor) temperature.
The control monitors ambient (outdoor air) temperature,
liquid line temperature and total compressor run time to de-
termine when a defrost cycle is required. Two temperature
probes are permanently attached to the control. The red
probe is used to measure ambient air temperature and the
blue probe is used to measure liquid line temperature. The
ambient probe is attached inside the ambient air port lo-
cated in the lower right corner of the compressor
compartment (see figure 1). The liquid line probe is at-
tached to the cooling liquid line adjacent to the expansion
valve.
The temperature probes cannot be detached from the con-
trol. The control and the attached probes MUST be
replaced as a unit. Do not attempt to cut or splice probe
wires.
Operation
On compressor startup, the control begins measuring com-
pressor run time. The control measures run time for the
current thermostat demand (cycle) and total run time since
the last defrost.
Two minutes after receiving a thermostat demand (after
two minutes of cycle run time) the control begins to mea-
sure liquid line temperature. When the liquid line
temperature drops below 39°F, the control begins to moni-
tor ambient temperature and to measure lockout time.
Lockout time is defined as the amount of time the heating
mode can continue before initiating a defrost. The amount
of lockout time which can be accumulated is determined by
Resistance Through Sensor
ohms
861
260
150
Page 17
ambient temperature as shown in table 11. When the con-
trol reaches the amount of lockout time determined by the
ambient temperature, the control proceeds to the next
step.
TABLE 11
Ambient
HP21−411/−511
°
Temperature F
HP21−413/−513
HP21−36−230/233
HP21−48−230/233
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34 and above
When the lockout time as shown in table 11 is satisfied, the
control begins to compare the temperature difference be-
tween the liquid line and ambient air. Liquid line
temperature and ambient air temperature are monitored in-
definitely (total run time continues to accumulate). When
the actual temperature difference between liquid line and
ambient air exceeds a calculated temperature difference,
the defrost relay is activated. Table 12 shows how the de-
frost control calculates the difference.
Defrost will last for for a maximum of 15 minutes or until coil
temperature reaches 70°F. An LED on the circuit board in-
dicates control is in defrost mode. Defrost timings are not
adjustable. When defrost relay contacts close, compressor
total run-time and lockout time are cleared. When defrost
terminates, timing cycles restart.
If thermostat demand is satisfied and a defrost was not initi-
ated during the cycle, compressor run-time will stop (but
not reset) when demand is satisfied and continue when a
new thermostat demand is received. If the control accumu-
lates a total compressor run-time of six hours, defrost will
be initiated.

Cooling Mode Operation

The defrost control uses the liquid line temperature sensor
to determine that the control is in cooling mode and defrost
is no longer needed. When the liquid line rises above 65°F,
the defrost control is turned off and cannot initiate defrost.
Lockout Time (Minutes)
HP21−651
HP21−653
HP21−60−230/233
120 60
98 49
85 42
74 37
67 33
61 30
56 28
52 26
49 24
47 23
45 22
44 22
43 21
42 21
41 20
40 20